Unit - 2
Physical Layer
The OSI model is shown in Fig. This model is based on a proposal developed by the International Standards Organization (ISO) as a first step toward international standardization of the protocols used in the various layers (Day and Zimmermann, 1983). It was revised in 1995. The model is called the ISO-OSI (Open Systems Interconnection) Reference Model because it deals with connecting open systems—that is, systems that are open for communication with other systems. The OSI model has seven layers. The principles that were applied to arrive at the seven layers can be briefly summarized as follows:
1. A layer should be created where a different abstraction is needed.
2. Each layer should perform a well-defined function.
3. The function of each layer should be chosen with an eye toward defining internationally standardized protocols.
4. The layer boundaries should be chosen to minimize the information flow across the interfaces.
5. The number of layers should be large enough that distinct functions need not be thrown together in the same layer out of necessity and small enough that the architecture does not become unwieldy.
Fig: OSI Reference Model
The Physical Layer: The physical layer is concerned with transmitting raw bits over a communication channel. The design issues have to do with making sure that when one side sends a 1 bit, it is received by the other side as a 1 bit, not as a 0 bit.
The Data Link Layer: The main task of the data link layer is to transform a raw transmission facility into a line that appears free of undetected transmission errors to the network layer. It accomplishes this task by having the sender break up the input data into data frames (typically a few hundred or a few thousand bytes) and transmits the frames sequentially. If the service is reliable, the receiver confirms correct receipt of each frame by sending back an acknowledgement frame. Another issue that arises in the data link layer (and most of the higher layers as well) is how to keep a fast transmitter from drowning a slow receiver in data. Some traffic regulation mechanism is often needed to let the transmitter know how much buffer space the receiver has at the moment. Frequently, this flow regulation and the error handling are integrated.
The Network Layer: The network layer controls the operation of the subnet. A key design issue is determining how packets are routed from source to destination. Routes can be based on static tables that are ''wired into'' the network and rarely changed. They can also be determined at the start of each conversation, for example, a terminal session (e.g., a login to a remote machine).
Finally, they can be highly dynamic, being determined anew for each packet, to reflect the current network load. If too many packets are present in the subnet at the same time, they will get in one another's way, forming bottlenecks. The control of such congestion also belongs to the network layer. More generally, the quality of service provided (delay, transit time, jitter, etc.) is also a network layer issue. When a packet has to travel from one network to another to get to its destination, many problems can arise.
The addressing used by the second network may be different from the first one. The second one may not accept the packet at all because it is too large. The protocols may differ, and so on. It is up to the network layer to overcome all these problems to allow heterogeneous networks to be interconnected. In broadcast networks, the routing problem is simple, so the network layer is often thin or even nonexistent.
The Transport Layer: The basic function of the transport layer is to accept data from above, split it up into smaller units, if need be, pass these to the network layer, and ensure that the pieces all arrive correctly at the other end. Furthermore, all this must be done efficiently and in a way that isolates the upper layers from the inevitable changes in the hardware technology. The transport layer also determines what type of service to provide to the session layer, and, ultimately, to the users of the network. The most popular type of transport connection is an error-free point-to-point channel that delivers messages or bytes in the order in which they were sent.
However, other possible kinds of transport service are the transporting of isolated messages, with no guarantee about the order of delivery, and the broadcasting of messages to multiple destinations. The type of service is determined when the connection is established. The transport layer is a true end-to-end layer, all the way from the source to the destination. In other words, a program on the source machine carries on a conversation with a similar program on the destination machine, using the message headers and control messages. In the lower layers, the protocols are between each machine and its immediate neighbours, and not between the ultimate source and destination machines, which may be separated by many routers.
The Session Layer: The session layer allows users on different machines to establish sessions between them. Sessions offer various services, including dialog control (keeping track of whose turn it is to transmit), token management (preventing two parties from attempting the same critical operation at the same time), and synchronization (check pointing long transmissions to allow them to continue from where they were after a crash).
The Presentation Layer: The presentation layer is concerned with the syntax and semantics of the information transmitted. In order to make it possible for computers with different data representations to communicate, the data structures to be exchanged can be defined in an abstract way, along with a standard encoding to be used ''on the wire.'' The presentation layer manages these abstract data structures and allows higher-level data structures (e.g., banking records), to be defined and exchanged.
The Application Layer: The application layer contains a variety of protocols that are commonly needed by users. One widely-used application protocol is HTTP (Hypertext Transfer Protocol), which is the basis for the World Wide Web. When a browser wants a Web page, it sends the name of the page it wants to the server using HTTP. The server then sends the page back. Other application protocols are used for file transfer, electronic mail, and network news.
Key takeaway
Guided Media
It is defined as the physical medium through which the signals are transmitted. It is also known as Bounded media.
Types of Guided media:
Twisted pair:
Twisted pair is a physical media made up of a pair of cables twisted with each other. A twisted pair cable is cheap as compared to other transmission media. Installation of the twisted pair cable is easy, and it is a lightweight cable. The frequency range for twisted pair cable is from 0 to 3.5 KHz.
A twisted pair consists of two insulated copper wires arranged in a regular spiral pattern.
The degree of reduction in noise interference is determined by the number of turns per foot. Increasing the number of turns per foot decreases noise interference.
Types of Twisted pair:
Unshielded Twisted Pair:
An unshielded twisted pair is widely used in telecommunication. Following are the categories of the unshielded twisted pair cable:
- Category 1: Category 1 is used for telephone lines that have low-speed data.
- Category 2: It can support upto 4Mbps.
- Category 3: It can support upto 16Mbps.
- Category 4: It can support upto 20Mbps. Therefore, it can be used for long-distance communication.
- Category 5: It can support upto 200Mbps.
Advantages of Unshielded Twisted Pair:
- It is cheap.
- Installation of the unshielded twisted pair is easy.
- It can be used for high-speed LAN.
Disadvantage:
- This cable can only be used for shorter distances because of attenuation.
Shielded Twisted Pair
A shielded twisted pair is a cable that contains the mesh surrounding the wire that allows the higher transmission rate.
Characteristics of Shielded Twisted Pair:
- The cost of the shielded twisted pair cable is not very high and not very low.
- An installation of STP is easy.
- It has higher capacity as compared to unshielded twisted pair cable.
- It has a higher attenuation.
- It is shielded that provides the higher data transmission rate.
Disadvantages
- It is more expensive as compared to UTP and coaxial cable.
- It has a higher attenuation rate.
Coaxial Cable
- Coaxial cable is very commonly used transmission media, for example, TV wire is usually a coaxial cable.
- The name of the cable is coaxial as it contains two conductors parallel to each other.
- It has a higher frequency as compared to Twisted pair cable.
- The inner conductor of the coaxial cable is made up of copper, and the outer conductor is made up of copper mesh. The middle core is made up of non-conductive cover that separates the inner conductor from the outer conductor.
- The middle core is responsible for the data transferring whereas the copper mesh prevents from the EMI (Electromagnetic interference).
Coaxial cable is of two types:
- Baseband transmission: It is defined as the process of transmitting a single signal at high speed.
- Broadband transmission: It is defined as the process of transmitting multiple signals simultaneously.
Advantages of Coaxial cable:
- The data can be transmitted at high speed.
- It has better shielding as compared to twisted pair cable.
- It provides higher bandwidth.
Disadvantages of Coaxial cable:
- It is more expensive as compared to twisted pair cable.
- If any fault occurs in the cable causes the failure in the entire network.
Fibre Optic
- Fibre optic cable is a cable that uses electrical signals for communication.
- Fibre optic is a cable that holds the optical fibres coated in plastic that are used to send the data by pulses of light.
- The plastic coating protects the optical fibres from heat, cold, electromagnetic interference from other types of wiring.
- Fibre optics provides faster data transmission than copper wires.
Diagrammatic representation of fibre optic cable:
Basic elements of Fibre optic cable:
- Core: The optical fibre consists of a narrow strand of glass or plastic known as a core. A core is a light transmission area of the fibre. The more the area of the core, the lighter will be transmitted into the fibre.
- Cladding: The concentric layer of glass is known as cladding. The main functionality of the cladding is to provide the lower refractive index at the core interface as to cause the reflection within the core so that the light waves are transmitted through the fibre.
- Jacket: The protective coating consisting of plastic is known as a jacket. The main purpose of a jacket is to preserve the fibre strength, absorb shock and extra fibre protection.
Following are the advantages of fibre optic cable over copper:
- Greater Bandwidth: The fibre optic cable provides more bandwidth as compared copper. Therefore, the fibre optic carries more data as compared to copper cable.
- Faster speed: Fibre optic cable carries the data in the form of light. This allows the fibre optic cable to carry the signals at a higher speed.
- Longer distances: The fibre optic cable carries the data at a longer distance as compared to copper cable.
- Better reliability: The fibre optic cable is more reliable than the copper cable as it is immune to any temperature changes while it can cause obstruct in the connectivity of copper cable.
- Thinner and Sturdier: Fibre optic cable is thinner and lighter in weight so it can withstand more pull pressure than copper cable.
Unguided Transmission
- An unguided transmission transmits the electromagnetic waves without using any physical medium. Therefore, it is also known as wireless transmission.
- In unguided media, air is the media through which the electromagnetic energy can flow easily.
Unguided transmission is broadly classified into three categories:
Radio waves
- Radio waves are the electromagnetic waves that are transmitted in all the directions of free space.
- Radio waves are omnidirectional, i.e., the signals are propagated in all the directions.
- The range in frequencies of radio waves is from 3Khz to 1 khz.
- In the case of radio waves, the sending and receiving antenna are not aligned, i.e., the wave sent by the sending antenna can be received by any receiving antenna.
- An example of the radio wave is FM radio.
Applications of Radio waves:
- A Radio wave is useful for multicasting when there is one sender and many receivers.
- An FM radio, television, cordless phones are examples of a radio wave.
Advantages of Radio transmission:
- Radio transmission is mainly used for wide area networks and mobile cellular phones.
- Radio waves cover a large area, and they can penetrate the walls.
- Radio transmission provides a higher transmission rate.
Microwaves
Microwaves are of two types:
- Terrestrial microwave
- Satellite microwave communication.
Terrestrial Microwave Transmission
- Terrestrial Microwave transmission is a technology that transmits the focused beam of a radio signal from one ground-based microwave transmission antenna to another.
- Microwaves are the electromagnetic waves having the frequency in the range from 1GHz to 1000 GHz.
- Microwaves are unidirectional as the sending and receiving antenna is to be aligned, i.e., the waves sent by the sending antenna are narrowly focused.
- In this case, antennas are mounted on the towers to send a beam to another antenna which is km away.
- It works on the line-of-sight transmission, i.e., the antennas mounted on the towers are the direct sight of each other.
Characteristics of Microwave:
- Frequency range: The frequency range of terrestrial microwave is from 4-6 GHz to 21-23 GHz.
- Bandwidth: It supports the bandwidth from 1 to 10 Mbps.
- Short distance: It is inexpensive for short distance.
- Long distance: It is expensive as it requires a higher tower for a longer distance.
- Attenuation: Attenuation means loss of signal. It is affected by environmental conditions and antenna size.
Advantages of Microwave:
- Microwave transmission is cheaper than using cables.
- It is free from land acquisition as it does not require any land for the installation of cables.
- Microwave transmission provides an easy communication in terrains as the installation of cable in terrain is quite a difficult task.
- Communication over oceans can be achieved by using microwave transmission.
Disadvantages of Microwave transmission:
- Eavesdropping: An eavesdropping creates insecure communication. Any malicious user can catch the signal in the air by using its own antenna.
- Out of phase signal: A signal can be moved out of phase by using microwave transmission.
- Susceptible to weather condition: A microwave transmission is susceptible to weather condition. This means that any environmental change such as rain, wind can distort the signal.
- Bandwidth limited: Allocation of bandwidth is limited in the case of microwave transmission.
Satellite Microwave Communication
- A satellite is a physical object that revolves around the earth at a known height.
- Satellite communication is more reliable nowadays as it offers more flexibility than cable and fibre optic systems.
- We can communicate with any point on the globe by using satellite communication.
How Does Satellite work?
The satellite accepts the signal that is transmitted from the earth station, and it amplifies the signal. The amplified signal is retransmitted to another earth station.
Advantages of Satellite Microwave Communication:
- The coverage area of a satellite microwave is more than the terrestrial microwave.
- The transmission cost of the satellite is independent of the distance from the centre of the coverage area.
- Satellite communication is used in mobile and wireless communication applications.
- It is easy to install.
- It is used in a wide variety of applications such as weather forecasting, radio/TV signal broadcasting, mobile communication, etc.
Disadvantages of Satellite Microwave Communication:
- Satellite designing and development requires more time and higher cost.
- The Satellite needs to be monitored and controlled on regular periods so that it remains in orbit.
- The life of the satellite is about 12-15 years. Due to this reason, another launch of the satellite has to be planned before it becomes non-functional.
Infrared
- An infrared transmission is a wireless technology used for communication over short ranges.
- The frequency of the infrared in the range from 300 GHz to 400 THz.
- It is used for short-range communication such as data transfer between two cell phones, TV remote operation, data transfer between a computer and cell phone resides in the same closed area.
Characteristics of Infrared:
- It supports high bandwidth, and hence the data rate will be very high.
- Infrared waves cannot penetrate the walls. Therefore, the infrared communication in one room cannot be interrupted by the nearby rooms.
- An infrared communication provides better security with minimum interference.
- Infrared communication is unreliable outside the building because the sun rays will interfere with the infrared waves.
Key takeaway
Topology refers to the network's structure and how all of the elements are linked to one another. Physical and logical topology are the two forms of topology.
The geometric representation of all the nodes in a network is known as physical topology.
● Bus topology: The bus topology is set up in such a way that all of the stations are linked together by a single backbone cable. Each node is either connected to the backbone cable through a drop cable or is directly connected to it.
When a node wishes to send a message across the network, it does so by sending a message across the network. Regardless of whether the message has been answered, it will be received by all available stations in the network.
The bus topology is commonly used in regular networks such as 802.3 (ethernet) and 802.4 (wireless). In comparison to other topologies, the configuration of a bus topology is very straightforward. The backbone cable is thought of as a "single path" from which the message is sent to all of the stations.
Fig: Bus topology
● Ring topology: The topology of a ring is similar to that of a bus, but with connected ends. The node that receives the previous computer's message will retransmit to the next node. The data is unidirectional, meaning it only moves in one direction.
The data is continually flowed in a single loop, which is referred to as an infinite loop. It has no terminated ends, which means that each node is connected to the next and has no point of termination.
Fig: Ring topology
● Star topology: The star topology is a network configuration in which each node is connected to a central hub, switch, or device. The central computer is referred to as a server, and the peripheral devices connected to it as clients.
The computers are connected via coaxial cable or RJ-45 cables. In a physical star topology, hubs or switches are primarily used as connection devices. The star topology is the most widely used network topology.
Fig: Star topology
● Tree topology: Tree topology incorporates the advantages of both bus and star topologies. A tree topology is a system in which all computers are related to one another in a hierarchical manner.
A root node is the top-most node in a tree topology, and all other nodes are descendants of the root node. For data transmission, there is only one path between two nodes. As a result, it creates a parent-child hierarchy.
Fig: Tree topology
● Mesh topology: Mesh technology is a network configuration in which computers are connected to one another through multiple redundant connections. There are many ways to get from one device to another. It is devoid of the turn, hub, or any central device that serves as a communication hub.
The mesh topology is exemplified by the Internet. Mesh topology is most commonly used in WAN deployments where communication failures are a major concern. Wireless networks commonly use the mesh topology.
Fig: Mesh topology
● Hybrid topology: Hybrid topology is the synthesis of many different topologies. A hybrid topology connects various links and nodes in order to transfer data. Hybrid topology is described as the combination of two or more different topologies, while similar topologies connecting to each other do not result in Hybrid topology.
For example, if a ring topology exists in one SBI bank branch and a bus topology exists in another SBI bank branch, connecting these two topologies would result in a Hybrid topology.
Fig: Hybrid topology
Key takeaway
Elementary data link protocol
Protocols in the data link layer are designed so that this layer can perform its basic functions: framing, error control and flow control. Framing is the process of dividing bit - streams from physical layer into data frames whose size ranges from a few hundred to a few thousand bytes. Error control mechanisms deals with transmission errors and retransmission of corrupted and lost frames. Flow control regulates speed of delivery and so that a fast sender does not drown a slow receiver.
Types of Data Link Protocols
Data link protocols can be broadly divided into two categories, depending on whether the transmission channel is noiseless or noisy.
- Simplex Protocol
The Simplex protocol is hypothetical protocol designed for unidirectional data transmission over an ideal channel, i.e., a channel through which transmission can never go wrong. It has distinct procedures for sender and receiver. The sender simply sends all its data available onto the channel as soon as they are available its buffer. The receiver is assumed to process all incoming data instantly. It is hypothetical since it does not handle flow control or error control.
b. Stop – and – Wait Protocol
Stop – and – Wait protocol is for noiseless channel too. It provides unidirectional data transmission without any error control facilities. However, it provides for flow control so that a fast sender does not drown a slow receiver. The receiver has a finite buffer size with finite processing speed. The sender can send a frame only when it has received indication from the receiver that it is available for further data processing.
c. Stop – and – Wait ARQ
Stop-and– wait Automatic Repeat Request (Stop – and – Wait ARQ) is a variation of the above protocol with added error control mechanisms, appropriate for noisy channels. The sender keeps a copy of the sent frame. It then waits for a finite time to receive a positive acknowledgement from receiver. If the timer expires or a negative acknowledgement is received, the frame is retransmitted. If a positive acknowledgement is received, then the next frame is sent.
d. Go – Back – N ARQ
Go – Back – N ARQ provides for sending multiple frames before receiving the acknowledgement for the first frame. It uses the concept of sliding window, and so is also called sliding window protocol. The frames are sequentially numbered and a finite number of frames are sent. If the acknowledgement of a frame is not received within the time period, all frames starting from that frame are retransmitted.
e. Selective Repeat ARQ
This protocol also provides for sending multiple frames before receiving the acknowledgement for the first frame. However, here only the erroneous or lost frames are retransmitted, while the good frames are received and buffered.
f. Sliding window protocols
The sliding window is a technique for sending multiple frames at a time. It controls the data packets between the two devices where reliable and gradual delivery of data frames is needed. It is also used in TCP (Transmission Control Protocol).
In this technique, each frame has sent from the sequence number. The sequence numbers are used to find the missing data in the receiver end. The purpose of the sliding window technique is to avoid duplicate data, so it uses the sequence number.
Types of Sliding Window Protocol
Sliding window protocol has two types:
Go-Back-N ARQ
Selective Repeat ARQ
Go-Back-N ARQ
Go-Back-N ARQ protocol is also known as Go-Back-N Automatic Repeat Request. It is a data link layer protocol that uses a sliding window method. In this, if any frame is corrupted or lost, all subsequent frames have to be sent again. The size of the sender window is N in this protocol. For example, Go-Back-8, the size of the sender window, will be 8. The receiver window size is always 1. If the receiver receives a corrupted frame, it cancels it. The receiver does not accept a corrupted frame. When the timer expires, the sender sends the correct frame again.
Selective Repeat ARQ
Selective Repeat ARQ is also known as the Selective Repeat Automatic Repeat Request. It is a data link layer protocol that uses a sliding window method. The Go-Back-N ARQ protocol works well if it has fewer errors. But if there is a lot of error in the frame, lots of bandwidth loss in sending the frames again. So, we use the Selective Repeat ARQ protocol. In this protocol, the size of the sender window is always equal to the size of the receiver window. The size of the sliding window is always greater than 1.
If the receiver receives a corrupt frame, it does not directly discard it. It sends a negative acknowledgment to the sender. The sender sends that frame again as soon as on the receiving negative acknowledgment. There is no waiting for any time-out to send that frame.
Key Takeaways
The medium access control (MAC) is a sublayer of the data link layer of the open system interconnections (OSI) reference model for data transmission.
Error Detection
When data is transmitted from one device to another device, the system does not guarantee whether the data received by the device is identical to the data transmitted by another device. An Error is a situation when the message received at the receiver end is not identical to the message transmitted.
Types Of Errors
Fig – Types of errors
Errors can be classified into two categories:
- Single-Bit Error
- Burst Error
Single-Bit Error:
The only one bit of a given data unit is changed from 1 to 0 or from 0 to 1.
Fig – Single bit error
In the above figure, the message which is sent is corrupted as single-bit, i.e., 0 bit is changed to 1.
Single-Bit Error does not appear more likely in Serial Data Transmission. For example, Sender sends the data at 10 Mbps, this means that the bit lasts only for 1? s and for a single-bit error to occurred, a noise must be more than 1? s.
Single-Bit Error mainly occurs in Parallel Data Transmission. For example, if eight wires are used to send the eight bits of a byte, if one of the wires is noisy, then single-bit is corrupted per byte.
Burst Error:
The two or more bits are changed from 0 to 1 or from 1 to 0 is known as Burst Error.
The Burst Error is determined from the first corrupted bit to the last corrupted bit.
Fig - Burst Error
The duration of noise in Burst Error is more than the duration of noise in Single-Bit.
Burst Errors are most likely to occur in Serial Data Transmission.
The number of affected bits depends on the duration of the noise and data rate.
Error Detecting Techniques:
The most popular Error Detecting Techniques are:
- Single parity check
- Two-dimensional parity check
- Checksum
- Cyclic redundancy check
Single Parity Check
- Single Parity checking is the simple mechanism and inexpensive to detect the errors.
- In this technique, a redundant bit is also known as a parity bit which is appended at the end of the data unit so that the number of 1s becomes even. Therefore, the total number of transmitted bits would be 9 bits.
- If the number of 1s bits is odd, then parity bit 1 is appended and if the number of 1s bits is even, then parity bit 0 is appended at the end of the data unit.
- At the receiving end, the parity bit is calculated from the received data bits and compared with the received parity bit.
- This technique generates the total number of 1s even, so it is known as even-parity checking.
Fig - Single Parity Check
Drawbacks Of Single Parity Checking
- It can only detect single-bit errors which are very rare.
- If two bits are interchanged, then it cannot detect the errors.
Two-Dimensional Parity Check
- Performance can be improved by using Two-Dimensional Parity Check which organizes the data in the form of a table.
- Parity check bits are computed for each row, which is equivalent to the single-parity check.
- In Two-Dimensional Parity check, a block of bits is divided into rows, and the redundant row of bits is added to the whole block.
- At the receiving end, the parity bits are compared with the parity bits computed from the received data.
Drawbacks Of 2D Parity Check
- If two bits in one data unit are corrupted and two bits exactly the same position in another data unit are also corrupted, then 2D Parity checker will not be able to detect the error.
- This technique cannot be used to detect the 4-bit errors or more in some cases.
Checksum
A Checksum is an error detection technique based on the concept of redundancy.
It is divided into two parts:
Checksum Generator
A Checksum is generated at the sending side. Checksum generator subdivides the data into equal segments of n bits each, and all these segments are added together by using one's complement arithmetic. The sum is complemented and appended to the original data, known as checksum field. The extended data is transmitted across the network.
Suppose L is the total sum of the data segments, then the checksum would be? L
Fig - Checksum
- The Sender follows the given steps:
- The block unit is divided into k sections, and each of n bits.
- All the k sections are added together by using one's complement to get the sum.
- The sum is complemented and it becomes the checksum field.
- The original data and checksum field are sent across the network.
Checksum Checker
A Checksum is verified at the receiving side. The receiver subdivides the incoming data into equal segments of n bits each, and all these segments are added together, and then this sum is complemented. If the complement of the sum is zero, then the data is accepted otherwise data is rejected.
- The Receiver follows the given steps:
- The block unit is divided into k sections and each of n bits.
- All the k sections are added together by using one's complement algorithm to get the sum.
- The sum is complemented.
- If the result of the sum is zero, then the data is accepted otherwise the data is discarded.
Cyclic Redundancy Check (CRC)
CRC is a redundancy error technique used to determine the error.
Following are the steps used in CRC for error detection:
- In CRC technique, a string of n 0s is appended to the data unit, and this n number is less than the number of bits in a predetermined number, known as division which is n+1 bits.
- Secondly, the newly extended data is divided by a divisor using a process is known as binary division. The remainder generated from this division is known as CRC remainder.
- Thirdly, the CRC remainder replaces the appended 0s at the end of the original data. This newly generated unit is sent to the receiver.
- The receiver receives the data followed by the CRC remainder. The receiver will treat this whole unit as a single unit, and it is divided by the same divisor that was used to find the CRC remainder.
If the resultant of this division is zero which means that it has no error, and the data is accepted.
If the resultant of this division is not zero which means that the data consists of an error. Therefore, the data is discarded.
Fig – Data CRC
Let's understand this concept through an example:
Suppose the original data is 11100 and divisor is 1001.
CRC Generator
- A CRC generator uses a modulo-2 division. Firstly, three zeroes are appended at the end of the data as the length of the divisor is 4 and we know that the length of the string 0s to be appended is always one less than the length of the divisor.
- Now, the string becomes 11100000, and the resultant string is divided by the divisor 1001.
- The remainder generated from the binary division is known as CRC remainder. The generated value of the CRC remainder is 111.
- CRC remainder replaces the appended string of 0s at the end of the data unit, and the final string would be 11100111 which is sent across the network.
CRC Checker
- The functionality of the CRC checker is similar to the CRC generator.
- When the string 11100111 is received at the receiving end, then CRC checker performs the modulo-2 division.
- A string is divided by the same divisor, i.e., 1001.
- In this case, CRC checker generates the remainder of zero. Therefore, the data is accepted.
Error Correction
Error Correction codes are used to detect and correct the errors when data is transmitted from the sender to the receiver.
Error Correction can be handled in two ways:
- Backward error correction: Once the error is discovered, the receiver requests the sender to retransmit the entire data unit.
- Forward error correction: In this case, the receiver uses the error-correcting code which automatically corrects the errors.
A single additional bit can detect the error, but cannot correct it.
For correcting the errors, one has to know the exact position of the error. For example, if we want to calculate a single-bit error, the error correction code will determine which one of seven bits is in error. To achieve this, we have to add some additional redundant bits.
Suppose r is the number of redundant bits and d is the total number of the data bits. The number of redundant bits r can be calculated by using the formula:
2r>=d+r+1
The value of r is calculated by using the above formula. For example, if the value of d is 4, then the possible smallest value that satisfies the above relation would be 3.
To determine the position of the bit which is in error, a technique developed by R.W Hamming is Hamming code which can be applied to any length of the data unit and uses the relationship between data units and redundant units.
Key takeaway
The most popular Error Detecting Techniques are:
- Single parity check
- Two-dimensional parity check
- Checksum
- Cyclic redundancy check
- Framing is a point-to-point connection between two computers or devices which consists of a wire in which data is transmitted as a stream of bits.
- However, these bits must be framed into discernible blocks of information.
- Framing is the function of the data link layer. It provides a way for a sender to transmit a set of bits that are meaningful to the receiver.
- Ethernet, token ring, frame relay, and other data link layer technologies have their own frame structures.
- Frames have headers that contain information such as error-checking codes.
At data link layer it extracts message from sender and provide it to the receiver by providing sender’s and receiver’s address.
The advantage of using frames is that data is broken up into recoverable chunks that can easily be checked for corruption.
Types of framing – There are two types of framing:
1. Fixed size – The frame is of fixed size and there is no need to provide boundaries to the frame, length of the frame itself acts as delimiter.
2. Variable size – In this there is need to define end of frame as well as beginning of next frame to distinguish.
This can be done in two ways:
- Length field – We can introduce a length field in the frame to indicate the length of the frame. Used in Ethernet (802.3).
- End Delimeter (ED) – We can introduce an ED (pattern) to indicate the end of the frame. Used in Token Ring.
Stop and Wait Protocol
Before understanding the stop and Wait protocol, we first know about the error control mechanism. The error control mechanism is used so that the received data should be exactly same whatever sender has sent the data. The error control mechanism is divided into two categories, i.e., Stop and Wait ARQ and sliding window. The sliding window is further divided into two categories, i.e., Go Back N, and Selective Repeat. Based on the usage, the people select the error control mechanism whether it is stop and wait or sliding window.
What is Stop and Wait protocol?
Here stop and wait means, whatever the data that sender wants to send, he sends the data to the receiver. After sending the data, he stops and waits until he receives the acknowledgment from the receiver. The stop and wait protocol is a flow control protocol where flow control is one of the services of the data link layer.
It is a data-link layer protocol which is used for transmitting the data over the noiseless channels. It provides unidirectional data transmission which means that either sending or receiving of data will take place at a time. It provides flow-control mechanism but does not provide any error control mechanism.
The idea behind the usage of this frame is that when the sender sends the frame then he waits for the acknowledgment before sending the next frame.
Primitives of Stop and Wait Protocol
The primitives of stop and wait protocol are:
Sender side
Rule 1: Sender sends one data packet at a time.
Rule 2: Sender sends the next packet only when it receives the acknowledgment of the previous packet.
Therefore, the idea of stop and wait protocol in the sender's side is very simple, i.e., send one packet at a time, and do not send another packet before receiving the acknowledgment.
Receiver side
Rule 1: Receive and then consume the data packet.
Rule 2: When the data packet is consumed, receiver sends the acknowledgment to the sender.
Therefore, the idea of stop and wait protocol in the receiver's side is also very simple, i.e., consume the packet, and once the packet is consumed, the acknowledgment is sent. This is known as a flow control mechanism.
Working of Stop and Wait protocol
Fig - Working of Stop and Wait protocol
The above figure shows the working of the stop and wait protocol. If there is a sender and receiver, then sender sends the packet and that packet is known as a data packet. The sender will not send the second packet without receiving the acknowledgment of the first packet. The receiver sends the acknowledgment for the data packet that it has received. Once the acknowledgment is received, the sender sends the next packet. This process continues until all the packet are not sent. The main advantage of this protocol is its simplicity but it has some disadvantages also. For example, if there are 1000 data packets to be sent, then all the 1000 packets cannot be sent at a time as in Stop and Wait protocol, one packet is sent at a time.
Disadvantages of Stop and Wait protocol
The following are the problems associated with a stop and wait protocol:
1. Problems occur due to lost data
Fig - Problems occur due to lost data
Suppose the sender sends the data and the data is lost. The receiver is waiting for the data for a long time. Since the data is not received by the receiver, so it does not send any acknowledgment. Since the sender does not receive any acknowledgment so it will not send the next packet. This problem occurs due to the lost data.
In this case, two problems occur:
- Sender waits for an infinite amount of time for an acknowledgment.
- Receiver waits for an infinite amount of time for a data.
2. Problems occur due to lost acknowledgment
Suppose the sender sends the data and it has also been received by the receiver. On receiving the packet, the receiver sends the acknowledgment. In this case, the acknowledgment is lost in a network, so there is no chance for the sender to receive the acknowledgment. There is also no chance for the sender to send the next packet as in stop and wait protocol, the next packet cannot be sent until the acknowledgment of the previous packet is received.
In this case, one problem occurs:
- Sender waits for an infinite amount of time for an acknowledgment.
3. Problem due to the delayed data or acknowledgment
Suppose the sender sends the data and it has also been received by the receiver. The receiver then sends the acknowledgment but the acknowledgment is received after the timeout period on the sender's side. As the acknowledgment is received late, so acknowledgment can be wrongly considered as the acknowledgment of some other data packet.
Go-Back-N ARQ
Before understanding the working of Go-Back-N ARQ, we first look at the sliding window protocol. As we know that the sliding window protocol is different from the stop-and-wait protocol. In the stop-and-wait protocol, the sender can send only one frame at a time and cannot send the next frame without receiving the acknowledgment of the previously sent frame, whereas, in the case of sliding window protocol, the multiple frames can be sent at a time. The variations of sliding window protocol are Go-Back-N ARQ and Selective Repeat ARQ. Let's understand 'what is Go-Back-N ARQ'.
What is Go-Back-N ARQ?
In Go-Back-N ARQ, N is the sender's window size. Suppose we say that Go-Back-3, which means that the three frames can be sent at a time before expecting the acknowledgment from the receiver.
It uses the principle of protocol pipelining in which the multiple frames can be sent before receiving the acknowledgment of the first frame. If we have five frames and the concept is Go-Back-3, which means that the three frames can be sent, i.e., frame no 1, frame no 2, frame no 3 can be sent before expecting the acknowledgment of frame no 1.
In Go-Back-N ARQ, the frames are numbered sequentially as Go-Back-N ARQ sends the multiple frames at a time that requires the numbering approach to distinguish the frame from another frame, and these numbers are known as the sequential numbers.
The number of frames that can be sent at a time totally depends on the size of the sender's window. So, we can say that 'N' is the number of frames that can be sent at a time before receiving the acknowledgment from the receiver.
If the acknowledgment of a frame is not received within an agreed-upon time period, then all the frames available in the current window will be retransmitted. Suppose we have sent the frame no 5, but we didn't receive the acknowledgment of frame no 5, and the current window is holding three frames, then these three frames will be retransmitted.
The sequence number of the outbound frames depends upon the size of the sender's window. Suppose the sender's window size is 2, and we have ten frames to send, then the sequence numbers will not be 1,2,3,4,5,6,7,8,9,10. Let's understand through an example.
- N is the sender's window size.
- If the size of the sender's window is 4 then the sequence number will be 0,1,2,3,0,1,2,3,0,1,2, and so on.
The number of bits in the sequence number is 2 to generate the binary sequence 00,01,10,11.
Working of Go-Back-N ARQ
Suppose there are a sender and a receiver, and let's assume that there are 11 frames to be sent. These frames are represented as 0,1,2,3,4,5,6,7,8,9,10, and these are the sequence numbers of the frames. Mainly, the sequence number is decided by the sender's window size. But, for the better understanding, we took the running sequence numbers, i.e., 0,1,2,3,4,5,6,7,8,9,10. Let's consider the window size as 4, which means that the four frames can be sent at a time before expecting the acknowledgment of the first frame.
Step 1: Firstly, the sender will send the first four frames to the receiver, i.e., 0,1,2,3, and now the sender is expected to receive the acknowledgment of the 0th frame.
Let's assume that the receiver has sent the acknowledgment for the 0 frame, and the receiver has successfully received it.
The sender will then send the next frame, i.e., 4, and the window slides containing four frames (1,2,3,4).
The receiver will then send the acknowledgment for the frame no 1. After receiving the acknowledgment, the sender will send the next frame, i.e., frame no 5, and the window will slide having four frames (2,3,4,5).
Now, let's assume that the receiver is not acknowledging the frame no 2, either the frame is lost, or the acknowledgment is lost. Instead of sending the frame no 6, the sender Go-Back to 2, which is the first frame of the current window, retransmits all the frames in the current window, i.e., 2,3,4,5.
Important points related to Go-Back-N ARQ:
- In Go-Back-N, N determines the sender's window size, and the size of the receiver's window is always 1.
- It does not consider the corrupted frames and simply discards them.
- It does not accept the frames which are out of order and discards them.
- If the sender does not receive the acknowledgment, it leads to the retransmission of all the current window frames.
Let's understand the Go-Back-N ARQ through an example.
Example 1: In GB4, if every 6th packet being transmitted is lost and if we have to spend 10 packets then how many transmissions are required?
Solution: Here, GB4 means that N is equal to 4. The size of the sender's window is 4.
Step 1: As the window size is 4, so four packets are transferred at a time, i.e., packet no 1, packet no 2, packet no 3, and packet no 4.
Step 2: Once the transfer of window size is completed, the sender receives the acknowledgment of the first frame, i.e., packet no1. As the acknowledgment receives, the sender sends the next packet, i.e., packet no 5. In this case, the window slides having four packets, i.e., 2,3,4,5 and excluded the packet 1 as the acknowledgment of the packet 1 has been received successfully.
Step 3: Now, the sender receives the acknowledgment of packet 2. After receiving the acknowledgment for packet 2, the sender sends the next packet, i.e., packet no 6. As mentioned in the question that every 6th is being lost, so this 6th packet is lost, but the sender does not know that the 6th packet has been lost.
Step 4: The sender receives the acknowledgment for the packet no 3. After receiving the acknowledgment of 3rd packet, the sender sends the next packet, i.e., 7th packet. The window will slide having four packets, i.e., 4, 5, 6, 7.
Step 5: When the packet 7 has been sent, then the sender receives the acknowledgment for the packet no 4. When the sender has received the acknowledgment, then the sender sends the next packet, i.e., the 8th packet. The window will slide having four packets, i.e., 5, 6, 7, 8.
Step 6: When the packet 8 is sent, then the sender receives the acknowledgment of packet 5. On receiving the acknowledgment of packet 5, the sender sends the next packet, i.e., 9th packet. The window will slide having four packets, i.e., 6, 7, 8, 9.
Step 7: The current window is holding four packets, i.e., 6, 7, 8, 9, where the 6th packet is the first packet in the window. As we know, the 6th packet has been lost, so the sender receives the negative acknowledgment NAK (6). As we know that every 6th packet is being lost, so the counter will be restarted from 1. So, the counter values 1, 2, 3 are given to the 7th packet, 8th packet, 9th packet respectively.
Step 8: As it is Go-BACK, so it retransmits all the packets of the current window. It will resend 6, 7, 8, 9. The counter values of 6, 7, 8, 9 are 4, 5, 6, 1, respectively. In this case, the 8th packet is lost as it has a 6-counter value, so the counter variable will again be restarted from 1.
Step 9: After the retransmission, the sender receives the acknowledgment of packet 6. On receiving the acknowledgment of packet 6, the sender sends the 10th packet. Now, the current window is holding four packets, i.e., 7, 8, 9, 10.
Step 10: When the 10th packet is sent, the sender receives the acknowledgment of packet 7. Now the current window is holding three packets, 8, 9 and 10. The counter values of 8, 9, 10 are 6, 1, 2.
Step 11: As the 8th packet has 6 counter value which means that 8th packet has been lost, and the sender receives NAK (8).
Step 12: Since the sender has received the negative acknowledgment for the 8th packet, it resends all the packets of the current window, i.e., 8, 9, 10.
Step 13: The counter values of 8, 9, 10 are 3, 4, 5, respectively, so their acknowledgments have been received successfully.
We conclude from the above figure that total 17 transmissions are required.
A Protocol Using Selective Repeat
Selective repeat protocol, also called Selective Repeat ARQ (Automatic Repeat request), is a data link layer protocol that uses sliding window method for reliable delivery of data frames. Here, only the erroneous or lost frames are retransmitted, while the good frames are received and buffered.
It uses two windows of equal size: a sending window that stores the frames to be sent and a receiving window that stores the frames receive by the receiver. The size is half the maximum sequence number of the frame. For example, if the sequence number is from 0 – 15, the window size will be 8.
Working Principle
Selective Repeat protocol provides for sending multiple frames depending upon the availability of frames in the sending window, even if it does not receive acknowledgement for any frame in the interim. The maximum number of frames that can be sent depends upon the size of the sending window.
The receiver records the sequence number of the earliest incorrect or un-received frame. It then fills the receiving window with the subsequent frames that it has received. It sends the sequence number of the missing frame along with every acknowledgement frame.
The sender continues to send frames that are in its sending window. Once, it has sent all the frames in the window, it retransmits the frame whose sequence number is given by the acknowledgements. It then continues sending the other frames.
Sender Site Algorithm of Selective Repeat Protocol
Begin
Frame s; //s denotes frame to be sent
Frame t; //t is temporary frame
S_window = power(2,m-1); //Assign maximum window size
SeqFirst = 0; // Sequence number of first frame in window
SeqN = 0; // Sequence number of Nth frame window
While (true) //check repeatedly
Do
Wait_For_Event(); //wait for availability of packet
If ( Event(Request_For_Transfer)) then
//check if window is full
If (SeqN–SeqFirst >= S_window) then
DoNothing();
End if;
Get_Data_From_Network_Layer();
s = Make_Frame();
s.seq = SeqN;
Store_Copy_Frame(s);
Send_Frame(s);
Start_Timer(s);
SeqN = SeqN + 1;
End if;
If ( Event(Frame_Arrival) then
r = Receive_Acknowledgement();
//Resend frame whose sequence number is with ACK
If ( r.type = NAK) then
If ( NAK_No > SeqFirst && NAK_No < SeqN ) then
Retransmit( s.seq(NAK_No));
Start_Timer(s);
End if
//Remove frames from sending window with positive ACK
Else if ( r.type = ACK ) then
Remove_Frame(s.seq(SeqFirst));
Stop_Timer(s);
SeqFirst = SeqFirst + 1;
End if
End if
// Resend frame if acknowledgement haven’t been received
If ( Event(Time_Out)) then
Start_Timer(s);
Retransmit_Frame(s);
End if
End
Receiver Site Algorithm of Selective Repeat Protocol
Begin
Frame f;
RSeqNo = 0; // Initialise sequence number of expected frame
NAKsent = false;
ACK = false;
For each slot in receive_window
Mark(slot)=false;
While (true) //check repeatedly
Do
Wait_For_Event(); //wait for arrival of frame
If ( Event(Frame_Arrival) then
Receive_Frame_From_Physical_Layer();
If ( Corrupted ( f.SeqNo ) AND NAKsent = false) then
SendNAK(f.SeqNo);
NAKsent = true;
End if
If ( f.SeqNo != RSeqNo AND NAKsent = false ) then
SendNAK(f.SeqNo);
NAKsent = true;
If ( f.SeqNo is in receive_window ) then
If ( Mark(RSeqNo) = false ) then
Store_frame(f.SeqNo);
Mark(RSeqNo) = true;
End if
End if
Else
While ( Mark(RSeqNo))
Extract_Data(RSeqNo);
Deliver_Data_To_Network_Layer();
RSeqNo = RSeqNo + 1;
Send_ACK(RSeqNo);
End while
End if
End if
End while
End
Sliding Window Protocol
The sliding window is a technique for sending multiple frames at a time. It controls the data packets between the two devices where reliable and gradual delivery of data frames is needed. It is also used in TCP (Transmission Control Protocol).
In this technique, each frame has sent from the sequence number. The sequence numbers are used to find the missing data in the receiver end. The purpose of the sliding window technique is to avoid duplicate data, so it uses the sequence number.
Types of Sliding Window Protocol
Sliding window protocol has two types:
- Go-Back-N ARQ
- Selective Repeat ARQ
Go-Back-N ARQ
Go-Back-N ARQ protocol is also known as Go-Back-N Automatic Repeat Request. It is a data link layer protocol that uses a sliding window method. In this, if any frame is corrupted or lost, all subsequent frames have to be sent again.
The size of the sender window is N in this protocol. For example, Go-Back-8, the size of the sender window, will be 8. The receiver window size is always 1.
If the receiver receives a corrupted frame, it cancels it. The receiver does not accept a corrupted frame. When the timer expires, the sender sends the correct frame again. The design of the Go-Back-N ARQ protocol is shown below.
The example of Go-Back-N ARQ is shown below in the figure.
Selective Repeat ARQ
Selective Repeat ARQ is also known as the Selective Repeat Automatic Repeat Request. It is a data link layer protocol that uses a sliding window method. The Go-back-N ARQ protocol works well if it has fewer errors. But if there is a lot of error in the frame, lots of bandwidth loss in sending the frames again. So, we use the Selective Repeat ARQ protocol. In this protocol, the size of the sender window is always equal to the size of the receiver window. The size of the sliding window is always greater than 1.
If the receiver receives a corrupt frame, it does not directly discard it. It sends a negative acknowledgment to the sender. The sender sends that frame again as soon as on the receiving negative acknowledgment. There is no waiting for any time-out to send that frame. The design of the Selective Repeat ARQ protocol is shown below.
The example of the Selective Repeat ARQ protocol is shown below in the figure.
Difference between the Go-Back-N ARQ and Selective Repeat ARQ?
Go-Back-N ARQ | Selective Repeat ARQ |
If a frame is corrupted or lost in it, all subsequent frames have to be sent again. | In this, only the frame is sent again, which is corrupted or lost. |
If it has a high error rate, it wastes a lot of bandwidth. | There is a loss of low bandwidth. |
It is less complex. | It is more complex because it has to do sorting and searching as well. And it also requires more storage. |
It does not require sorting. | In this, sorting is done to get the frames in the correct order. |
It does not require searching. | The search operation is performed in it. |
It is used more. | It is used less because it is more complex. |
Key takeaways
Before understanding the stop and Wait protocol, we first know about the error control mechanism. The error control mechanism is used so that the received data should be exactly same whatever sender has sent the data. The error control mechanism is divided into two categories, i.e., Stop and Wait ARQ and sliding window. The sliding window is further divided into two categories, i.e., Go Back N, and Selective Repeat. Based on the usage, the people select the error control mechanism whether it is stop and wait or sliding window.
The Network Devices are Hub, Repeater, Bridge, Switch, Router and Gateways.
1. Repeater – A repeater operates at the physical layer. Its job is to regenerate the signal over the same network before the signal becomes too weak or corrupted so as to extend the length to which the signal can be transmitted over the same network. An important point to be noted about repeaters is that they do no amplify the signal. When the signal becomes weak, they copy the signal bit by bit and regenerate it at the original strength. It is a 2-port device. A single Ethernet segment can have a maximum length of 500 meters with a maximum of 100 stations (in a cheaper net segment it is 185m). To extend the length of the network, a repeater may be used as shown in Figure. Functionally, a repeater can be considered as two transceivers joined together and connected to two different segments of coaxial cable.
The repeater passes the digital signal bit-by-bit in both directions between the two segments. As the signal passes through a repeater, it is amplified and regenerated at the other end. The repeater does not isolate one segment from the other, if there is a collision on one segment, it is regenerated on the other segment. Therefore, the two segments form a single LAN and it is transparent to rest of the system. Ethernet allows five segments to be used in cascade to have a maximum network span of 2.5 km. With reference of the ISO model, a repeater is considered as a level-1 relay as depicted in Figure. It simply repeats, retimes and amplifies the bits it receives. The repeater is merely used to extend the span of a single LAN. Important features of a repeater are as follows: • A repeater connects different segments of a LAN • A repeater forwards every frame it receives • A repeater is a regenerator, not an amplifier • It can be used to create a single extended LAN
Fig: Repeater connecting two LAN segments
Fig: Operation of a repeater as a level-1 relay
2. Hub – A hub is basically a multiport repeater. A hub connects multiple wires coming from different branches, for example, the connector in star topology which connects different stations. Hubs cannot filter data, so data packets are sent to all connected devices. In other words, collision domain of all hosts connected through Hub remains one. Also, they do not have intelligence to find out best path for data packets which leads to inefficiencies and wastage. Hub is a generic term, but commonly refers to a multiport repeater.
It can be used to create multiple levels of hierarchy of stations. The stations connect to the hub with RJ-45 connector having maximum segment length is 100 meters. This type of interconnected set of stations is easy to maintain and diagnose. Figure shows how several hubs can be connected in a hierarchical manner to realize a single LAN of bigger size with a large number of nodes.
Fig: Hub as a multi-port repeater can be connected in a hierarchical manner to form a single LAN with many nodes
3. Bridge – A bridge operates at data link layer. A bridge is a repeater, with add on functionality of filtering content by reading the MAC addresses of source and destination. It is also used for interconnecting two LANs working on the same protocol. It has a single input and single output port, thus making it a 2-port device. The device that can be used to interconnect two separate LANs is known as a bridge. It is commonly used to connect two similar or dissimilar LANs as shown in Figure below. The bridge operates in layer 2, that is data-link layer and that is why it is called level-2 relay with reference to the OSI model.
It links similar or dissimilar LANs, designed to store and forward frames, it is protocol independent and transparent to the end stations. The flow of information through a bridge is shown in Figure below. Use of bridges offer a number of advantages, such as higher reliability, performance, security, convenience and larger geographic coverage. But it is desirable that the quality of service (QOS) offered by a bridge should match that of a single LAN. The parameters that define the QOS include availability, frame mishaps, transit delay, frame lifetime, undetected bit errors, frame size and priority.
Key features of a bridge are mentioned below:
- A bridge operates both in physical and data-link layer
- A bridge uses a table for filtering/routing
- A bridge does not change the physical (MAC) addresses in a frame
Types of bridges:
- Transparent Bridges
- Source routing bridges
A bridge must contain addressing and routing capability. Two routing algorithms have been proposed for a bridged LAN environment. The first, produced as an extension of IEEE 802.1 and applicable to all IEEE 802 LANs, is known as transparent bridge. And the other, developed for the IEEE 802.5 token rings, is based on source routing approach. It applies to many types of LAN including token ring, token bus and CSMA/CD bus.
Fig: A bridge connecting two separate LANs
Fig: Information flow through a bridge
4. Switch – A switch is a multi port bridge with a buffer and a design that can boost its efficiency (large number of ports imply less traffic) and performance. Switch is data link layer device. Switch can perform error checking before forwarding data, that makes it very efficient as it does not forward packets that have errors and forward good packets selectively to correct port only. In other words, switch divides collision domain of hosts, but broadcast domain remains same. A switch is essentially a fast bridge having additional sophistication that allows faster processing of frames.
Some of important functionalities are:
- Ports are provided with buffer
- Switch maintains a directory: #address - port#
- Each frame is forwarded after examining the #address and forwarded to the proper port#
- Three possible forwarding approaches: Cut-through, Collision-free and fully buffered as briefly explained below.
Cut-through: A switch forwards a frame immediately after receiving the destination address. As a consequence, the switch forwards the frame without collision and error detection.
Collision-free: In this case, the switch forwards the frame after receiving 64 bytes, which allows detection of collision. However, error detection is not possible because switch is yet to receive the entire frame.
Fully buffered: In this case, the switch forwards the frame only after receiving the entire frame. So, the switch can detect both collision and error free frames are forwarded.
5 Router
A router is a network layer hardware device that transmits data from one LAN to another if both networks support the same set of protocols. So, a router is typically connected to at least two LANs and the internet service provider (ISP). It receives its data in the form of packets, which are data frames with their destination address added. Router also strengthens the signals before transmitting them. That is why it is also called repeater.
Routing Table
A router reads its routing table to decide the best available route the packet can take to reach its destination quickly and accurately. The routing table may be of these two types −
- Static − In a static routing table the routes are fed manually. So, it is suitable only for very small networks that have maximum two to three routers.
- Dynamic − In a dynamic routing table, the router communicates with other routers through protocols to determine which routes are free. This is suited for larger networks where manual feeding may not be feasible due to large number of routers.
6 Gateway is a network device used to connect two or more dissimilar networks. In networking parlance, networks that use different protocols are dissimilar networks. A gateway usually is a computer with multiple NICs connected to different networks. A gateway can also be configured completely using software. As networks connect to a different network through gateways, these gateways are usually hosts or end points of the network.
Gateway uses packet switching technique to transmit data from one network to another. In this way it is similar to a router, the only difference being router can transmit data only over networks that use same protocols.
7 Wi-Fi Card
Wi-Fi is the acronym for wireless fidelity. Wi-Fi technology is used to achieve wireless connection to any network. Wi-Fi card is a card used to connect any device to the local network wirelessly. The physical area of the network which provides internet access through Wi-Fi is called Wi-Fi hotspot. Hotspots can be set up at home, office or any public space. Hotspots themselves are connected to the network through wires. A Wi-Fi card is used to add capabilities like teleconferencing, downloading digital camera images, video chat, etc. to old devices. Modern devices come with their in-built wireless network adapter.
Key takeaway
Comparison between a switch and a hub Although a hub and a switch apparently look similar, they have significant differences. As shown in Figure, both can be used to realize physical star topology, the hubs work like a logical bus, because the same signal is repeated on all the ports. On the other hand, a switch functions like a logical star with the possibility of the communication of separate signals between any pair of port lines. As a consequence, all the ports of a hub belong to the same collision domain, and in case of a switch each port operates on separate collision domain. Moreover, in case of a hub, the bandwidth is shared by all the stations connected to all the ports. On the other hand, in case of a switch, each port has dedicated bandwidth. Therefore, switches can be used to increase the bandwidth of a hub-based network by replacing the hubs by switches.
References:
1. Forouzan, Data Communication & Networking, McGraw-Hill Education
2. Lathi, B. P. & Ding, Z., (2010), Modern Digital and Analog Communication Systems, Oxford University Press
3. Stallings, W., (2010), Data and Computer Communications, Pearson.
4. Andrew S. Tanenbaum, “Computer Networks” Pearson.
5. Ajit Pal, “Data Communication and Computer Networks”, PHI
6. Dimitri Bertsekas, Robert G. Gallager, “Data Networks”, Prentice Hall, 1992
