Link Layer Services in computer networks
- 1. Link Layer
- 2. Agenda • Link Layer And Local Area Networks: Data Link-Layer Services and Principles, • Introduction, • Error Detection and Correction Techniques, • Multiple Access Protocols, • LANAddresses andARP, • Ethernet, Hubs, Bridges, Switches, • wireless Links: IEEE 802.11, Bluetooth, • PPP:The Point-to-Point Protocol, • AsynchronousTransfer Mode (ATM), • X.25 and Frame Relay.
- 3. Purpose of Link Layer • the basic service of any link layer is to move a datagram from one node to an adjacent node over a single communication link
- 4. Services of Link Layer • Framing • Link Access • Reliable Delivery • Error Detection and Correction
- 5. Framing • Almost all link-layer protocols encapsulate each network-layer datagram within a link-layer frame before transmission over the link. • A frame consists of a data field, in which the network-layer datagram is inserted, and a number of header fields.
- 6. Link Access • A medium access control (MAC) protocol specifies the rules by which a frame is transmitted onto the link. • For point-to-point links that have a single sender at one end of the link and a single receiver at the other end of the link, the MAC protocol is simple (or nonexistent)—the sender can send a frame whenever the link is idle.
- 7. Reliable Delivery • When a link-layer protocol provides reliable delivery service, it guarantees to move each network-layer datagram across the link without error. • A link-layer reliable delivery service is often used for links that are prone to high error rates, such as a wireless link, with the goal of correcting an error locally—on the link where the error occurs—rather than forcing an end- toend retransmission of the data by a transport- or application-layer protocol. H
- 8. Error detection and correction. • The link-layer hardware in a receiving node can incorrectly decide that a bit in a frame is zero when it was transmitted as a one, and vice versa. • Such bit errors are introduced by signal attenuation and electromagnetic noise. Because there is no need to forward a datagram that has an error, many link-layer protocols provide a mechanism to detect such bit errors. • This is done by having the transmitting node include error-detection bits in the frame, and having the receiving node perform an error check. • Error detection in the link layer is usually more sophisticated and is implemented in hardware. • Error correction is similar to error detection, except that a receiver not only detects when bit errors have occurred in the frame but also determines exactly where in the frame the errors have occurred (and then corrects these errors).
- 10. • At the sending node, data, D, to be protected against bit errors is augmented with error- detection and -correction bits (EDC). • Typically, the data to be protected includes not only the datagram passed down from the network layer for transmission across the link, but also link-level addressing information, sequence numbers, and other fields in the link frame header. • Both D and EDC are sent to the receiving node in a link-level frame. • At the receiving node, a sequence of bits, D and EDC is received. Note that D and EDC may differ from the original D and EDC as a result of in-transit bit flips. • The receiver’s challenge is to determine whether or not D is the same as the original D, given that it has only received D and EDC.The exact wording of the receiver’s decision is important.
- 11. Error Detection and CorrectionTechniques • There are three methods for error detection and correction as follows • Parity Checks • Checksum Methods • Cyclic Redundancy Checks
- 12. Parity Checks • Parity checks is two types • Single bit parity check • Two dimensional parity bit check
- 13. Single bit parity check • Let we have some data d to be sent as follows In an even parity scheme, the sender simply includes one additional bit and chooses its value such that the total number of 1s in the d + 1 bits (the original information plus a parity bit) is even. For odd parity schemes, the parity bit value is chosen such that there is an odd number of 1s. Receiver operation is also simple with a single parity bit. The receiver need only count the number of 1s in the received d + 1 bits. If an odd number of 1- valued bits are found with an even parity scheme, the receiver knows that at least one bit error has occurred.
- 14. Two-dimensional Parity check Parity check bits are calculated for each row, which is equivalent to a simple parity check bit. Parity check bits are also calculated for all columns, then both are sent along with the data. At the receiving end these are compared with the parity bits calculated on the received data.
- 15. Checksum •In checksum error detection scheme, the data is divided into k segments each of m bits. •In the sender’s end the segments are added using 1’s complement arithmetic to get the sum. The sum is complemented to get the checksum. •The checksum segment is sent along with the data segments. •At the receiver’s end, all received segments are added using 1’s complement arithmetic to get the sum. The sum is complemented. •If the result is zero, the received data is accepted; otherwise discarded.
- 17. Cyclic redundancy check (CRC) • Unlike checksum scheme, which is based on addition, CRC is based on binary division. • In CRC, a sequence of redundant bits, called cyclic redundancy check bits, are appended to the end of data unit so that the resulting data unit becomes exactly divisible by a second, predetermined binary number. • At the destination, the incoming data unit is divided by the same number. If at this step there is no remainder, the data unit is assumed to be correct and is therefore accepted. • A remainder indicates that the data unit has been damaged in transit and therefore must be rejected.
- 20. Multiple Access Links & protocols • Types of Network Links • Point to Point Link • Broadcast Network Link
- 21. Point to Point Link • In point to point link we have only one sender and one receiver. • There are two protocols designed for Point to point link as follows • Point to point(ppp) protocol • High Level Data link Control (HDLC)
- 22. PPP Link Protocol • It defines the format of frames through which the transmission occurs. • It defines the link establishment process. If user establishes a link with a server, then "how this link establishes" is done by the PPP protocol. • It defines data exchange process, i.e., how data will be exchanged, the rate of the exchange. • The main feature of the PPP protocol is the encapsulation. It defines how network layer data and information in the payload are encapsulated in the data link frame. • It defines the authentication process between the two devices. The authentication between the two devices, handshaking and how the password will be exchanged between two devices are decided by the PPP protocol.
- 23. Frame format of PPP protocol • Flag: The flag field is used to indicate the start and end of the frame. The flag field is a 1-byte field that appears at the beginning and the ending of the frame. The pattern of the flag is similar to the bit pattern in HDLC, i.e., 01111110. • Address: It is a 1-byte field that contains the constant value which is 11111111. These 8 ones represent a broadcast message. • Control: It is a 1-byte field which is set through the constant value, i.e., 11000000. It is not a required field as PPP does not support the flow control and a very limited error control mechanism. The control field is a mandatory field where protocol supports flow and error control mechanism. • Protocol: It is a 1 or 2 bytes field that defines what is to be carried in the data field. The data can be a user data or other information. • Payload: The payload field carries either user data or other information. The maximum length of the payload field is 1500 bytes. • Checksum: It is a 16-bit field which is generally used for error detection.
- 24. HDLC • The HDLC primarily supports two types of transfer models, which are: • Normal Response Model (NRM) - This transfer model combines primary and secondary stations in point-to-point or multipoint network configurations to exchange commands from primary stations and responses from secondary stations. • Asynchronous Balanced Model (ABM) - In this transfer model, combined stations are installed in a point-to-point configuration for exchange commands and responses in a balanced format.
- 25. HDLC Frame The data unit for sharing information in HDLC is known as frames. HDLC frame consists of multiple frame fields, which may vary according to the type of frame used, and are as follows: •Flag Field - In HDLC, each frame starts and ends with the flag field in the configuration and is defined by an 8-bit octet sequence 01111110 in the flag field. •Address Field - It encapsulates the receiver's address in the field. For example, if the frame is sent from the primary station, it contains the secondary station’s address and vice versa. •Control Field - This field contains the flow and error control information in byte format. •Payload/Information Field - It carries information from the network layer, and the data size may vary depending on the network. •FCS Field - This field stands for Frame Check Sequence and acts as an error detection field in the HDLC protocol, which includes a 16-bit CRC check bit.
- 26. Broadcast Network Link • In this we can have multiple sending and receiving nodes all connected to the same, single, shared broadcast channel. • The term broadcast is used here because when any one node transmits a frame, the channel broadcasts the frame and each of the other nodes receives a copy. • There are different types of Multiple Access Protocols as follows • Channel Partitioning Protocols • RandomAccess Protocols • Taking-Turns Protocols
- 27. Asynchronous Transfer Mode (ATM) in Computer Network • It is an International Telecommunication Union- Telecommunications Standards Section (ITU-T) efficient for call relay and it transmits all information including multiple service types such as data, video, or voice which is conveyed in small fixed- size packets called cells. Cells are transmitted asynchronously and the network is connection-oriented. • ATM is a technology that has some event in the development of broadband ISDN in the 1970s and 1980s, which can be considered an evolution of packet switching. Each cell is 53 bytes long – 5 bytes header and 48 bytes payload. Making an ATM call requires first sending a message to set up a connection.
- 28. ATM Cell Format • As information is transmitted in ATM in the form of fixed-size units called cells. As known already each cell is 53 bytes long which consists of a 5 bytes header and 48 bytes payload.
- 29. • Asynchronous Transfer Mode can be of two format types which are as follows: 1.UNI Header: This is used within private networks of ATMs for communication between ATM endpoints and ATM switches. It includes the Generic Flow Control (GFC) field. 2.NNI Header: is used for communication between ATM switches, and it does not include the Generic Flow Control(GFC) instead it includes a Virtual Path Identifier (VPI) which occupies the first 12 bits.
- 30. ATM Layers
- 31. ATM Adaption Layer (AAL) It is meant for isolating higher-layer protocols from details of ATM processes and prepares for conversion of user data into cells and segments it into 48-byte cell payloads. AAL protocol excepts transmission from upper-layer services and helps them in mapping applications, e.g., voice, data to ATM cells.
- 32. Physical Layer – It manages the medium-dependent transmission and is divided into two parts physical medium-dependent sublayer and transmission convergence sublayer. The main functions are as follows: 1.It converts cells into a bitstream. 2.It controls the transmission and receipt of bits in the physical medium. 3.It can track the ATM cell boundaries. 4.Look for the packaging of cells into the appropriate type of frames.
- 33. ATM Layer It handles transmission, switching, congestion control, cell header processing, sequential delivery, etc., and is responsible for simultaneously sharing the virtual circuits over the physical link known as cell multiplexing and passing cells through an ATM network known as cell relay making use of the VPI and VCI information in the cell header.
- 34. X.25 is generally a protocol that was developed by Telecommunication Standardization Sector (ITU-T) of International Telecommunication Union. It usually allows various logical channels to make use of same physical line. It basically defines a series of documents particularly issued by ITU. These documents are also known as X.25 Recommendations. X.25 also supports various conversations by multiplexing packets and also with the help of virtual communication channels. X.25 basically encompasses or suits to the lower three layers of the Open System Interconnection (OSI) reference model for networking. These three protocol layers are : X.25 Structure 1.Physical Layer 2.Frame Layer 3.Packet Layer
- 36. Physical Layer : This layer is basically concerned with electrical or signaling. The physical layer interface of X.25 also known as X.21 bis was basically derived from RS-232 interface for serial transmission. This layer provides various communication lines that transmit or transfer some electrical signals. X.21 implementer is usually required for linking.
- 37. Frame Layer Data link layer is also known as Frame Layer. This layer is an implementation or development of ISO High-Level Data Link Layer (HDLC) standard which is known as LAPB (Link Access Procedure Balanced). It also provides a communication link and transmission that is error-free among any two physically connected nodes or X.25 nodes. LAPB also allows DTE (Data Terminal Equipment) or DCE (Data Circuit-Terminating Equipment) simply to start or end a communication session or start data transmission. This layer is one of the most important and essential parts of X.25 Protocol. This layer also provides a mechanism for checking in each hop during the transmission. This service also ensures a bit-oriented, error-free, and also sequenced and ordered delivery of data frames or packets. There are many protocols that can be used in frame-level as given below :
- 38. •Link Access Procedure Balanced (LAPB) – It is specified by ITU-T Recommendation X usually derived from HDLC. It is the most commonly used protocol that allows establishing a logical connection. •Link Access Protocol (LAP) – This protocol is very rarely used. This is usually used for framing and transferring data packets across point-to-point links. •Link Access Procedure D-channel (LAPD) – It is used to convey or transfer data over D- channel. It also enables and allows transmission of data among DTEs through D channel especially among a DTE and an ISDN node. •Logical Link Control (LLC) – It is used to manage and ensure the integrity of transmissions of data. It also allows transmission of X.25 data packets or frames through a LAN (Local Area Network) channel.
- 39. Packet Layer : Packet layer is also known as Network Layer protocol of X.25. This layer generally governs the end-to-end communications among various DTE devices. It also defines how to address and deliver X.25 packets among end nodes and switches on a network with the help of PVCs (Permanent Virtual Circuits) or SVCs (Switched Virtual Circuits). This layer also governs and manages set-up and teardown and also flow control among DTE devices as well as various routing functions along with multiplexing multiple logical or virtual connections. This layer also defines and explains the format of data packets and also the procedures for control and transmission of data frames. This layer is also responsible for establishing a connection, transmitting data frames or packets, ending or terminating a connection, error and flow control, transmitting data packets over external virtual circuits.