Module-III(4).pdf

Data Communications and Computer Networks Assistant Professor Department of Computer Science & Engineering IIIT Naya Raipur  Module III Data Link Layer  HDLC High-level Data Link Control (HDLC) is a bit-oriented protocol for communication over...

Data Communications and Computer Networks Assistant Professor Department of Computer Science & Engineering IIIT Naya Raipur  Module III Data Link Layer  HDLC High-level Data Link Control (HDLC) is a bit-oriented protocol for communication over point-to-point and multipoint links. It implements the Stop-and-Wait protocol we discussed earlier. HDLC provides two common transfer modes that can be used in different configurations: normal response mode (NRM) and asynchronous. balanced mode (ABM). HDLC defines three types of frames: information frames (frames), supervisory frames (S-frames), and unnumbered frames (U-frames). Each type of frame serves as an envelope for the transmission of a different type of message. Normal response mode Asynchronous balanced mode HDLC frames Control field format for the different frame types  U-frame control command and response 11.9  Example Figure shows how U-frames can be used for connection establishment and connection release. Node A asks for a connection with a set asynchronous balanced mode (SABM) frame; node B gives a positive response with an unnumbered acknowledgment (UA) frame. After these two exchanges, data can be transferred between the two nodes (not shown in the figure). After data transfer, node A sends a DISC (disconnect) frame to release the connection; it is confirmed by node B responding with a UA (unnumbered acknowledgment). 11.10  Point to Point Protocol Although HDLC is a general protocol that can be used for both point-to-point and. multipoint configurations, one of the most common protocols for point-to-point access is the Point-to-Point Protocol (PPP). PPP is a byte-oriented protocol. Frame format: Note PPP is a byte-oriented protocol using byte stuffing with the escape byte 01111101. Transition phases Figure PAP packets encapsulated in a PPP frame Figure CHAP packets encapsulated in a PPP frame Figure LCP packet encapsulated in a frame  Media Access control The Medium access control or Media access control (MAC) sublayer provides addressing and channel access control mechanisms that make it possible for several terminals or network nodes to communicate within a multiple accessnetwork that incorporates a shared medium. If there is a dedicated link between the sender and the receiver then data link control layer is sufficient. however if there is no dedicated link present then multiple stations can access the channel simultaneously. Hence multiple access protocols are required to decrease collision and avoid crosstalk.  Random Access In random access or contention methods, no station is superior to another station and none is assigned the control over another. No station permits, or does not permit, another station to send. At each instance, a station that has data to send uses a procedure defined by the protocol to make a decision on whether or not to send. Some common examples of random access protocols are as follows:  ALOHA [Pure and slotted ALOHA]  Carrier Sense Multiple Access(CSMA)  Carrier Sense Multiple Access with Collision Detection(CSMA/CD)  Carrier Sense Multiple Access with Collision Avoidance(CSMA/CA)  Pure ALOHA P u r e ALOH A a l l ows s t at i on s t o transmit whenever they have data to be sent. When a station sends data it waits for an acknowledgement. If the acknowledgement doesn't come within the allotted time then the station waits for a random amount of time called back-off time (Tb) and resends the data. Figur Vulnerable time for pure ALOHA protocol e  Note The throughput for pure ALOHA is S = G × e −2G. The maximum throughput Smax = 0.184 when G= (1/2). 12.7 Figure Procedure for pure ALOHA protocol  Slotted ALOHA It was developed just to improve the efficiency of pure aloha as the chances for collision in pure aloha are high. The time of the shared channel is divided into discrete time intervals called slots. Sending of data is allowed only at the beginning of these slots. If a station misses out the allowed time, it must wait for the next slot. This reduces the probability of collision. Figure Vulnerable time for slotted ALOHA protocol  Note The throughput for slotted ALOHA is S = G × e−G. The maximum throughput Smax = 0.368 when G = 1. 12.11 Example A pure ALOHA network transmits 200-bit frames on a shared channel of 200 kbps. What is the requirement to make this frame collision-free? Solution Average frame transmission time Tfr is 200 bits/200 kbps or 1 ms. The vulnerable time is 2 × 1 ms = 2 ms. This means no station should send later than 1 ms before this station starts transmission and no station should start sending during the one 1-ms period that this station is sending.