Presentation on theme: "LAN Protocol Architecture. Lower layers of OSI model IEEE 802 reference model, is a standardized protocol architecture for LANs, which describes: –Physical."— Presentation transcript:
LAN Protocol Architecture
Lower layers of OSI model IEEE 802 reference model, is a standardized protocol architecture for LANs, which describes: –Physical layer. –Logical link control (LLC) sub-layer, –Media access control (MAC) sub-layer.
IEEE 802 v OSI
IEEE 802 Protocol Layers Physical layer, includes such functions as: –Encoding and decoding of signals –Preamble generation and removal (for synchronization) –Bit transmission and reception Logical Link Control, functions: –Assemble data into a frame with address and error-detection fields –Disassemble frame and perform address recognition and error detection –Govern access to the LAN transmission medium –Interface to higher levels and performs flow and error control
Logical Link Control LLC is concerned with transmission of link- level PDUs between two stations LLC has two special characteristics: –Must support multiaccess, shared medium (no primary node as in multidrop line) –Relieved of some link access details by MAC layer Addressing involves specifying source and destination LLC users –Referred to as service access points (SAP) –Service users are typically higher level protocols
LLC Services The operation and format of this standard is based on HDLC. Provide three different services for attached devices: –Unacknowledged connectionless service: Datagram-style service. No flow-control and no error-control mechanisms. Reliability depend to some higher layer of software. –Connection mode service: Similar to that of HDLC. Provides flow- and error-control. –Acknowledged connectionless service Datagrams are to be acknowledged. No prior logical connection is set up.
LLC Protocol Is modeled after HDLC, and has similar functions and formats. LLC protocol operation: –LLC use asynchronous balanced mode of operation of HDLC to support connection mode LLC service (type 2 operation) –LLC supports an unacknowledged connectionless service using unnumbered information PDUs (type 1 operation) –LLC supports acknowledged connectionless service using unnumbered information PDUs (type 3 operation) –LLC permits multiplexing using LLC service access points (LSAPs)
Media Access Control Key parameters of MAC technique is where and how: Where –Central Adv.: Greater control and simple access logic at station (no coordination complexities) Disadv.: Single point of failure and potential bottleneck –Distributed: opposite of central. How –Synchronous: specific capacity dedicated to connection –Asynchronous: in response to demand; can be subdivided into three categories (round robin, reservation, contention)
Asynchronous MAC Techniques Round robin: –Each station is given the opportunity to transmit. The right to transmit passes in a logical sequence. Reservation: a node will reserves future slots from the medium time to transmit, this is good for stream traffic. Contention –Good for bursty traffic –All stations contend for time –Distributed control –Simple to implement –Efficient under moderate load –Tend to collapse under heavy load
MAC Frame Format MAC layer receives data from LLC layer The fields of MAC frame: –MAC control: protocol control like priority. –Destination MAC address –Source MAC address –LLC: data from next higher layer. –CRC: FCS for error detection. MAC layer detects errors and discards frames LLC optionally retransmits unsuccessful frames