2LAN Protocol Architecture Lower layers of OSI modelIEEE 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.
4IEEE 802 Protocol Layers Physical layer, includes such functions as: Encoding and decoding of signalsPreamble generation and removal (for synchronization)Bit transmission and receptionLogical Link Control, functions:Assemble data into a frame with address and error-detection fieldsDisassemble frame and perform address recognition and error detectionGovern access to the LAN transmission mediumInterface to higher levels and performs flow and error control
5Logical Link ControlLLC is concerned with transmission of link-level PDUs between two stationsLLC has two special characteristics:Must support multiaccess, shared medium (no primary node as in multidrop line)Relieved of some link access details by MAC layerAddressing involves specifying source and destination LLC usersReferred to as service access points (SAP)Service users are typically higher level protocols
6LLC ServicesThe 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 serviceDatagrams are to be acknowledged.No prior logical connection is set up.
7LLC ProtocolIs 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)
8Media Access Control Key parameters of MAC technique is where and how: CentralAdv.: Greater control and simple access logic at station (no coordination complexities)Disadv.: Single point of failure and potential bottleneckDistributed: opposite of central.HowSynchronous: specific capacity dedicated to connectionAsynchronous: in response to demand; can be subdivided into three categories (round robin, reservation, contention)
9Asynchronous 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.ContentionGood for bursty trafficAll stations contend for timeDistributed controlSimple to implementEfficient under moderate loadTend to collapse under heavy load
10MAC Frame Format MAC layer receives data from LLC layer The fields of MAC frame:MAC control: protocol control like priority.Destination MAC addressSource MAC addressLLC: data from next higher layer.CRC: FCS for error detection.MAC layer detects errors and discards framesLLC optionally retransmits unsuccessful frames