LAN.pdf - CS4344 – Computer Networks and Distributed Systems Local Area Network Dr Padmaraj Nair Bus Topology • Topology – Refers to the way in

LAN.pdf - CS4344 – Computer Networks and Distributed...

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CS4344 – Computer Networks and Distributed Systems Local Area Network Dr. Padmaraj Nair
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Bus Topology Topology Refers to the way in which the endpoints, or stations, attached to the network are interconnected Ø Bus topology All stations attach, through a tap, directly to a linear transmission medium, or bus Full-duplex operation between the station and the tap allows data to be transmitted onto the bus and received from the bus A transmission from any station propagates the length of the medium in both directions and can be received by all other stations At each end of the bus is a terminator, which absorbs any signal, removing it from the bus
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A A C transmits frame addressed to A Frame is not addressed to B; B ignores it A copies frame as it goes by A A B C A B C A B C
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Star Topology Each station connects to common central node Usually via two point-to-point links One for transmission and one for reception Operate in broadcast fashion Physical star, logical bus Only one station can transmit at a time (hub) Can act as frame switch Central node
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Central Hub, switch, or repeater
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Physical Data Link Medium Network Transport Session Presentation Application OSI Reference Model Physical Medium Access Control Medium Logical Link Control ( ) ( ) ( ) Upper Layer Protocols LLC Service Access Point (LSAP) Scope of IEEE 802 Standards IEEE 802 Reference Model IEEE 802 protocol layers
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IEEE 802 Reference Model Lowest layer corresponds to the physical layer of the OSI model Includes a specification of the transmission medium and the topology Includes functions such as: Encoding/decoding of signals Preamble generation/removal Bit transmission/reception
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IEEE 802 Layers Logical Link Control Layer (LLC) Provide interface to higher levels Perform flow and error control Media Access Control (MAC) On transmit assemble data into frame On reception disassemble frame, perform address recognition and error detection Govern access to LAN transmission medium
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TCP segment IP datagram LLC protocol data unit MAC frame Application data TCP header IP header LLC header MAC header MAC trailer Figure 11.4 LAN Protocols in Context Application Layer TCP Layer IP Layer LLC Layer MAC Layer
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Logical Link Control Transmission of link level PDUs between stations Must support multi-access, shared medium Relieved of some details of link access by the MAC layer Addressing involves specifying source and destination LLC users A user is a higher-layer protocol or a network management function in the station Referred to as service access points (SAPs)
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LLC Services Unacknowledged connectionless service Data-gram style service Delivery of data is not guaranteed
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