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Local Area Networks

Local Area Networks. hub. hub. hub. hub. LAN characteristics. confined within geographical area relatively high data rate under single management. stations. stations. LAN Ingredients. Topology bus, star, ring Transmission medium twisted pair, coaxial, fiber optics, wireless

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Local Area Networks

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  1. Local Area Networks

  2. hub hub hub hub LAN characteristics • confined within geographical area • relatively high data rate • under single management stations stations LAN Ingredients... • Topology • bus, star, ring • Transmission medium • twisted pair, coaxial, fiber optics, wireless • Medium access techniques • protocol to coordinate the sharing of media station router server

  3. Topology • Topology defines how nodes/stations are connected • LAN typical topology : • bus/tree : all nodes connected to a common medium • star : all nodes are joined to a central nodes • ring : nodes form a ring by point-to-point links to adjacent neighbors ring star bus

  4. Topology : Bus C D B A D B and C D Application Application Presentation Presentation Session Session Transport Transport Network Network Data Link Data Link Physical Physical

  5. A A A A Topology : Ring A A • nodes function as a repeater • only destination copies frame to it, all other nodes have to discarded the frame • Unidirectional link C B C B B transmits frame addressed to A C ignores frame A A C B C B A copies frame C absorbs returning frame

  6. A A A A Topology : Star • Each station attaches to the central node • Two point-to-point links for tranmitting and receiving • Two alternatives operation • broadcasting • switching shared hub broadcast packet to every ports but only destination copies frame A C B A B C switch hub retransmitts packet to the destination only C transmitts frame addressed to A A B C

  7. Transmission media • Twisted pair • Cat 3 -- voice grade UTP, data rate up to 10Mbps • Cat 5 -- data grade UTP, data rate 100 Mbps • Coaxial cable • baseband coaxial support data rates 20 Mbps at distance of up to 2 km • Fiber optics • support gigabit data rate and longer distances

  8. Medium Access Control • Protocol for controlling access to transmission medium • Defined as part of Data Link layer • The protocol performs: • perform functions related to medium access (MAC sublayer) • concerned with the transmission of a link-level between two nodes (LLC sublayer) Network LLC Logical Link Control sublayer Data Link MAC Medium Access Control sublayer Physical

  9. LLC Sublayer Functions • Provides a compatibility interface, irrespective of the MAC layer used. • Service Access Points (SAPs) interface sublayers to upper-layer functions • Optional connection, flow control, and sequencing services

  10. Destination Address Source Address Type LLC PDU CRC MAC control DSAP SSAP CTRL Data MAC frame format MAC Frame (802.3) SAPs define a protocol for upper layer LLC PDU DSAP: Destination Service Access Point SSAP: Source Service Access Point Control: Command and response functions Data: LLC SDU

  11. 24 bits 24 bits Vendor Code Serial Number MAC Address 0000.0c12. 3456 ROM RAM • MAC address is burned into ROM on a network interface card

  12. MAC Strategies • Where? • centralized : a designated station has an authurity to grant access to the network • distributed : stations can dynamically determined transmission order • How? • Sychronous : dedicated specific capacity to a conection • Asynchronous : dynamic allocate capacity (3 categories available) • Round-robin • Reservation • Contention

  13. MAC techniques • Contention • Medium is free for all • A node senses the free medium and occupies it • Example: Ethernet, IEEE 802.3 • Round robin • Give every body a turn • Inefficient for lightly loaded network • Example: Token Ring/IEEE 802.5, Token Bus/IEEE 802.4, FDDI • Reservation • schedule a time slot like TDM • Example: DQDB

  14. 802.2 LLC FDDI 802.3 CSMA/CD 802.4 Token Bus 802.5 Token Ring 802.6 DQDB Overview of LAN Standard • 802.1 Overview • 802.2 Logical Link Control • 802.3 CSMA/CD (Ethernet) • 802.4 Token Bus • 802.5 Token Ring • 802.6 Distributed Queue Dual BUS --MAN standard • FDDI Fiber Distributed Data Interface

  15. A A A Ethenet/802.3 Operation • Every node can receive a transmission by all other nodes • need addressing scheme to identify a destination • only destination copies frame to it, all other nodes have to discarded the frame C B A C B A terminator C finds the bus is free C transmits frame addressed to A C C B B A A A copies frame B ignores frame signal is absorbed by the terminators

  16. Token Ring/802.5 Operation T T = 0 T = 0 A A T T = 1 A T Data • Token Ring LANs continuously pass a token or a Token Ring frame

  17. FDDI FDDI Dual Ring

  18. DSAP 1 SSAP 1 CTRL 1 Data >=0 Preamble 16 SD 2 FC 2 DA 12 SA 12 802.2 LLC FCS 8 ED 1 or 2 FS 1 SD 1 AC 1 FC 1 DA 6 SA 6 802.2 LLC FCS 4 ED 1 FS 1 Preamble 8 DA 6 SA 6 Length 2 802.2 LLC FCS 4 Preamble 8 DA 6 SA 6 Type 2 Data FCS 4 LAN Frame Format Summary 802.2 LLC FDDI FDDI uses 4-bits symbols 802.5 802.3 Ethernet

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