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Computer Networks

Computer Networks. Chapter 2 – Data Link Layer Issues. LAN Properties. Shared medium High data rate Low delay Low error rate Native broadcast support Limited physical extent (a few kms) Limited number of stations (100’s) STAs are peers Local management (not under PTT regulation).

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Computer Networks

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  1. Computer Networks Chapter 2 – Data Link Layer Issues CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  2. LAN Properties • Shared medium • High data rate • Low delay • Low error rate • Native broadcast support • Limited physical extent (a few kms) • Limited number of stations (100’s) • STAs are peers • Local management (not under PTT regulation) CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  3. Medium Access • Access allocation so that • One STA at a time accesses medium • Each STA gets a fair share • Delays are reasonable • Overhead and waste are minimized • Approaches • Tokens • Contention CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  4. Token-based Approaches • Token Ring • STAs linked (simplex) to two neighbors • Token circulates physical ring • Add STA by insertion into ring • Token Bus • STAs attached to bus • Token circulates logical ring • Add STA to bus and insert into logical ring CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  5. Contention Approaches • ALOHA • Can’t sense medium, so just talk! • CSMA • Listen before talk • CSMA/CD • Add collision detection (need sensitive PCS) • CSMA/CA • Use collision avoidance (when VCS used) CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  6. IEEE 802 • 802.1 – common issues • Addressing, management, bridging, security • 802.2 – LLC • Type 1 – best effort / Type 2 - reliable • 802.3 – CSMA/CD LAN • From Xerox Ethernet • 802.4 – Token Bus • 802.5 – Token Ring • 802.11 – Wireless LAN • 802.16 – Wimax • Note – FDDI standardized by ANSI CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  7. Names, Addresses, Routes • Name – what • Location-independent identifier • May be human-friendly or not • Address – where • Independent of source location, but will change if destination moves • Route – how to get there • Depends on both source and destination CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  8. LAN Addresses • LANs are broadcast medium – need both • Source address (for return messages) • Destination address (to filter) • IEEE 802 addresses • 16 and 48 bits (also 60 for 802.6 DQDB) • 48 bit addresses managed by IEEE • Pay to get 224 address block, Vendor Code (OUI) • G/L bit is 0 if global, 1 if locally managed OUI 0 2nd Octet 3rd Octet 4th Octet 5th Octet 6th Octet G/L bit (global/local) G/I bit (group/individual) CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  9. Multicast Addresses • Service Discovery • Solicitation (client mcast to Z-Servers address) • Advertisement (Z-Servers mcast to Z-Client addr) • Why Multicast (group) addresses? • Reduce interrupt handling by hosts • Hardware filter • Why G/I bit? • Allow filtering by hash buckets in HW • SW filters all hits in relevant hash buckets CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  10. Broadcast • The all 1’s address (0xffffffffffff) is bcast • Means that all STAs must receive • Really, though, only those implementing the protocol used in the broadcast packet have to… • Broadcast address interrupts everyone anyway CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  11. Multiplexing Field • Multiple higher layer protocols • Format and data alone cannot distinguish • Multiplexing field selects which one • Protocol type in Ethernet • DSAP and SSAP in IEEE 802 • Service Access Point (SAP) Structure • G/L and G/I bits also, hence 6 bits • All 1’s = all SAPs (!!!) • Others assigned by IEEE – too few! • SNAP (Subnetwork Access Protocol) • When DSAP = SSAP = 0xaa • extra protocol type field (5 octets) • 3 OUI octets, 3 vendor-assigned octets CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  12. Bit Order • Bit order is order in which bits are put onto the medium • Not standard () • LSB first canonical and for most LANs • MSB first for 802.5, FDDI • Bridges must convert • Shuffle bit order within octets • Impact on ARP and higher layer protocols CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  13. LLC • 802.2 LLC has two significant types • Type 1 – datagrams (best effort) • Type 2 – reliable (connection oriented) • Type 1 Control – 1 octet • UI – unnumbered info (datagram) • XID – Exchange ID (command/response) • ID of transmitter • LLC types supported • Test – (Cmd/Rsp) – Rsp echo data in Cmd CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  14. LLC Type 2 • Control is 1 or 2 octets, per type • 2-octet control fields contain 7-bit seq #s • I = Information (data) – SN plus ACK SN • RR = Receive Ready (ACK) – ACK SN • RNR = Rcv Not Ready (Busy) – ACK SN • REJ = Reject – ACK SN CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  15. LLC Type 2 • Control is 1 or 2 octets, per type • 1-octet control field types • SABME = Set Asynchronous Balanced Mode Extended (start connection) • DISC = Disconnect (end connection) • DM = Disconnected Mode (confirm DISC) • FRMR = Frame Reject (receipt of invalid pkt) • UA = Unnumbered ACK (for DISC/SABME) CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  16. 802.3 Issues • CSMA/CD • Listen before talk • Detect collisions • Binary exponential backoff • Minimum frame length • So all STAs detect collision • Slot time = 2t (512 bits at 2.5km and 10Mbps) Start Tx End Tx Start Tx Start Rx Detect Collision End Tx End Rx CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  17. 802.5 Issues • Each packet traverses every station in physical ring • Each STA has transceiver buffer with special HW to recognize token, addresses • Two bits at end of each frame for ACK: • A bit (address recognized) • C bit (frame copied) • Each STA may modify bits • Sender sees A/C bits when frame returns CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  18. 802.5 Issues • A/C Bits on return: • A=0: Address not recognized (give up) • A=1, C=0: Address recognized, but STA busy (try again) • A=1, C=1: Address recognized and frame copied (success!) • What does a bridge do with these? • Clear both? • Leave unmodified • Set A and C if bridge forwards • Clear A and set C if bridge forwards • A/C used for other purposes: • Ring order (bcast frame with A bit clear indicates predecessor) • Only 31 functional addresses for multicast • Mapping • Oversubscription CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  19. Packet Bursts • Station processing rate may be slower than the LAN data rate • While OK on average, packets may be sent in a burst • Early packets received, later ones lost • Problem if naïve protocol retransmits whole burst every time CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  20. Why Bridges? • Limit number of stations in LAN • Packet lengths • Delay • Size limitations • 802.3 collision detection • Traffic • Capacity is shared • Simple, high performance, allow limited location transparency (keep IP address) CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  21. Point-to-Point Links flag address control data checksum flag • Flag • Special pattern to delimit frames • In HDLC, 01111110 • In DDCMP, DLE-SOF & DLE-EOF • Bit-stuffing/character-stuffing for data transparency • In HDLC, 011111… -> 0111110… on Transmit • In DDCMP, … DLE … -> … DLE DLE … on Transmit • Addressing • Needed if multiple stations on medium • Traditionally assume master/slave • Control – Like LLC Type 2 • Checksum – 16 bit CRC HDLC format CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  22. Point-to-Point Links flag Addr=0xff Ctl=0x03 protocol data checksum flag • Multiplexing • Protocol field in PPP (16 bits – see RFC 1700) • Service • If links not reliable, need reliable transfer per hop (HDLC, DDCMP, LLC Type 1) • What is probability of success for k hops with FER P? • What is cost per attempt for k hops with FER P? • What is overall cost for success for k hops with FER P? • If links reliable, then datagram service OK (PPP, LLC Type 2) PPP format CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  23. Link Reliability Issues CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  24. Link Reliability Issues CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  25. Link Reliability Issues CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  26. Link Reliability Issues CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

  27. Per Hop vs. End-to-End BEC Pkt Sent Pkt Sent Pkt Recv Pkt Recv ACK Recv ACK Recv Per Hop End-to-End CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman

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