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20: Other Technologies used at the Link Layer

20: Other Technologies used at the Link Layer. Last Modified: 9/20/2014 2:41:53 AM. Token Passing: IEEE802.5 standard. 4 Mbps max token holding time: 10 ms, limiting frame length. SD, ED mark start, end of packet AC: access control byte:

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20: Other Technologies used at the Link Layer

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  1. 20: Other Technologies used at the Link Layer Last Modified: 9/20/2014 2:41:53 AM 5: DataLink Layer

  2. Token Passing: IEEE802.5 standard • 4 Mbps • max token holding time: 10 ms, limiting frame length • SD, ED mark start, end of packet • AC: access control byte: • token bit: value 0 means token can be seized, value 1 means data follows FC • priority bits: priority of packet • reservation bits: station can write these bits to prevent stations with lower priority packet from seizing token after token becomes free 5: DataLink Layer

  3. Token Passing: IEEE802.5 standard • FC: frame control used for monitoring and maintenance • source, destination address: 48 bit physical address, as in Ethernet • data: packet from network layer; checksum: CRC • FS: frame status: set by dest., read by sender • set to indicate destination up, frame copied OK from ring • limited number of stations: 802.5 have token passing delays at each station 5: DataLink Layer

  4. Point to Point Data Link Control • one sender, one receiver, one link: easier than broadcast link: • no need for explicit MAC addressing • full-duplex simultaneous bi-directional operation = no need for media access control • e.g., dialup link, ISDN line • popular point-to-point protocols: • PPP (point-to-point protocol) • HDLC: High level data link control 5: DataLink Layer

  5. PPP Design/Features • packet framing: encapsulation of network-layer datagram in data link frame • carry network layer data of any network layer protocol (not just IP) at same time • ability to demultiplex upwards • bit transparency: must carry any bit pattern in the data field • error detection (no correction) • connection liveness: detect, signal link failure to network layer • network layer address negotiation: endpoint can learn/configure each other’s network address 5: DataLink Layer

  6. PPP non-requirements • no error correction/recovery • no flow control • no need to support multipoint links (e.g., polling) Error recovery, flow control, data re-ordering all relegated to higher layers!| 5: DataLink Layer

  7. PPP Data Frame • Flag: delimiter (framing) • Address: does nothing (only one option) • Control: does nothing; in the future possible multiple control fields • Protocol: upper layer protocol to which frame delivered (eg. IP, PPP-LCP, IPCP, etc) 5: DataLink Layer

  8. PPP Data Frame • info: upper layer data being carried • check: cyclic redundancy check for error detection 5: DataLink Layer

  9. Byte Stuffing • “data transparency” requirement: data field must be allowed to include flag pattern <01111110> • Q: is received <01111110> data or flag? • Sender: adds (“stuffs”) extra < 01111110> byte after each < 01111110> data byte • Receiver: • two 01111110 bytes in a row: discard first byte, continue data reception • single 01111110: flag byte 5: DataLink Layer

  10. Byte Stuffing flag byte pattern in data to send flag byte pattern plus stuffed byte in transmitted data 5: DataLink Layer

  11. PPP Data Control Protocol Before exchanging network-layer data, data link peers must • configure PPP link (max. frame length, authentication) • learn/configure network layer information • for IP: carry IP Control Protocol (IPCP) msgs (protocol field: 8021) to configure/learn IP address 5: DataLink Layer

  12. IP over Other Wide Area Network Technologies • ATM • Frame Relay • X-25 5: DataLink Layer

  13. ATM architecture • Adaptation layer (AAL): only at edge of ATM network • roughly analogous to Internet transport layer • ATM layer: “network” layer • Virutal circuits, routing, cell switching • physical layer 5: DataLink Layer

  14. ATM Layer: ATM cell • 5-byte ATM cell header • 48-byte payload • Why?: small payload -> short cell-creation delay for digitized voice • halfway between 32 and 64 (compromise!) Cell header Cell format 5: DataLink Layer

  15. ATM cell header • VCI: virtual channel ID • will change from link to link thru net • PT:Payload type (e.g. RM cell versus data cell) • CLP: Cell Loss Priority bit • CLP = 1 implies low priority cell, can be discarded if congestion • HEC: Header Error Checksum • cyclic redundancy check 5: DataLink Layer

  16. ATM: network or link layer? Vision:end-to-end transport: “ATM from desktop to desktop” • ATM is a network technology Reality:used to connect IP backbone routers • “IP over ATM” • ATM as switched link layer, connecting IP routers 5: DataLink Layer

  17. Datagram Journey in IP-over-ATM Network • at Source Host: • IP layer finds mapping between IP, ATM dest address • passes datagram to AAL5 • AAL5 encapsulates data, segments to cells, passes to ATM layer • ATM network:moves cell along VC to destination (uses existing one or establishes another) • at Destination Host: • AAL5 reassembles cells into original datagram • if CRC OK, datgram is passed to IP 5: DataLink Layer

  18. X.25 and Frame Relay Like ATM: • wide area network technologies • virtual circuit oriented • origins in telephony world • can be used to carry IP datagrams and can thus be viewed as Link Layers by IP protocol just like ATM 5: DataLink Layer

  19. X.25 • X.25 builds VC between source and destination for each user connection • Per-hop control along path • error control (with retransmissions) on each hop • per-hop flow control using credits • congestion arising at intermediate node propagates to previous node on path • back to source via back pressure 5: DataLink Layer

  20. IP versus X.25 • X.25: reliable in-sequence end-end delivery from end-to-end • “intelligence in the network” • IP: unreliable, out-of-sequence end-end delivery • “intelligence in the endpoints” • 2000: IP wins • gigabit routers: limited processing possible 5: DataLink Layer

  21. Frame Relay • Designed in late ‘80s, widely deployed in the ‘90s • Frame relay service: • no error control • end-to-end congestion control 5: DataLink Layer

  22. Frame Relay (more) • Designed to interconnect corporate customer LANs • typically permanent VC’s: “pipe” carrying aggregate traffic between two routers • switched VC’s: as in ATM • corporate customer leases FR service from public Frame Relay network (eg, Sprint, ATT) 5: DataLink Layer

  23. address data CRC flags flags Frame Relay (more) • Flag bits, 01111110, delimit frame • Address = address and congestion control • 10 bit VC ID field • 3 congestion control bits • FECN: forward explicit congestion notification (frame experienced congestion on path) • BECN: congestion on reverse path • DE: discard eligibility 5: DataLink Layer

  24. Frame Relay -VC Rate Control • Committed Information Rate (CIR) • defined, “guaranteed” for each VC • negotiated at VC set up time • customer pays based on CIR • DE bit: Discard Eligibility bit • Edge FR switch measures traffic rate for each VC; marks DE bit • DE = 0: high priority, rate compliant frame; deliver at “all costs” • DE = 1: low priority, eligible for discard when congestion 5: DataLink Layer

  25. principles behind data link layer services: error detection, correction sharing a broadcast channel: multiple access link layer addressing, ARP various link layer technologies Ethernet hubs, bridges, switches IEEE 802.11 LANs PPP ATM, X.25, Frame Relay journey down the protocol stack now OVER! Summary 5: DataLink Layer

  26. A bit about physical connections 5: DataLink Layer

  27. Rating wide area internet connections • T0, DS0 – 1 voice channel, 65 Kbps • What homes get for 1 telephone line • T1 (Level 1 transmission line) or DS1 • 1.544 Mbps, 24 voice channels at 64 Kbps • T3 or DS3 = 28 T1 lines, 44.746 Mbps • OC3 = 3 DS3s • OC12 = 12 DS3s • OC48 = 48 DS3s, 2488 Mbps • OC192 = 192 DS3s 5: DataLink Layer

  28. SONET and SDH • Higher data rates often achieved using synchronous optical networking (SONET) and Synchronous Digital Hierarchy (SDH) • SONET in the US and Canada and SDH in the rest of the world • Transport over optical fiber using lasers/ LEDs • Transporting large amounts of telephone calls and data traffic over the same fiber without synchronization problems 5: DataLink Layer

  29. T0 = typical phoneline connection • DS3 delivered native on a copper trunk or converted to an optical fiber run when needing longer distances between termination points • DS3 transported over SONET is encapsulated in a STS-1 SONET channel • Still analog when delivered over fiber • When delivering data over an OC3 or greater SONET is used. • OC-3 SONET link contains three STS-1s, and therefore may carry three DS3s. • Likewise, OC-12, OC-48, and OC-192 may carry 12, 48, and 192 DS3s respectively. 5: DataLink Layer

  30. More on SONET • Designed to carry multiple real-time, uncompressed, circuit-switched voice lines encoded in Pulse-Code Modulation (PCM) format • Also multiple digital bit streams of differing origin within single framing protocol • Multiplex circuit mode communications (T1, T3, DS1, DS3,etc.) from a variety of different sources over same fiber • Emphasis is on merging many different flow into one quickly 5: DataLink Layer

  31. STM-1 (Synchronous Transport Module, level 1) frame is the basic transmission format for SDH. • STM-1 frame is transmitted in exactly 125 µs, therefore, there are 8,000 frames per second on a 155.52 Mbit/s OC-3 fiber-optic circuit 5: DataLink Layer

  32. Protocol neutral • Not communications protocols in and of themselves • Generic, all-purpose transport containers for moving both voice and data. • Used to carry ATM, Ethernet, TCP/IP etc. 5: DataLink Layer

  33. SONET standard defined by Telcordia and American National Standards Institute (ANSI) standard T1.105 and T1.119 • SDH standard specified in International Telecommunication Union (ITU) standards G.707, G.783, G.784, and G.803 • SDH originally defined by the European Telecommunications Standards Institute (ETSI) 5: DataLink Layer

  34. Carrier Pricing • Two simple components: local loop and port • Local loop = cost to transport the signal from the end user's central office (CO) to the point of presence (POP) of the carrier • Local loop cost based on geography/distance from CO to POP • Port = cost to access the network through the carrier's network • Port cost based on access speed and yearly commitment level 5: DataLink Layer

  35. Fiber cable runs • One example from the North Country 5: DataLink Layer

  36. Undersea cables 5: DataLink Layer

  37. multiple SONET signals can be transported over multiple wavelengths on a single fiber pair by means of wave length-division multiplexing, including dense wavelength-division multiplexing (DWDM) and coarse wavelength-division multiplexing (CWDM). • DWDM circuits are the basis for all modern submarine communications cable systems and other long-haul circuits. 5: DataLink Layer

  38. Other • Satellite Links • Pros and Cons 5: DataLink Layer

  39. Outtakes 5: DataLink Layer

  40. IEEE 802.11 MAC Protocol 802.11 CSMA Protocol: others • NAV: Network Allocation Vector • 802.11 frame has transmission time field • others (hearing data) defer access for NAV time units 5: DataLink Layer

  41. IEEE 802.11 MAC Protocol: CSMA/CA 802.11 CSMA: sender - if sense channel idle for DISF sec. then transmit entire frame (no collision detection) -ifsense channel busy then binary backoff 802.11 CSMA receiver: if received OK return ACK after SIFS 5: DataLink Layer

  42. IP-Over-ATM IP over ATM • replace “network” (e.g., LAN segment) with ATM network • IP addresses -> ATM addresses just like IP addresses to 802.3 MAC addresses! Classic IP only • 3 “networks” (e.g., LAN segments) • MAC (802.3) and IP addresses ATM network Ethernet LANs Ethernet LANs 5: DataLink Layer

  43. ARP in ATM Nets • ATM network needs destination ATM address • just like Ethernet needs destination Ethernet address • IP/ATM address translation done by ATM ARP (Address Resolution Protocol) • ARP server in ATM network performs broadcast of ATM ARP translation request to all connected ATM devices • hosts can register their ATM addresses with server to avoid lookup 5: DataLink Layer

  44. Access Control • 802.11 working group considered 2 proposals for a MAC algorithm • Distributed access protocols • Centralized access protocols 5: DataLink Layer

  45. Distributed Access Protocols • Distribute the decision to transmit over all the notes • Like Carrier-sense mechanisms in Ethernet • Makes sense especially for an ad hoc network of peer workstations • Can also be good for busty traffic 5: DataLink Layer

  46. Centralized Access Protocols • Regulation of transmission by a centralized decision maker • Natural for networks with a base station • Especially good if network is highly utilized ( avoid fighting it out among peers) • Also good if some data is time sensitive/high priority 5: DataLink Layer

  47. Distributed Foundation Wireless MAC • Compromise was Distributed Foundation Wireless MAC (DFWMAC) • Distributed Access control mechanism with an optional centralized control layer on top of that • Distributed Coordination Function (DCF) on top of physical layer • On top of that is optional Point Coordination Function (PCF) that provides contention free service 5: DataLink Layer

  48. Access Control

  49. CSMA • DCF uses Carrier Sense Multiple Access (CSMA) • CSMA means listen before you send to make sure the medium is idle • No Collision Detection - Not CSMA/CD like Ethernet • CD based on listening while you send to make sure you hear only your signal • Wireless HW not made to send and listen at same time • Large dynamic range of possible signals – cannot effectively distinguish incoming weak signals from noise and the effects of its own transmission 5: DataLink Layer

  50. Medium Access Control Logic IFS = interframe space Each time fail increase time to wait before send

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