Data Link Layer Functions and Link Devices

1. Data-link layer Functions, specific link layers and devices CSE524: Lecture 16

2. Administrative • Reading assignment • Chapter 5 • Homework #5 • See web site • Sample final exam • See web site • Send me questions to cover by next Monday (November 29th) and I will cover them in class

3. Where we’re at… • Internet architecture and history • Internet protocols in practice • Application layer • Transport layer • Network layer • Data-link layer • Functions • Digital to analog conversion, Framing, Physical addressing, Demux to upper protocol, Flow control, Reliable delivery, Error detection and correction, Security • Media access and quality of service • Channel Partitioning (TDMA, FDMA, CDMA) • Random access protocols (Slotted Aloha, Pure Aloha, CSMA, CSMA/CD) • Taking turns protocols • Specific link layer examples and devices • Physical layer

4. DL: Random access MAC protocols • CSMA: Carrier Sense Multiple Access • Listen before transmitting • If channel sensed idle: transmit entire pkt • If channel sensed busy, defer transmission • Persistent CSMA: retry immediately with probability p when channel becomes idle • Non-persistent CSMA: retry after random interval

5. DL: CSMA collisions spatial layout of nodes along ethernet collisions can still occur: propagation delay means two nodes may not hear each other’s transmission collision: entire packet transmission time wasted note: role of distance and propagation delay in determining collision prob.

6. DL: CSMA/CD (Collision Detection) • Same carrier sensing deferral as in CSMA • Add collision detection • For wired LANs: measure signal strengths, compare transmitted, received signals • Collisions detected within short time • Abort transmission as soon as collision detected to reduce channel waste • Can be used with persistent or non-persistent retransmission

7. DL: CSMA/CD collision detection

8. DL: CSMA/CD problems • Can CSMA/CD work over wireless LANs? • Difficult in wireless LANs: receiver shut off while transmitting • Hidden terminal problem

9. DL: Hidden Terminal effect • A, C cannot hear each other • obstacles, signal attenuation • Neither A or C can tell if they collide at B

10. DL: CSMA/CA • Use base CSMA • Add acknowledgements • Receiver acknowledges receipt of data • Avoids hidden terminal problem • Avoid collisions explicitly via channel reservation • Sender sends “request-to-send” (RTS) messages • Transmitted without reservation using CSMA with ACKs • Receiver sends “clear-to-send” (CTS) messages • Transmitted without reservation using CSMA with ACKs • Sender sends data packet using reservation • Explicitly indicates length of so others know how long to back off • Used in 802.11 wireless LAN networks

11. DL: “Taking Turns” MAC protocols • Recall.... • Channel partitioning MAC protocols: • share channel efficiently at high load • inefficient at low load: delay in channel access, 1/N bandwidth allocated even if only 1 active node! • Random access MAC protocols • efficient at low load: single node can fully utilize channel • high load: collision overhead • “Taking Turns” protocols • Best of both worlds?

12. DL: “Taking Turns” MAC protocols Token passing: • control token passed from one node to next sequentially. • concerns: • token overhead • latency • single point of failure (token) Polling: • master node continuously “invites” slave nodes to transmit in turn • RTS, CTS messages • concerns: • polling overhead • latency • single point of failure (master)

13. DL: Taking-turns protocols Distributed Polling: • time divided into slots • begins with N short reservation slots • reservation slot time equal to channel end-end propagation delay • station with message to send posts reservation • reservation seen by all stations • after reservation slots, message transmissions ordered by known priority

14. DL: Media access protocols summary • Managing access to shared media • Channel Partitioning, by time, frequency or code • Time Division, Code Division, Frequency Division • Random partitioning (dynamic), • ALOHA, S-ALOHA, CSMA, CSMA/CD • carrier sensing: easy in some technoligies (wire), hard in others (wireless) • CSMA/CD used in Ethernet • Taking Turns • polling from a central cite, token passing

15. DL: Specific data-link layers and devices • Specific data-link layers • Ethernet (802.3) • Token Ring (802.5) • WiFi (802.11) • PPP • ATM • X.25 • Frame relay • Specific data-link layer devices • Hubs • Bridges • Switches

16. DL: Ethernet Metcalfe’s Ethernet sketch “Dominant” LAN technology: • First practical local area network, built at Xerox PARC in 70’s • Cheap: $3 for 100Mbs NIC • Simpler, cheaper than token LANs and ATM • Kept up with speed race: 10, 100, 1000 Mbps

17. DL: Ethernet's implementation of data-link layer • Digital to analog conversion (Manchester encoding) • Framing (special pre-amble within frame) • Physical addressing (6 byte hardware addresses) • Demux to upper protocol (type field in header) • Flow control (none) • Error detection and correction (CRC-32) • Reliable delivery (none) • Security (none) • Media access and quality of service (CSMA/CD with adaptive, randomized wait)

18. DL: Ethernet Frames • Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame • Original frame format circa 1970s by Xerox • Not used anymore • Ethernet II frame • Most common type • Other Ethernet frame formats • IEEE 802.3 LLC frame • IEEE 802.3 SNAP frame • Novell Proprietary • Not all compatible with each other

19. DL: Ethernet II Frame Structure • Preamble – 8 bytes • 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 • Used to synchronize receiver, sender clock rates • Src/Dst Address – 6 bytes • Globally unique, allocated to manufacturers • All adapters listening receive frame, discard if not destined for itself • Type/length – 2 bytes • If field < 1536, then the field indicates packet length • If field > 1536, it indicates higher layer protocol type (mostly IP) • How to determine frame boundaries? • IFS between frames with no transitions indicates frame boundaries • http://www.cavebear.com/CaveBear/Ethernet/type.html • Data – 46 to 1500 bytes • CRC – 4 bytes • Checked at receiver, dropped if doesn’t match • CRC-32 (x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1)

20. DL: Ethernet: uses CSMA/CD if packet then { A: sense channel if idle then { transmit and monitor the channel; if detect another transmission then { abort and send jam signal; update # collisions; delay as required by exponential backoff algorithm; goto A } else {done with the frame; set collisions to zero} } else {wait until ongoing transmission is over and goto A} }

21. DL: Ethernet CSMA/CD Packet? No Send Sense Carrier Detect Collision Yes Discard Packet b=CalcBackoff(); wait(b); attempts++; attempts < 16 attempts == 16

22. DL: Ethernet Backoff Calculation • If deterministic delay after collision, collision will occur again in lockstep • If random delay with fixed mean • Few senders  needless waiting • Too many senders  too many collisions • Exponentially increasing random delay • Infer senders from # of collisions • More senders  increase wait time

23. DL: Ethernet’s CSMA/CD (more) Exponential Backoff: • Goal: adapt retransmission attempts to estimated current load • heavy load: random wait will be longer • first collision: choose K from {0,1}; delay is K x 512 bit transmission times • after second collision: choose K from {0,1,2,3}… • after ten or more collisions, choose K from {0,1,2,3,4,…,1023}

24. DL: Ethernet CSMA/CD and Packet Size • What if two people sent really small packets • How do you find collision? • Must have a minimum packet size

25. DL: Ethernet Collision Detect & Packet Size • Min packet length > 2x max prop delay • If A, B are at opposite sides of link, and B starts one link prop delay after A • Jam signal • Jam network for 32-48 bits after collision, then stop sending • Ensures that everyone notices collision

26. DL: Propagation delay & packet size • Propagation delay determines min. packet size to prevent undetected collisions • Modern 10Mb Ethernet • Minimum packet size calculation • 500m maximum segment length • Can add repeaters up to a maximum 5 segments (2500m) • c in cable = 60% * c in vacuum = 1.8 x 10^8 m/s • ~ 12.5us one-way delay • Add repeater and tranceiver delay • To be safe IEEE specifies a 512 “bit-time” slot for Ethernet = 51.2us • 512 bits = 64 bytes (data payload = 46 bytes)

27. DL: Minimum packet size • What about scaling? 100Mbit, 1Gbit... • Make network smaller? • Solution for 100BaseT • Make min pkt size larger? • 512bits @ 1Gbps = 512ns • 512ns * 1.8 * 10^8 = 92meters • Gigabit ethernet uses collision extension for small pkts

28. DL: Ethernet Problems • Ethernet unstable at high loads • Peak utilization = 1/e = 37% • Peak throughput worse with • More hosts – more collisions needed to identify single sender • Smaller packet sizes – more frequent arbitration • Longer links – collisions take longer to observe, more wasted bandwidth

29. DL: 10Base2 Ethernet • Sifting through the jargon (10Base2) • 10: 10Mbps; 2: under 200 meters max cable length • thin coaxial cable in a bus topology • repeaters used to connect up to multiple segments • repeater repeats bits it hears on one interface to its other interfaces: physical layer device only!

30. DL: 10BaseT and 100BaseT Ethernet • 10/100 Mbps rate; latter called “fast ethernet” • T stands for Twisted Pair cabling • Nodes connected to hubs or switches in a “star topology” • Max distance from node to Hub is 100 meters based on electrical properties of TP • Smart hubs • Disconnect “jabbering adapter” versus 10Base2

31. DL: Gbit Ethernet • Use standard Ethernet frame format • Allows for point-to-point links and shared broadcast channels • In shared mode, CSMA/CD is used • shhort distances required to be efficient • Full-Duplex at 1 Gbps for point-to-point links

32. DL: Token Rings • Packets broadcast around ring • Token “right to send” rotates around ring • Fair, real-time bandwidth allocation • Every host holds token for limited time • Higher latency when only one sender • Higher bandwidth • Point to point links electrically simpler than bus

33. DL: Token Passing: IEEE802.5 standard • 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 • 4 Mbps • max token holding time: 10 ms (limits frame length)

34. DL: Why Did Ethernet Win? • Better failure modes • Token rings – network unusable • Ethernet – node detached • Good performance in common case • Volume  lower cost  higher volume …. • Adaptable • To higher bandwidths (vs. FDDI) • To switching (vs. ATM) • Completely distributed, easy to maintain/administer • Easy incremental deployment • Cheap cabling, etc

35. DL: IEEE 802.11 Wireless LAN • Wireless LANs: untethered (often mobile) networking • IEEE 802.11 standard: • Defines specific implementations of data-link functions • Framing, error detection, MAC, etc. • Unlicensed frequency spectrum: 900Mhz, 2.4Ghz • Organized into cells called Basic Service Sets • Wireless hosts • Access Point (base station) • Combined to form distribution system

36. DL: Ad Hoc Networks • Ad hoc network: IEEE 802.11 stations can dynamically form network without AP • Applications: • “laptop” meeting in conference room, car • interconnection of “personal” devices • battlefield • IETF MANET (Mobile Ad hoc Networks) working group

37. IEEE 802.11 MAC • Allows for several modes • CSMA (with explicit ACK to indicate collision) • CSMA/CA: reservations • Polling from AP

38. DL: IEEE 802.11 MAC Protocol: CSMA 802.11 CSMA sender - if sense channel idle for DIFS sec. then transmit entire frame (no collision detection) -ifsense channel busy then backoff (random, exponential) 802.11 CSMA receiver if received OK return ACK after SIFS 802.11 CSMA others • NAV: Network Allocation Vector • 802.11 frame has transmission time field • others (hearing data) defer access for NAV time units SIFS and DIFS • Used to separate frames and prioritize them • SIFS < DIFS allows acks to grab channel with higher priority

39. DL: IEEE 802.11 MAC Protocol CSMA/CA • Same as previous mode but with explicit channel reservation • Send short reservation messages via CSMA to reserve channel • Sender RTS (request to send), Receiver CTS (clear to send) • CTS notifies all hidden stations of sender's reservation • Short messages so that collision less likely and of short duration • Send data unobstructed on reserved channel • End result similar to CSMA/CD

40. DL: Point to Point Data Link Control • Point-to-point links • One sender, one receiver, one link • Easier than shared broadcast links • No media access control • No need for explicit MAC addressing (ie ARP) • Goal of Point-to-Point protocols • Layer generic “higher-level” data-link layer functions on top of a variety of point-to-point links • Dial-up phone line, DSL, ISDN etc. • Each different link does its own digital-analog conversion (ie provides bits) • Implement common pseudo-link layer on top that implements common functions • Framing, Demux to upper layer, etc. • Examples • PPP (point-to-point protocol) • HDLC: High level data link control (Data link used to be considered “high layer” in protocol stack!)

41. DL: PPP functions • http://www.rfc-editor.org/rfc/rfc1548.txt • 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 • 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

42. DL: PPP non-requirements Error recovery, flow control, data re-ordering all relegated to higher layers! • no error correction/recovery • no flow control • out of order delivery OK • no need to support multipoint links

43. DL: 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, PPP-LCP, IP, IPCP, etc) • info: upper layer data being carried • check: cyclic redundancy check for error detection

44. DL: Byte stuffing in PPP • “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

45. DL: Byte Stuffing flag byte pattern in data to send flag byte pattern plus stuffed byte in transmitted data

46. DL: 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

47. DL/NL: Asynchronous Transfer Mode (ATM) • 1980s/1990’s standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architecture • Take strengths of IP, learn from its shortcomings • Packet switching good • Packet switching without explicit network-level connections and reservations bad • Packet switching using large headers for small packets bad (voice) • Design new network to address emerging applications while allowing for efficient support for non-real-time data applications • Goal:integrated, end-end transport of carry voice, video, data • meeting timing/QoS requirements of voice, video (versus Internet best-effort model) • “next generation” telephony: technical roots in telephone world • packet-switching (fixed length packets, called “cells”) using virtual circuits • Covered now since it is used mostly as a data-link layer

48. DL/NL: ATM architecture (whole 9 yards) • adaptation layer: only at edge of ATM network • data segmentation/reassembly • roughly analagous to Internet transport layer • ATM layer: “network” layer • cell switching, routing • physical layer

49. DL/NL: ATM Adaptation Layer (AAL) • ATM Adaptation Layer (AAL): “adapts” upper layers (IP or native ATM applications) to ATM layer below • AAL present only in end systems, not in switches • Segment (header/trailer fields, data) fragmented across multiple ATM cells • Analogy: TCP segment in many IP packets

50. DL/NL: ATM Adaption Layer (AAL) [more] User data AAL PDU ATM cell Different versions of AAL layers, depending on ATM service class: • AAL1: for CBR (Constant Bit Rate) services, e.g. circuit emulation • AAL2: for VBR (Variable Bit Rate) services, e.g., MPEG video • AAL5: for data (eg, IP datagrams)