1. 5: DataLink Layer5a-1 Interconnecting LANs Q: Why not just one big LAN? r Limited amount of supportable traffic: on single LAN, all stations must share bandwidth r limited length: 802.3 specifies maximum cable length r large “collision domain” (can collide with many stations) r limited number of stations: 802.5 have token passing delays at each station

2. 5: DataLink Layer5a-2 Hubs r Physical Layer devices: essentially repeaters operating at bit levels: repeat received bits on one interface to all other interfaces r Hubs can be arranged in a hierarchy (or multi-tier design), with backbone hub at its top

3. 5: DataLink Layer5a-3 Hubs (more) r Each connected LAN referred to as LAN segment r Hubs do not isolate collision domains: segments form a large collision domain m if a node in CS and a node EE transmit at same time: collision r Hub Advantages: m simple, inexpensive device m Multi-tier provides graceful degradation: portions of the LAN continue to operate if one hub malfunctions m extends maximum distance between node pairs (100m per Hub)

4. 5: DataLink Layer5a-4 Hub limitations r single collision domain results in no increase in max throughput m multi-tier throughput same as single segment throughput r individual LAN restrictions pose limits on number of nodes in same collision domain and on total allowed geographical coverage r cannot connect different Ethernet types (e.g., 10BaseT and 100baseT)

5. 5: DataLink Layer5a-5 Bridges r Link layer device m stores and forwards Ethernet frames m examines frame header and selectively forwards frame based on MAC dest address m when frame is to be forwarded on segment, uses CSMA/CD to access segment m can connect different type Ethernet r transparent m hosts are unaware of presence of bridges r plug-and-play, self-learning m bridges do not need to be configured

6. 5: DataLink Layer5a-6 Bridges: traffic isolation r Bridge installation breaks LAN into LAN segments r bridges filter packets: m same-LAN-segment frames not usually forwarded onto other LAN segments m segments become separate collision domains bridge collision domain collision domain = hub = host LAN (IP network) LAN segment

7. 5: DataLink Layer5a-7 Forwarding How do determine to which LAN segment to forward frame? Looks like a routing problem...

8. 5: DataLink Layer5a-8 Self learning r bridge has a bridge table r entry in bridge table: m (Node LAN Address, Bridge Interface, Time Stamp) m stale entries in table dropped (TTL can be 60 min) r bridges learn which hosts can be reached through which interfaces m when frame received, bridge “learns” location of sender: incoming LAN segment m records sender/location pair in bridge table

9. 5: DataLink Layer5a-9 Filtering/Forwarding When bridge receives a frame: index bridge table using MAC dest address if entry found for destination then { if dest on segment from which frame arrived then drop the frame else forward the frame on interface indicated } else flood forward on all but the interface on which the frame arrived

10. 5: DataLink Layer5a-10 Bridge example Scenario: r C sends frame to D r D replies back with frame to C bridge ABCHIJDEF address port AHIFAHIF 12231223 1 2 3 C 1 D 3 Bridge Table

11. 5: DataLink Layer5a-11 Interconnection without backbone r Not recommended for two reasons: - single point of failure at Computer Science hub - all traffic between EE and SE must pass over CS segment

12. 5: DataLink Layer5a-12 Backbone configuration Recommended !

13. 5: DataLink Layer5a-13 Bridges Spanning Tree r for increased reliability, desirable to have redundant, alternative paths from source to dest r with multiple paths, cycles result - bridges may multiply and forward frame forever r solution: organize bridges in a spanning tree by disabling subset of interfaces Disabled

14. 5: DataLink Layer5a-14 Bridges vs. Routers r both store-and-forward devices m routers: network layer devices (examine network layer headers) m bridges are link layer devices r routers maintain routing tables, implement routing algorithms r bridges maintain bridge tables, implement filtering, learning and spanning tree algorithms

15. 5: DataLink Layer5a-15 Routers vs. Bridges Bridges + and - + Bridge operation is simpler requiring less packet processing + Bridge tables are self learning - All traffic confined to spanning tree, even when alternative bandwidth is available - Bridges do not offer protection from broadcast storms

16. 5: DataLink Layer5a-16 Routers vs. Bridges Routers + and - + arbitrary topologies can be supported, cycling is limited by TTL counters (and good routing protocols) + provide protection against broadcast storms - require IP address configuration (not plug and play) - require higher packet processing r bridges do well in small (few hundred hosts) while routers used in large networks (thousands of hosts)

17. 5: DataLink Layer5a-17 Ethernet Switches r Essentially a multi- interface bridge r layer 2 (frame) forwarding, filtering using LAN addresses r Switching: A-to-A’ and B- to-B’ simultaneously, no collisions r large number of interfaces r often: individual hosts, star-connected into switch m Ethernet, but no collisions!

18. 5: DataLink Layer5a-18 Ethernet Switches r cut-through switching: frame forwarded from input to output port without awaiting for assembly of entire frame m slight reduction in latency r combinations of shared/dedicated, 10/100/1000 Mbps interfaces

19. 5: DataLink Layer5a-19 Not an atypical LAN (IP network) Dedicated Shared

20. 5: DataLink Layer5a-20 Summary comparison

21. 5: DataLink Layer5a-21 IEEE 802.11 Wireless LAN r 802.11b m 2.4-5 GHz unlicensed radio spectrum m up to 11 Mbps m widely deployed, using base stations r 802.11a m 5-6 GHz range m up to 54 Mbps r 802.11g m 2.4-5 GHz range m up to 54 Mbps r All use CSMA/CA for multiple access r All have base-station and ad-hoc network versions

22. 5: DataLink Layer5a-22 Base station approach r Wireless host communicates with a base station m base station = access point (AP) r Basic Service Set (BSS) (a.k.a. “cell”) contains: m wireless hosts m access point (AP): base station r BSS’s combined to form distribution system (DS)

23. 5: DataLink Layer5a-23 Ad Hoc Network approach r No AP (i.e., base station) r wireless hosts communicate with each other m to get packet from wireless host A to B may need to route through wireless hosts X,Y,Z r Applications: m “laptop” meeting in conference room, car m interconnection of “personal” devices m battlefield r IETF MANET (Mobile Ad hoc Networks) working group

24. 5: DataLink Layer5a-24 IEEE 802.11: multiple access r Collision if 2 or more nodes transmit at same time r CSMA makes sense: m get all the bandwidth if you’re the only one transmitting m shouldn’t cause a collision if you sense another transmission r Collision detection doesn’t work: m hidden terminal problem

25. 5: DataLink Layer5a-25 Hidden terminal problem r hidden terminals: A, C cannot hear each other m obstacles, signal attenuation m collisions at B r goal: avoid collisions at B r CSMA/CA: CSMA with Collision Avoidance

26. 5: DataLink Layer5a-26 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) -if sense channel busy then binary backoff 802.11 CSMA receiver - if received OK return ACK after SIFS (ACK is needed due to hidden terminal problem)

27. 5: DataLink Layer5a-27 Collision avoidance mechanisms r Problem: m two nodes, hidden from each other, transmit complete frames to base station m wasted bandwidth for long duration ! r Solution: m small reservation packets m nodes track reservation interval with internal “network allocation vector” (NAV)

28. 5: DataLink Layer5a-28 Collision Avoidance: RTS-CTS exchange r sender transmits short RTS (request to send) packet: indicates duration of transmission r receiver replies with short CTS (clear to send) packet m notifying (possibly hidden) nodes r hidden nodes will not transmit for specified duration: NAV

29. 5: DataLink Layer5a-29 Collision Avoidance: RTS-CTS exchange r RTS and CTS short: m collisions less likely, of shorter duration m end result similar to collision detection r IEEE 802.11 allows: m CSMA m CSMA/CA: reservations m polling from AP

30. 5: DataLink Layer5a-30 Point to Point Data Link Control r one sender, one receiver, one link: easier than broadcast link: m no Media Access Control m no need for explicit MAC addressing m e.g., dialup link, ISDN line r popular point-to-point DLC protocols: m PPP (point-to-point protocol) m HDLC: High-level data link control (Data link used to be considered “high layer” in protocol stack!)

31. 5: DataLink Layer5a-31 PPP Design Requirements [RFC 1557] r packet framing: encapsulation of network-layer datagram in data link frame m carry network layer data of any network layer protocol (not just IP) at same time m ability to demultiplex upwards r bit transparency: must carry any bit pattern in the data field r error detection (no correction) r connection liveness: detect, signal link failure to network layer r network layer address negotiation: endpoint can learn/configure each other’s network address

32. 5: DataLink Layer5a-32 PPP non-requirements r no error correction/recovery r no flow control r out of order delivery OK r no need to support multipoint links (e.g., polling) Error recovery, flow control, data re-ordering all relegated to higher layers!

33. 5: DataLink Layer5a-33 PPP Data Frame r Flag: delimiter (framing) r Address: does nothing (only one option) r Control: does nothing; in the future possible multiple control fields r Protocol: upper layer protocol to which frame delivered (eg, PPP-LCP, IP, IPCP, etc)

34. 5: DataLink Layer5a-34 PPP Data Frame r info: upper layer data being carried r check: cyclic redundancy check for error detection

35. 5: DataLink Layer5a-35 Byte Stuffing r “data transparency” requirement: data field must be allowed to include flag pattern m Q: is received data or flag? r Sender: adds (“stuffs”) extra byte after each data byte r Receiver: m two 01111110 bytes in a row: discard first byte, continue data reception m single 01111110: flag byte

36. 5: DataLink Layer5a-36 Byte Stuffing flag byte pattern in data to send flag byte pattern plus stuffed byte in transmitted data

37. 5: DataLink Layer5a-37 PPP Data Control Protocol Before exchanging network- layer data, data link peers must r configure PPP link (max. frame length, authentication) r learn/configure network layer information m for IP: carry IP Control Protocol (IPCP) msgs (protocol field: 8021) to configure/learn IP address

38. 5: DataLink Layer5a-38 Asynchronous Transfer Mode: ATM r 1990’s/00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architecture r Goal: integrated, end-end transport of carry voice, video, data m meeting timing/QoS requirements of voice, video (versus Internet best-effort model) m “next generation” telephony: technical roots in telephone world m packet-switching (fixed length packets, called “cells”) using virtual circuits

39. 5: DataLink Layer5a-39 ATM architecture r adaptation layer: only at edge of ATM network m data segmentation/reassembly m roughly analagous to Internet transport layer r ATM layer: “network” layer m cell switching, routing r physical layer

40. 5: DataLink Layer5a-40 ATM: network or link layer? Vision: end-to-end transport: “ATM from desktop to desktop” m ATM is a network technology Reality: used to connect IP backbone routers m “IP over ATM” m ATM as switched link layer, connecting IP routers

41. 5: DataLink Layer5a-41 ATM Adaptation Layer (AAL) r ATM Adaptation Layer (AAL): “adapts” upper layers (IP or native ATM applications) to ATM layer below r AAL present only in end systems, not in switches r AAL layer segment (header/trailer fields, data) fragmented across multiple ATM cells m analogy: TCP segment in many IP packets

42. 5: DataLink Layer5a-42 ATM Adaptation Layer (AAL) [more] Different versions of AAL layers, depending on ATM service class: r AAL1: for CBR (Constant Bit Rate) services, e.g. circuit emulation r AAL2: for VBR (Variable Bit Rate) services, e.g., MPEG video r AAL5: for data (eg, IP datagrams) AAL PDU ATM cell User data

43. 5: DataLink Layer5a-43 AAL5 - Simple And Efficient AL (SEAL) r AAL5: low overhead AAL used to carry IP datagrams m 4 byte cyclic redundancy check m PAD ensures payload multiple of 48bytes m large AAL5 data unit to be fragmented into 48- byte ATM cells

44. 5: DataLink Layer5a-44 ATM Layer Service: transport cells across ATM network r analagous to IP network layer r very different services than IP network layer Network Architecture Internet ATM Service Model best effort CBR VBR ABR UBR Bandwidth none constant rate guaranteed rate guaranteed minimum none Loss no yes no Order no yes Timing no yes no Congestion feedback no (inferred via loss) no congestion no congestion yes no Guarantees ?

45. 5: DataLink Layer5a-45 ATM Layer: Virtual Circuits r VC transport: cells carried on VC from source to dest m call setup, teardown for each call before data can flow m each packet carries VC identifier (not destination ID) m every switch on source-dest path maintain “state” for each passing connection m link,switch resources (bandwidth, buffers) may be allocated to VC: to get circuit-like perf. r Permanent VCs (PVCs) m long lasting connections m typically: “permanent” route between to IP routers r Switched VCs (SVC): m dynamically set up on per-call basis

46. 5: DataLink Layer5a-46 ATM VCs r Advantages of ATM VC approach: m QoS performance guarantee for connection mapped to VC (bandwidth, delay, delay jitter) r Drawbacks of ATM VC approach: m Inefficient support of datagram traffic m one PVC between each source/dest pair) does not scale (N*2 connections needed) m SVC introduces call setup latency, processing overhead for short lived connections

47. 5: DataLink Layer5a-47 ATM Layer: ATM cell r 5-byte ATM cell header r 48-byte payload m Why?: small payload -> short cell-creation delay for digitized voice m halfway between 32 and 64 (compromise!) Cell header Cell format

48. 5: DataLink Layer5a-48 ATM cell header r VCI: virtual channel ID m will change from link to link thru net r PT: Payload type (e.g. RM cell versus data cell) r CLP: Cell Loss Priority bit m CLP = 1 implies low priority cell, can be discarded if congestion r HEC: Header Error Checksum m cyclic redundancy check

49. 5: DataLink Layer5a-49 ATM Physical Layer Two pieces (sublayers) of physical layer: r Transmission Convergence Sublayer (TCS): adapts ATM layer above to PMD sublayer below r Physical Medium Dependent: depends on physical medium being used TCS Functions: m Header checksum generation: 8 bits CRC m Cell delineation m With “unstructured” PMD sublayer, transmission of idle cells when no data cells to send

50. 5: DataLink Layer5a-50 ATM Physical Layer (more) Physical Medium Dependent (PMD) sublayer r SONET/SDH: transmission frame structure (like a container carrying bits); m bit synchronization; m bandwidth partitions (TDM); m several speeds: OC3 = 155.52 Mbps; OC12 = 622.08 Mbps; OC48 = 2.45 Gbps, OC192 = 9.6 Gbps r TI/T3: transmission frame structure (old telephone hierarchy): 1.5 Mbps/ 45 Mbps r unstructured: just cells (busy/idle)

51. 5: DataLink Layer5a-51 IP-Over-ATM Classic IP only r 3 “networks” (e.g., LAN segments) r MAC (802.3) and IP addresses IP over ATM r replace “network” (e.g., LAN segment) with ATM network r ATM addresses, IP addresses ATM network Ethernet LANs Ethernet LANs

52. 5: DataLink Layer5a-52 IP-Over-ATM Issues: r IP datagrams into ATM AAL5 PDUs r from IP addresses to ATM addresses m just like IP addresses to 802.3 MAC addresses! ATM network Ethernet LANs

53. 5: DataLink Layer5a-53 Datagram Journey in IP-over-ATM Network r at Source Host: m IP layer maps between IP, ATM dest address (using ARP) m passes datagram to AAL5 m AAL5 encapsulates data, segments cells, passes to ATM layer r ATM network: moves cell along VC to destination r at Destination Host: m AAL5 reassembles cells into original datagram m if CRC OK, datagram is passed to IP

54. 5: DataLink Layer5a-54 ARP in ATM Nets r ATM network needs destination ATM address m just like Ethernet needs destination Ethernet address r IP/ATM address translation done by ATM ARP (Address Resolution Protocol) m ARP server in ATM network performs broadcast of ATM ARP translation request to all connected ATM devices m hosts can register their ATM addresses with server to avoid lookup

55. 5: DataLink Layer5a-55 X.25 and Frame Relay Like ATM: r wide area network technologies r Virtual-circuit oriented r origins in telephony world r can be used to carry IP datagrams m can thus be viewed as Link Layers by IP protocol

56. 5: DataLink Layer5a-56 X.25 r X.25 builds VC between source and destination for each user connection r Per-hop control along path m error control (with retransmissions) on each hop using LAP-B variant of the HDLC protocol m per-hop flow control using credits congestion arising at intermediate node propagates to previous node on path back to source via back pressure

57. 5: DataLink Layer5a-57 IP versus X.25 r X.25: reliable in-sequence end-end delivery from end-to-end m “intelligence in the network” r IP: unreliable, out-of-sequence end-end delivery m “intelligence in the endpoints” r gigabit routers: limited processing possible r 2000: IP wins

58. 5: DataLink Layer5a-58 Frame Relay r Designed in late ‘80s, widely deployed in the ‘90s r Frame relay service: m no error control m end-to-end congestion control

59. 5: DataLink Layer5a-59 Frame Relay (more) r Designed to interconnect corporate customer LANs m typically permanent VC’s: “pipe” carrying aggregate traffic between two routers m switched VC’s: as in ATM r corporate customer leases FR service from public Frame Relay network (eg, Sprint, ATT)

60. 5: DataLink Layer5a-60 Frame Relay (more) r Flag bits, 01111110, delimit frame r address: m 10 bit VC ID field m 3 congestion control bits FECN: forward explicit congestion notification (frame experienced congestion on path) BECN: congestion on reverse path DE: discard eligibility address flags data CRC flags

61. 5: DataLink Layer5a-61 Frame Relay -VC Rate Control r Committed Information Rate (CIR) m defined, “guaranteed” for each VC m negotiated at VC set up time m customer pays based on CIR r DE bit: Discard Eligibility bit m Edge FR switch measures traffic rate for each VC; marks DE bit m DE = 0: high priority, rate compliant frame; deliver at “all costs” m DE = 1: low priority, eligible for congestion discard

62. 5: DataLink Layer5a-62 Frame Relay - CIR & Frame Marking r Access Rate: rate R of the access link between source router (customer) and edge FR switch (provider); 64Kbps < R < 1,544Kbps r Typically, many VCs (one per destination router) multiplexed on the same access trunk; each VC has own CIR r Edge FR switch measures traffic rate for each VC; it marks (ie DE = 1) frames which exceed CIR (these may be later dropped) r Internet’s more recent differentiated service uses similar ideas

63. 5: DataLink Layer5a-63 Chapter 5: Summary r principles behind data link layer services: m error detection, correction m sharing a broadcast channel: multiple access m link layer addressing, ARP r link layer technologies: Ethernet, hubs, bridges, switches, IEEE 802.11 LANs, PPP, ATM, Frame Relay r Finished journey down the protocol stack m next stops: multimedia, security, network management