1. 5: DataLink Layer5c-1 Today r Assign Homework m Ch5 #1,4,5,7,11,12 Due Wednesday October 22 m Ch5 #13-16,18,20 Due Monday, October 27 r Project #2 due this Monday Oct 20! r Opportunities for undergraduate research and employment r Exam #2 rescheduled: m Now Friday, Oct 31 (covering Ch4-5) r Continue with Chapter 5

2. 5: DataLink Layer5c-2 Ch4 True/False Quiz 1) When employing virtual-circuits, packet switches are involved in virtual circuit setup, and each packet switch is aware of the VCs passing through it. 2) In RPF, a node will receive multiple copies of the same packet. 3) In the BGP routing algorithm, each AS advertises to its neighbors its estimates of the shortest distances from the AS to all possible destination ASs. 4) Suppose your computer has been configured with an IP address, and you move (along with your computer) to an office down the hall. If afterwards your computer is connected to the same IP net, then it is not necessary to reconfigure the IP address in your computer. 5) Single-homed hosts have one interface and routers typically have two or more interfaces.

3. 5: DataLink Layer5c-3 Chapter 5 outline r 5.1 Introduction and services r 5.2 Error detection and correction r 5.3Multiple access protocols r 5.4 LAN addresses and ARP r 5.5 Ethernet r 5.6 Hubs, bridges, and switches r 5.7 Wireless links and LANs r 5.8 PPP r 5.9 ATM r 5.10 Frame Relay

4. 5: DataLink Layer5c-4 Interconnecting LAN segments r Hubs r Bridges r Switches m Remark: switches are essentially multi-port bridges. m What we say about bridges also holds for switches!

5. 5: DataLink Layer5c-5 Interconnecting with hubs r Backbone hub interconnects LAN segments r Extends max distance between nodes r But individual segment collision domains become one large collision domain m if a node in CS and a node EE transmit at same time: collision r Can’t interconnect 10BaseT & 100BaseT with hubs

6. 5: DataLink Layer5c-6 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 r transparent m hosts are unaware of presence of bridges r plug-and-play, self-learning m bridges do not need to be configured

7. 5: DataLink Layer5c-7 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

8. 5: DataLink Layer5c-8 Forwarding How do determine to which LAN segment to forward frame? Looks like a routing problem...

9. 5: DataLink Layer5c-9 Self learning r A 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

10. 5: DataLink Layer5c-10 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

11. 5: DataLink Layer5c-11 Bridge example Suppose C sends frame to D and D replies back with frame to C. r Bridge receives frame from from C m notes in bridge table that C is on interface 1 m because D is not in table, bridge sends frame into interfaces 2 and 3 r frame received by D

12. 5: DataLink Layer5c-12 Bridge Learning: example r D generates frame for C, sends r bridge receives frame m notes in bridge table that D is on interface 2 m bridge knows C is on interface 1, so selectively forwards frame to interface 1

13. 5: DataLink Layer5c-13 Interconnection without backbone r Not recommended for two reasons: - single point of failure at Computer Science hub - all traffic between EE and SE must path over CS segment

14. 5: DataLink Layer5c-14 Backbone configuration Recommended !

15. 5: DataLink Layer5c-15 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

16. 5: DataLink Layer5c-16 Some bridge features r Isolates collision domains resulting in higher total max throughput r Limitless number of nodes and geographical coverage r Can connect different Ethernet types r Transparent (“plug-and-play”): no configuration necessary

17. 5: DataLink Layer5c-17 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

18. 5: DataLink Layer5c-18 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

19. 5: DataLink Layer5c-19 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)

20. 5: DataLink Layer5c-20 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!

21. 5: DataLink Layer5c-21 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 (e.g., full-duplex), 10/100/1000 Mbps interfaces

22. 5: DataLink Layer5c-22 Typical LAN (IP network) Dedicated Shared

23. 5: DataLink Layer5c-23 Summary comparison

24. 5: DataLink Layer5c-24 Chapter 5 outline r 5.1 Introduction and services r 5.2 Error detection and correction r 5.3Multiple access protocols r 5.4 LAN addresses and ARP r 5.5 Ethernet r 5.6 Hubs, bridges, and switches r 5.7 Wireless links and LANs r 5.8 PPP r 5.9 ATM r 5.10 Frame Relay

25. 5: DataLink Layer5c-25 IEEE 802.11 Wireless LAN r 802.11b m 2.4-5 GHz unlicensed radio spectrum m up to 11 Mbps m direct sequence spread spectrum (DSSS) in physical layer all hosts use same chipping code 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

26. 5: DataLink Layer5c-26 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 BSSs combined to form distribution system (DS)

27. 5: DataLink Layer5c-27 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

28. 5: DataLink Layer5c-28 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: hidden terminal problem

29. 5: DataLink Layer5c-29 IEEE 802.11 MAC Protocol: CSMA/CA 802.11 CSMA: sender - if sense channel idle for DIFS 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)

30. 5: DataLink Layer5c-30 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)