1. DataLink Layer1 Ethernet Technologies: 10Base2 10: 10Mbps; 2: 200 meters (actual is 185m) max distance between any two nodes without repeaters thin coaxial cable in a bus topology max 30 nodes in a segment max 4 repeaters, max network diameter 925m has become a legacy technology

2. DataLink Layer2 10BaseT and 100BaseT 10/100 Mbps rate; latter called “fast ethernet” T stands for Twisted Pair Nodes connect to a hub: “star topology”; 100 m max distance between nodes and hub Hubs are essentially physical-layer repeaters: – bits coming in one link go out all other links – no frame buffering – no CSMA/CD at hub: adapters speak CSMA/CD – provides network management functionality hub nodes

3. DataLink Layer3 Gigabit Ethernet (IEEE 802.3z) 1Gbps data rate use standard Ethernet frame format star topology, allows for point-to-point links (use switches) and shared broadcast channels (use hubs) – Full-Duplex at 1 Gbps for point-to-point links – in shared mode, CSMA/CD is used

4. DataLink Layer4 Interconnecting LAN segments Hubs Bridges Switches – Remark: switches are essentially high performance multi-interface bridges. – What we say about bridges also holds for switches!

5. DataLink Layer5 Interconnecting with hubs Hubs are physical layer devices: operate on bits Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large collision domain – if a node in CS and a node in EE transmit at same time: collision Can’t interconnect 10BaseT & 100BaseT

6. DataLink Layer6 Bridges Link layer device: operate on frames – stores and forwards Ethernet frames – examines frame header and forwards frame based on destination MAC address – when frame is to be forwarded on a segment, uses CSMA/CD to access the segment – can interconnect different LAN technologies plug-and-play, self-learning – bridges do not need to be configured

7. DataLink Layer7 Bridges: traffic isolation Bridge installation breaks LAN into LAN segments bridges filter packets: – same-LAN-segment frames not usually forwarded onto other LAN segments – segments become separate collision domains bridge collision domain collision domain = hub = host LAN LAN segment

8. DataLink Layer8 Forwarding How do determine to which LAN segment to forward frame? Looks like a routing problem...

9. DataLink Layer9 Self learning A bridge has a bridge table entry in bridge table: – (Node LAN Address, Bridge Interface, Time Stamp) – stale entries in table dropped (TTL typically 60 min) bridges learn which hosts can be reached through which interfaces – bridge table initially empty – when frame received, bridge “learns” location of sender – records sender’s LAN address, arriving interface, and current time in bridge table – delete an address in table if no frames are received with that address as the source address after some period of time

10. DataLink Layer10 Filtering/Forwarding When bridge receives a frame: index bridge table using destination MAC 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. DataLink Layer11 Bridge example Suppose C sends a frame to D and D replies back with a frame to C. r Bridge receives frame 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. DataLink Layer12 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 forwards frame to interface 1 C 1

13. DataLink Layer13 Spanning Tree for increased reliability, desirable to have redundant, alternative paths from source to dest with multiple paths, cycles result - bridges may multiply and forward frame forever solution: bridges determine a spanning tree by disabling subset of interfaces Disabled

14. DataLink Layer14 Some bridge features isolates collision domains resulting in higher total max throughput limitless number of nodes and geographical coverage can connect different Ethernet types transparent (“plug-and-play”): no configuration necessary

15. DataLink Layer15 Interconnection without backbone Not recommended for two reasons: - single point of failure at Computer Science hub - all traffic between EE and SE must pass through CS segment

16. DataLink Layer16 Backbone configuration Recommended ! With a backbone, each pair of LAN segments can communicate without passing through a third-party LAN segment

17. DataLink Layer17 Bridges vs. Routers both store-and-forward devices – routers: network layer devices (forward packets using network layer addresses) – bridges: link layer devices (forward packets using LAN addresses) routers maintain routing tables, implement routing algorithms bridges maintain bridge tables, implement learning and spanning tree algorithms

18. DataLink Layer18 Routers vs. Bridges Bridges + and - + Bridges are plug-and-play + Bridge operation is simpler requiring less packet processing  high packet filtering and forwarding rates - All traffic confined to spanning tree, even when alternative paths are available - Bridges do not offer protection from broadcast storms

19. DataLink Layer19 Routers vs. Bridges Routers + and - + arbitrary topologies can be supported, cycling is limited by TTL counters (and good routing protocols)  packets can use the best path + provide protection against broadcast storms - require IP address configuration (not plug and play) - require larger packet processing time bridges do well in small (few hundred hosts) networks while routers used in large (thousands of hosts) networks

20. DataLink Layer20 Ethernet Switches Essentially high-performance multi- interface bridges – switches have large number of interfaces layer 2 (frame) forwarding, filtering using LAN addresses automatically build forwarding tables often: individual hosts star-connected into switch – Switching: A-to-A’, B-to-B’, and C-to-C’ simultaneously – Full duplex, no collisions! combinations of 10/100/1000 Mbps interfaces

21. DataLink Layer21 Ethernet Switches cut-through switching: frame forwarded from input to output port without waiting for assembly of entire frame – slight reduction in latency – store-and-forward and cut-through switching differ only when the output buffer becomes empty before the entire packet has arrived PreambleDASAtypeDataCRC Cut-though forwards after DA Store-and-forward sends after CRC

22. DataLink Layer22 An Example LAN Dedicated Shared

23. DataLink Layer23 Summary comparison

24. DataLink Layer24 IEEE 802.11 Wireless LANs 802.11b – operate at 2.4 GHz, 11 Mbps – widely deployed 802.11a – 5-6 GHz range – up to 54 Mbps 802.11g – 2.4 GHz – up to 54 Mbps All have base-station and ad-hoc network versions All use CSMA/CA for multiple access