DataLink Layer 1

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

DataLink Layer 2

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

DataLink Layer 3

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

DataLink Layer 4

Interconnecting LAN segments

• Hubs• Bridges• Switches– Remark: switches are essentially high

performance multi-interface bridges.– What we say about bridges also holds for

switches!

DataLink Layer 5

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

DataLink Layer 6

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

DataLink Layer 7

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 LAN segment

DataLink Layer 8

Forwarding

How do determine to which LAN segment to forward frame?• Looks like a routing problem...

DataLink Layer 9

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

DataLink Layer 10

Filtering/ForwardingWhen bridge receives a frame:

index bridge table using destination MAC addressif 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

DataLink Layer 11

Bridge example

Suppose C sends a frame to D and D replies back with a frame to C.

Bridge receives frame from C notes in bridge table that C is on interface 1 because D is not in table, bridge sends frame into interfaces 2 and

3 frame received by D

DataLink Layer 12

Bridge Learning: example

D generates frame for C, sends bridge receives frame

notes in bridge table that D is on interface 2 bridge knows C is on interface 1, so forwards frame to interface 1

C 1

DataLink Layer 13

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

DataLink Layer 14

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

DataLink Layer 15

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

DataLink Layer 16

Backbone configuration

Recommended !With a backbone, each pair of LAN segments can communicate without passing through a third-party LAN segment

DataLink Layer 17

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

DataLink Layer 18

Routers vs. Bridges

Bridges + and - + Bridges are plug-and-play+ Bridge operation is simpler requiring less packet

processinghigh 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

DataLink Layer 19

Routers vs. BridgesRouters + 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

DataLink Layer 20

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

DataLink Layer 21

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

Preamble DA SA type Data CRC

Cut-though forwards after DA

Store-and-forward sends after CRC

DataLink Layer 22

An Example LANDedicated

Shared

DataLink Layer 23

Summary comparison

hubs bridges routers switches

traffi cisolation

no yes yes yes

plug & play yes yes no yes

optimalrouting

no no yes no

cutthrough

yes no no yes

DataLink Layer 24

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