Ethernet Technologies: 10Base2

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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

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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

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

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Interconnecting LAN segments

• Hubs
• Bridges
• Switches
• Remark switches are essentially high performance
  multi-interface bridges.
• What we say about bridges also holds for switches!

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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
• Cant interconnect 10BaseT 100BaseT

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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

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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

LAN
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Forwarding

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

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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 senders 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

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Filtering/Forwarding

• When bridge receives a frame
• index bridge table using destination MAC address
• if entry found for destinationthen
• 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
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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

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Bridge Learning example
C 1

• 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

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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

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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

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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

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

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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

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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

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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

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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
Store-and-forward sends after CRC
Cut-though forwards after DA
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An Example LAN
Dedicated
Shared
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Summary comparison
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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