CS3505: Loca Area Networks CSMA/CD : IEEE 802.3 and LAN Internetworks

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CS3505Loca Area Networks CSMA/CD IEEE
802.3andLAN Internetworks

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CSMA/CD

• basic LAN architecture and protocol
• most widespread LAN estimate more that 50 of
  all LANs on the Internet... implemented by many
  different companies
• developed by Robert Metcalfe, XEROX PARC, early
  1970s..... led to founding of 3COM company,
  (Santa Clara). later Metcalfe sold his company
  for 400M)

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CSMA / CD basic protocol

• broadcast medium
• first version used bus
• when the MAC receives a packet to transmit
• 1. sense carrier (listen)
• if no signal detected
• then begin Xmitting message, continue
  sensing
• if collision detected
• then Xmit jam, stop Xmitting, wait, then
  goto (1)
• when end-of-packet Xmitted, END.
• else carrier is busy go to (1)

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CSMA / CD basic protocol

• the wait random time is precisely defined
  binary exponential backoff
• physical encoding digital signals, manchester
  encoding. Broadcast medium.
• because collisions are detected, wasted time is
  short

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CSMA / CD backoff algorithm

• binary exponential backoff (BACKOFF)
• 1 slot 51.2 ?s
• time following collision measured in slots
• a random slot between 1 and 1023 is chosen, and
  the packet is retransmitted then

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CSMA / CD backoff algorithm

• example suppose 2 stations collide. What
  happens?
• what is Prob another collision?
• why measure time in 51.2 ?s slots?

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CSMA / CD frame format

• length 64 to 1518 bytes
• preamble 7 bytes SOF 1 byte
• DA,SA 2/6 bytes length 2 bytes
• data 0-1500 pad 0-46 FCS 4

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CSMA / CD topologies, media , etc.

• transmission media
• coaxial cable (decreasing)
• twisted pair (Cat 5, widely used)
• fiber (less common but increasing)
• topologies
• bus - original design used for many years
• star with dumb or smart hub now usual
• data rates
• 10 Mbps
• 100 Mbps
• Gbps where needed available

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CSMA / CD network components

• medium (coax, tp, fiber)
• transceivers
• drop cable station to coax
• NIC (MAC protocol logic)
• repeaters (needed to extend coax)
• test equipment
• hub (multiport repeater) for star configuration
• bridges (to connect to other LANs)

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CSMA / CD physical layout

• coax. cable, physical(and logical) bus CSMA/CD

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CSMA / CD physical layout

• star/hub configuration. physical star, logical
  bus
• 2-twisted pair connections hub is a repeater

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CSMA / CD some specifics

• prop speed 0.77 c on coax, 0.59 c on t.p.
• at most 4 repeaters between 2 stations so at
  most 5 cable segments
• 500 m /segment max, or 2000 with repeaters
  (coax)
• drop cables 25 m max
• max 100 stations per segment on coax
• at least 2.5 m between adjacent receivers on coax
• at most 1024 stations per ethernet

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CSMA / CD standard s

• IEEE 802.3 several physical configurations
• 10BASE5 baseband coaxial cable original
• 10BASE2 thin coaxial cable cheaper alternative
• 10BASE-T twisted pair, hub configuration
• 10BROAD36 uses broadband coax (TV cable)
• 10BASE-F fiber
• 100BASE-X fiber OR twisted pair
• NOTE all use the same frame format and basic MAC
  protocol

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CSMA / CD practical considerations

• you have 3 PCs in your house what will it take
  (equipment/dollars) to connect them together ?
• design or plan a network for a small business
  with 30-40 machines (PCs, Macs, etc), which is
  located in a single building. (eqmt, dollars)

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CS3505Bridges / LAN internetworks

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Bridges connecting LANs together

• why do we need to connect LANs
• what is a bridge?
• types of bridges
• routing in LAN internetworks
• comparison bridges, routers, repeaters
• connecting similar LANs
• connecting dissimilar LANs

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why LANs need to be connected

• 1. connect 2 existing LANs (CS, math)
• 2. LAN too big split it, but stay connected
• -- too many stations or traffic for one LAN
• 3. connect geographically separate LANs.
• -- 2 offices in different towns
• 4. reduce collisions
• --increase efficiency
• 5. security
• --help restrict traffic to one LAN

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bridge what is it?

• low level switch that connects two or more
  LANs. low level gt MAC layer
• transparent there is no change in the LANs or
  in the protocols of the networks
• able to do simple routing
• retains the simplicity and flexibility of LANs
• faster than software switches (routers)
• reasonable cost cheaper than routers

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bridge or switch? terminology

• 1st bridges sold in 1984, were 2-ports
• early 1990s, multiport bridges appeared were
  called switches by marketing vendors
  technically no difference between a switch
  (layer 2) and a bridge
• multiple LANs connected by high port density
  bridges commonly called switched LANs -
  actually an internet of LANs
• switching hub, LAN switch - other terms for a
  multiport bridge

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bridges LAN connectors types

• local bridge
• remote (2 half bridges)
• same LAN, different LAN
• two port, multiport
• hub (not a bridge)
• repeater (not a bridge)
• router(not a bridge)

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bridge basic function

• suppose a bridge B connects networks X Y.
• Then B
• 1. reads all packets on X and Y, noting the
  destination, source addresses (DA, SA)
• 2. each packet on Y with DA on X is copied and
  transmitted on X.
• 3. each packet on X with DA on Y is copied,
  transmitted on Y.
• the bridge operates on X, Y using the MAC
  protocol of those LANs.

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bridge connecting 2 ethernets
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notes on bridges

• all stations have unique MAC addresses
• bridge must know which LAN station is on
• multi-port bridges - similar extends to multiple
  LANs
• no change or adjustment in NIC needed bridge
  completely transparent
• bridge operates on each LAN using the MAC
  protocol
• remote (half bridge) - may use another protocol
  between the 2 half bridges, while using MAC on
  each LAN

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LAN internet
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half - bridge, connecting 2 LANs

• 2 halves communicate through some other protocol,
  e.g., PPP, HDLC.

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bridges - routing

• how do bridges know which packets to forward,
  and in which direction?
• 2 basic techniques
• 1. fixed routing - the information is loaded
  manually into the bridge (typing it in, etc.).
  This info is then stored in a routing table.
• 2. dynamic routing learning bridges -
• the bridge learns where the stations are by
  watching the traffic on its ports

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bridges - routing

• for fixed routing, many topologies possible
• dynamic routing - the internet must be configured
  as a tree this simplifies routing
• tree LANs and bridges are the nodes, and the
  links between them are the edges, and
• LANs can be connected only to bridges, not
  (directly) to other LANs
• if a cycle exists, the bridges will detect it and
  remove one from the active network, so that a
  tree structure is maintained

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bridges - dynamic routing

• bridge has a routing table, 3 fields
• dest.address, next port , time
• when bridge receives a packet DA,SA on port X
• 1. if SA found in table, reset timer,
• else add SA, port, time to table.
• 2. if DA found in table send packet out on
  next port indicated else send packet on all
  ports except X.

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bridge dynamic routing

• timer typical value 300 seconds (why have the
  timer? is this a good default value?)
• given the tree structure, bridges will learn a
  stations direction (explain how?)
• MAC addresses could be divided into (network,
  station) parts. If so, tables can be made
  smaller, but same algorithm used.

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bridges, routers, repeaters, hubs

• repeaters simply connect 1 cable to another,
  repeat the bits. No routing decisions or
  filtering.
• hubs serve to extend the ethernet. No routing
  or filtering of messages.
• bridges - connect LANs together at the MAC layer
  filter and rout messages at the MAC layer.
• routers -. Layer 3/3.5 (internet). Software, IP
  protocol.. Usually more expensive. Discussed in
  CS4550 ....