CS3502: Data and Computer Networks Local Area Networks 2 CSMACD : IEEE 802'3

1
CS3502Data and Computer NetworksLocal Area
Networks - 2CSMA/CD IEEE 802.3

2
LANs Aloha/CSMA , summary

• describe these using a CFSM model
• pure Aloha
• slotted Aloha
• 1-persistent CSMA
• p-persistent CSMA
• nonpersistent CSMA
• Compare approximate utilization of the above

3
LANs CSMA

• when collisions occur, how much time is wasted?
• what is approximate relative likelihood of
  repeating the collision, with
• CSMA, 1-persistent
• CSMA, 0.1 persistent
• CSMA, nonpersistent
• How can time wasted be reduced?

4
LANs CSMA/CD

• adds collision detection capability to CSMA
  greatly reduces time wasted due to collisions
• standardized as IEEE 802.3 most widespread LAN,
  perhaps most widespread network protocol
  estimate more that 50 of all LANs on the
  Internet use this protocol... 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)

5
CSMA / CD basic protocol

• big difference from CSMA is that transmissions
  are stopped when collisions are detected.
• first version used bus -broadcast topology
• when the MAC receives a packet to transmit
• (1) sense the carrier LISTEN
• if no signal is detected
• then begin Xmitting message continue sensing
• if collision detected
• then Xmit jam, stop Xmitting,
  wait(BACKOFF), goto (1)
• when end-of-packet Xmitted, END.
• else carrier is busy go to (1)

6
CSMA / CD basic protocol

• the wait random time is precisely defined uses
  the binary exponential backoff algorithm
• physical encoding digital signals, Manchester
  encoding. Broadcast medium.
• if collisions are detected, then wasted time is
  relatively short
• Q what if the packet length is very short?
  (consider propagation delay)
• How long should the packet be?

7
CSMA / CD backoff algorithm

• binary exponential backoff (BACKOFF)
• 1 slot 51.2 ?s
• time following collision measured in slots
• after ith collision (i 1, ..., 10) chose a
  random slot in 0, 2i - 1.
• for i 11..16, choose from 0..1023
• when i reaches 16, give up.

8
CSMA / CD backoff algorithm

• example suppose 2 stations collide. What
  happens?
• what is Prob another collision?
• if 2nd collision occurs, what is Prob 3rd?
• why measure time in 51.2 ?s slots?

9
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
• exercise specify the protocol using a CFSM model

10
CSMA / CD topologies, media , etc.

• media
• coaxial cable
• twisted pair
• fiber (less common)
• topologies
• bus - original design widespread for many years
• star with hub in middle now becoming common
• data rates
• 10 Mbps
• 100 Mbps
• Gbps on the way

11
CSMA / CD network components

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

12
CSMS / CD physical layout

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

13
CSMA / CD physical layout

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

14
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) 185 m /segment on thin cable.
• drop cables 25 m max
• max 100 stations per segment on coax
• at least 2.5 m between adjacent receivers on coax
• at most 30 stations per segment on thin cable
• at most 1024 stations per ethernet

15
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

16
CSMA / CD practical considerations

• For the following consider only the LAN hardware
  needed dont yet consider internet connections
• you have 3 PCs in your house what will it take
  (equipment/dollars) to connect them together ?
• design a network for a small business with 30-40
  machines (PCs, Macs, linux), which is located in
  a single building. (eqmt, dollars)
• ditto, but its in 2 different buildings 10 miles
  apart