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CS3502: Data and Computer Networks Local Area Networks - 2 CSMA/CD : IEEE 802.3 . 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.

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lans aloha csma summary
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
lans csma
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?
lans csma cd
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)
csma cd basic protocol
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)

csma cd basic protocol6
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?

csma cd backoff algorithm
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.
csma cd backoff algorithm8
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?
csma cd frame format
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

csma cd topologies media etc
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
csma cd network components
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)
csms cd physical layout
CSMS / CD : physical layout

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

csma cd physical layout
CSMA / CD : physical layout

star/hub configuration. physical star, logical bus

2-twisted pair connections; hub is a repeater

csma cd some specifics
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
csma cd standard s
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

csma cd practical considerations
CSMA / CD : practical considerations

For the following: consider only the LAN hardware needed; don’t 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 it’s in 2 different buildings 10 miles apart