1. CS3502: Data and Computer Networks Local Area Networks - 2 CSMA/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 = , choose from [ ]
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 : ; pad : 0-46; FCS : 4
exercise: specify the protocol using a CFSM model

10. CSMA / CD : topologies, media , etc.
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; 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 machines (PCs, Macs, linux), which is located in a single building. (eqmt, dollars)
ditto, but it’s in 2 different buildings 10 miles apart