Ethernet MAC-Advance Computer Networking-Lecture Slides, Slides of Computer Networks

Download Ethernet MAC-Advance Computer Networking-Lecture Slides and more Slides Computer Networks in PDF only on Docsity!

• Lecture No.

3

Ethernet MAC – CSMA/CD

• Multiple access
  • Nodes send and receive frames over a

shared link

• Carrier sense
  • Nodes can

distinguish

between an idle

and busy link

• Collision detection
  • A node

listens as it transmits

to detect

collision

5

CSMA/CD MAC Algorithm

• If line is busy (

carrier sensed

) …

• Wait until the line becomes idle and

then transmit immediately

• Called

1-persistent

(special case of

p-

persistent

)

• If

collision detected

• Stop sending data and jam signal– Try again later

6

Collision Detection

How to ensure that my-machine knows about the collision?

start

transmission

at time 0

my-machine

your-machine

start

transmission

at time T

almost there

at time T

collision !!!

8

Constraints on Collision Detection

• Specifics of IEEE 802.
  • bounds 2T to 51.2 microseconds– packet must be at least 64B long
    • Jam after the collision, for 32 bits, then

stop transmitting frame (runt frame of96 bits)– ensures that

all hosts notice collision

9

Ethernet Min. Frame Size

-^

RTT

on a maximally configured Ethernet

of 2500m, with 4 repeaters is about 51.

μ

s

• 2500m / 2 x 10

8

m/s = 12.5 us

• 2 x 12.5 = 25 us + repeater delays

μ

s on 10 Mbps corresponds to 512 bits

(64 bytes)

• Therefore, the

minimum frame length

for

Ethernet is 64 bytes (header +46 bytes data)

11

Retry After the Collision

• How long should a host wait to

retry after a collision ?–

Binary exponential backoff

• maximum backoff doubles with

each failure (exponential)

• after N failures, pick an N-bit

number

• 2

N

discrete possibilities from 0 to maximum

12

Retry After the Collision

Delay and try again algorithm

–1st time: 0 or 51.2us–2nd time: 0, 51.2, or 102.4us–3rd time51.2, 102.4, or 153.6us–

nth

time:

k

x

51.2us, for randomly

selected

k

n^

–Give up after several tries

(usually 16)

14

Binary Exponential Backoff

• Ts is 51.2 microseconds for IEEE 802.3• Consider that

k

hosts collide

• each picks

random

number from 0 to 2

( N-1)

• if minimum value is

unique

• all other hosts see busy line• remember that Ethernet RTT < 51.

microseconds

15

Binary Exponential Backoff

• Consider that

k

hosts collide

• if minimum value is

not unique

• hosts in minimum value slot

collide again

• following slot idle• consider next smallest backoff

value

17

Ethernet Frame Reception

• Acceptable frames
  • addressed to host, or;– to multicast address to which host

belongs or;

• to a broadcast address– anything if in promiscuous mode

(packet sniffing and

tcpdump

requirement)

18

Ethernet Collision Detection

• Bus topology Ethernets
  • Transceiver handles
    • Carrier detection• Collision detection• Jamming after a collision
      • Transceiver sees voltage sum
        • Outgoing signal + Incoming signal
          • Looks for voltages impossible for local alone
            • Attenuation can prevent detection• Limits segment length

20

Experience With Ethernet

• Number of

hosts limited to 200

in

practice, standard allows 1024

• Range much

shorter than 2.5 km

limit in standard

• Round-trip time is typically

5 or 10

μ

s

, not 50

μ

s

21

Experience With Ethernet

• Higher-level flow control (

i.e.

, TCP) limits

load–

30% maximum load

recommended

• Large packets recommended
  • Star topologies easier to administer than Bus
    • Failure isolation, node addition w/o complex

topology

• (Backwards) Compatibility more important

than elegance or raw performance (1 > 0)