Enhanced CSMA

• Additional improvement Use CSMA access Listen while transmitting Stop immediately if collision sensed Called collision detection Reduces time lost due to collisions

• Carrier Sense Multiple Access/Collision Detection protocols (CSMA/CD)

CSMA/CDHow long should a station listen while transmitting to ensure no collision?

Shared channel

A B

- Suppose A begins to transmit

- Suppose B wants to transmit before hearing A’s transmission

☼☼ collision

- How long before A hears collision?

Collision Detection

• Let p = maximum propagation delayp = time for signal to travel full length of channel

• To ensure detection of a collision, transmitter must listen 2 x propagation delay

• Thus, the time we need listen depends on size of network

• If we set a limit to size of network, we could define a maximum time to listen

• On wired networks we consider propagation delay as 2 * 108 meters per second

Collision Detection

• Suppose time to transmit frame is longer than

2 * propagation delay• What can we say about the channel if we have

not heard a collision after 2 * propagation delay? • By that time, station will have seized the channel• What if we required a minimum frame size?• What if the time to transmit a frame of minimum

size was ≥ 2 * propagation delay?• Under this condition, if transmission completes

without collision, we are assured none will occur

Ethernet/IEEE 802.3• 1975 – Original Ethernet

• Based on a paper by Robert Metcalfe• Based on Aloha concepts• 2.94 Mbps (3 Mbps)• Maximum size of 1 Km• Used CSMA/CD protocol• Used coaxial cable as media

• First version developed in collaboration with• Xerox• Intel• Digital Equipment Corporation

Ethernet/IEEE 802.3• 1980 Ethernet Version 1.0

• 10 Mbps• 500 meter segment• Used 50 ohm coaxial cable• CSMA/CD – 1-persistent

• 1982 Ethernet Version 2.0• Some electrical revisions

• 1985 IEEE 802.3• Defined a family of networks• Minor differences with Ethernet

– Some additional electrical functions– Frame format variations

IEEE 802.3• Family of Networks

CSMA/CD 1-persistent Different speeds Different media

• 10BASE5 Thick Coax 500 meters• 10BASE2 Thin Coax 185 meters• 10BASET Twisted pairs 100 meters• 10BASEF Fiber 1 & 2 Km• 100BASET twisted pairs 100 meters• 1000BaseTtwisted pairs 100 meters• Others

10BASE5

• 500 meters

• 10 Mbps

• Maximum 100 stations on a segment

• Minimum 2.5 meters apart

• Manchester encoding ± .85 volts High to low = 1 bit Low to high = 0 bit

10BASE5500 meters

host

Transceiver – EthernetMedia Access Unit (MAU) - IEEE

Transceiver/AUI Cable50 meters maximum

Attachment Unit Interface (AUI)

How long must we listen to assure no collision?How large a minimum frame do we need to detect any collision before end of transmission?

10BASE5500 meters

500 meters

RepeaterTransceiver/AUI Cable50 meters maximum

A

BHow long must we listen to assure no collision?

Calculating Propagation Delay

• 100 meters of cable adds .5 µs to delay or 1 µs roundtrip

• Repeater adds approximately .75 µs one-way or 1.5 µs roundtrip

• Transceiver adds 1.0 µs one-way or 2.0 µs roundtrip (if not connected to host)

• On a 10 Mbps Ethernet, 1 µs is equivalent to transmitting 10 bits

• We can calculate delay in time units or in bits

Ethernet/IEEE 802.3

• Use CSMA/CD

• Detect collisions while transmitting frame

• Requires a minimum size frame Transmission rate (speed) Frame size Network size

All related

10BASE5

B

A

500 meters

500 meters

500 meters

500 meters

500 meters

R

R

R

R

5 * 500 segments

= 2500 meters

= 12.5 µs

8 * 50 Transceiver cables

= 400 meters

= 2 µs

4 repeaters * .75 µs

= 3 µs

8 transceivers * 1 µs

= 8 µs

Total one way delay = 25.5 µs

Total round trip = 51 µs

Ethernet/IEEE 802.3

• Minimum frame size = 512 bits

= 64 bytes• Vulnerable time or contention slot = 51.2 µs• After 51.2 µs, station has seized channel• What happens if a station sends unlimited size

frame?• Maximum size frame also specified at 1518 bytes• Maximum size frame enforced by transceiver

Original Ethernet Diagram

Ethernet Wiring Topologies

Star wired hubs

Shared Channeln Mbps capacity

A B F GC ED

n Mbpsn Mbps

All stations share n Mbps

A shared hub is actually a repeater

Efficiency of CSMA/CD

• Let p = propagation delay (one way)• A = probability a station acquires channel during

a contention slot• Tx = time to transmit an average size frame• It can be shown that

• Optimal utilization when

Ap

Tx

TxU

2

eA

1

speed

framebitsTx

/

Ethernet/IEEE802.3 Utilization

Ethernet/IEEE 802.3

• Advantages Most widely used network architecture New stations can be installed on live network Low delay at low load Simple access mechanism

• Disadvantages Limited distances Non deterministic High collisions rate at high loads No provisions for priority traffic

Collisions• Minimum frame defined as 512 bits• At 10 Mbps contention slot = 51.2 µs• After a collision

each station waits 0 or 1 contention slots

• After a second collisioneach station waits 0, 1, 2, or 3 contention slots

• After j collisions, each station waits

0, 1, 2, . . . . . ., 2j-1 contention slots

until j = 10

then continue until 16 total tries

• This is called binary exponential backoff

Ethernet/IEEE 802.3 Addressing• Each station on a multiple access LAN must be

uniquely identified• For Ethernet, addresses are assigned to network

interfaces by vendors• Each address is 48 bits

XXXXXX XXXXXX (X = 4 bits) vendors address

• There is a special broadcast address FFFFFF FFFFFF (all 1s)

All stations on shared channel

• There are provisions for subgroups (multicast)• This is called the Media Access Control (MAC)

address

Ethernet/IEEE 802.3 Addressing

• You can look up the vendor of the adaptor for your machine

http://standards.ieee.org/regauth/oui/index.shtml

Ethernet Frame Format

Preamble

SFD

Destination Address

Source Address

Type

Data46–1500 bytes

FCSCRC - 32

Ethernet

7

1

6

6

2

4

Preamble

SFD

Destination Address

Source Address

Length

LLC Data46– 496bytes

FCSCRC - 32

IEEE 802.3

7

1

6

6

2

4

10101010

………..

10101011