enhanced csma additional improvement use csma access listen while transmitting stop immediately...
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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)
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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?
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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
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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
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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
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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
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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
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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
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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?
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10BASE5500 meters
500 meters
RepeaterTransceiver/AUI Cable50 meters maximum
A
BHow long must we listen to assure no collision?
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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
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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
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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
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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
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Original Ethernet Diagram
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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
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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
/
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Ethernet/IEEE802.3 Utilization
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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
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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
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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
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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
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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