[Med_Access_0.doc]
Time division multiplexing
Statistical time division multiplexing
Frequency division multiplexing
Polling
Bus broadcast sharing - message sense signaling
Ring/loop sharing - extension of time division multiplexing usage is dictated by system protocols
. pre-allocated time slots ; empty time slots - slotted rings; token passing
Network Access Control
Data is send in packets. A packet consists of
Contention-based access methods
Multiple access
. Data packets are send to all terminal stations.
. Terminal station with matching address receive the packet while all other ignore it.
. Collision when more than one terminal send packets to the host simultaneously.
. If the station has not received an ACK from the host within a specific time, it knows that the packet has been destroyed. It then waits for a random period before re-transmitting.
Carrier Sense Multiple Access (CSMA)
. Carrier sense - node check the status of the network before transmission
. Collision - all transmitting nodes will detect a garbled signal on the network.
In CSMA all stations share the same physical medium or channel in a broadband system. All stations have equal priority and must listen to the media to detect if it is idle or not before starting its own transmission.
The problem is that two stations could detect an idle medium and transmit simultaneously. It also takes time for the signal to propagate from one station to another. One station may have transmitted for a time before all other station could detect the transmission. In the mean time, another station may also have started transmission. To solve the problem two schemes are normally used ie. CSMA/CA and CSMA/CD.
Carrier Sense Multiple Access with Collision Detect - CSMA/CD
Listen while transmitting
When the LAN is idle, a station can transmit but at the same time the transmitter also monitor the network for collision. On detecting collision, the transmitter sends out a jamming signal.
Collision consensus enforcement
If the collision only lasts a short time, it is possible that some nodes on the network would not detect it. To avoid this, the colliding nodes transmit a number of bytes of random data (called a jam). This last long enough to propagate to all nodes on the network so that there is no doubt that a collision has taken place.
Back off with out transmitting
The transmitting stations involved in the collision will each stop trying to transmit for random time intervals, before trying again. This is called “backing off”. Delay is computed in each individual node using an algorithm designed to minimize collision. After a fixed number of retries, no further attempts will be made to transmit.
Collisions are much more likely when there is a heavy demand on the network.
Binary exponential backoff
If a second collision is detected the station will double its mean random delay time and backoff again. This is known as binary exponential backoff and a station will attempt to transmit for up to 16 times before reporting an error condition.
n = no of retransmission attempt
k = min(n,10)
The delay is an integer of multiple of slot time. The number of slot times to delay before the n th retransmission attempt is chosen as a uniform distributed random integer r in the range
0 r 2k
Worst Case Collision
B detects collision first, stop sending message and sends a jamming signal.
Jamming signal must reach A before the end of A's frame to ensure A detects the collision. This imposes a minimum packet size with the maximum end to end transmission time.
ie. min. packet transmission time > 2 x max. propagation delay time.
In ethernet running at 10 Mbit/s, minimum packet size = 64 bits
max. end to end transmission time = 44.95 usec
overall distance of coaxial cable < 1.5 km
example: CSMA/CD
The network’s slot time is an important parameter when considering its overall efficiency. From a performance point of view, time spent in the handling of collisions represents channel bandwidth wasted and should be minimized. When a node begins transmission, it is exposed to the risk of a collision occurring for the duration of one slot interval (ie. one round trip time).
Thus the ratio of frame transmission time to slot length should be maximized. The slot time is fixed by physical and electrical considerations. The only way to improve network efficiency is to increase the packet length or to decrease the transmission speed. Reducing speed is not suitable as it would also reduce system throughput.
It has been shown in practice that the efficiency of a heavily loaded Ethernet can vary between 97% with a packet size of 512 bytes to 54% with a 4 byte packet. It is normal to engineer CSMA/CD networks so that they will run with a sustained load of no more than 50% of available capacity. This means that the access method provides reliable transmission with very low delay periods.
CSMA/CA ( Carrier Sense multiple access with collision avoidance )
CSMA/CA attempts to detect a collision before any actual data is transmitted. A station tries to claim the LAN by transmitting a short frame, called a carrier burst. Stations could detect a collision in this initial burst and backoff as normal. Any station seeing a carrier burst will not try to transmit until the end of the subsequent data packet has been seen.
The transmitting station then listens to the LAN to detect any carrier burst coming from remote stations. When it is sure that every station has seen the carrier burst it can transmit the data.
If the first station's carrier burst is swamped by a similar burst from a remote station, by the time it reaches the remote station, the carrier burst going in the opposite direction will have been seen by the first station, which will be in its listening gap. It will then backoff and let the second station transmit. This technique allows very short packets to be used.
example
transmitter listens before send
if cable free, tenasmitter sends initial carier burst
transmitter listen to detect carrier burst
if cable still free then send second carrier burst
short time delay
data packet
time reserved for a response frame from receiver
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CSMA - Carrier Sense Multiple Access
In this scheme, a station that wishes to transmit attempts to avoid collisions by first listening to the medium to determine if another transmission is in progress.
The scheme is further divided into the followings.
Non-persistent CSMA - if medium is idle, then transmit
if busy, then wait random amount of time and re-sense channel
p-persistent CSMA - if medium idle, transmit with probability p
if busy, continue to listen until channel is idle then transmit with probability p
1-persistent CSMA - if medium idle, then transmit
if busy, then continue to listen until channel idle and then transmit immediately
(special case of p-persistent)
When the transmission is unsuccessful, the packet will be retransmitted according to a randomly distributed retransmission delay.
IEEE802.3 [ use 1-persistent ]
refer to ethernet notes for detailed description.
Essentially similar to CSMA/CD. On detecting collision, the station ceases transmission and sends a brief jamming signal. Collisions are detected by monitoring the analog waveform. The waveform is distorted with larger-than-normal voltage amplitude on the cable.
The deference mechanism depends on the non/1/p-persistent being used when the channel is idle or busy.
After the last bit of a frame, the MAC sublayer wait for a period called inter-frame spacing before transmission.
A random delay is used after collision. Stations involved in the collision are not likely to collide on the next transmission. Truncated binary exponential backoff is used in Ethernet. In this method, a station will persist in trying to transmit when there are repeated collisions. After 16 failures, the packet is discarded and the event is reported as error.
Backoff strategy- after n failure, the next delay = r x base backoff time ( ie 2 x end to end propagation delay)
0 £ r £ 2k k = min(n,10)
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NON CONTENTION ACCESS METHOD - nodes waiting for permission to transmit
Polling
The technique is widespread in traditional wide area networks. It relies a master-slave relationship between a central server and the other stations on the LAN.
The central server maintains a list of the stations and asks each one in turn if it has any transmission to make. If it has, that station is permitted to send one or more packet to the central station.
The station must then wait until it is polled again before it can send any more. Data can also be transmitted from the master to the slave during the same transaction.
Disadvantages - . station has to be polled even it has nothing to send.
. any station to station data transfer has to pass through the server.
. reliability depends on the central server.
Advantages –
. simple to implement requiring no complex hardware or software
. priority polling can easily be implemented
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