1. Channel Access Methods
When several devices are connected to a single channel, there must be some rules to govern these devices when they access, transmit, and release the channel .
There are three basic channel access methods to provide for an orderly and efficient use of that capacity:
Contention
Polling
Token passing
Different access methods have different overhead effects on network traffic.

2. Contention With contention systems, network devices may transmit
whenever they want.
With this technique, no control is exercised to determine whose
turn it is, all stations contend for time. No referee mandates
when a device may or may not use the channel.
This scheme is simple to design.
The scheme provides equal access rights to all stations.
Stations simply transmit whenever they are ready, without
considering what other stations are doing.
Unfortunately, the "transmit whenever ready" strategy has one
important shortcoming.
Stations can transmit at the same time.
When this happens, the resulting co-mingling of signals usually
damages both to the point that a frame's information is lost.
This unhappy event is called a "collision."

3. Collision
Newer contention protocols were developed that called for stations
to listen to the channel first before transmitting.
If the listening station detects a signal, it will not start
transmitting and try again later.
These protocols are called CSMA (Carrier Sense, Multiple Access
with collision detection) protocols.
These protocols will reduce collisions.
However, collisions may still occur when two stations sense the
cable, detect nothing, and subsequently transmit.
In order to reduce collisions, CSMA/CD protocols compute a
random backoff time before retransmitting the frame (as shown in
the flow diagram). Examples of CSMA/CD protocols :
IEEE (Ethernet )

4. CSMA / CD collision schematic

5. Frame format of a CSMA/CD bus Networks
Preamble
SFD
Destination address
Source address
Length indicator
Data
Pad (optional)
Frame Check Sequence
7 octets
1 octet
2 or 6 octets
2 or 6 octets
2 octets
2- octet field
<= 1500
4 octets

6. Operational parameters of a CSMA/CD bus Networks
Bit rate
10 Mbps (Manchester encode)
Slot time
512 bit times
Interframe gap
9.6 ms
Attempt limit 16
Backoff limit 10
Jam size
32 bits
Maximum frame size
1518 octets
Minimum frame size
512 bits

7. Frame format & operational parameters
A medium access control (MAC) unit is responsible for the encapsulation and de-encapsulation of frames for transmission and reception on the cable, error detection, and implementation of MAC algorithm.
Each frame consists of 8 fields. All the fields are of fixed length except the data and padding fields.
Preamble=> Sent at the head of all frames. ?used to achieve bit synchronization before the actual frame contents are received. It is a sequence of seven octets. (each equal to the binary pattern ).
SFD (start of frame delimiter)=> Single octet , signals the start of a valid frame t the receiver, immediately follows the preamble.

8. Frame format & operational parameters
Destination & Source addresses => Specify the intended destination station & originating station. Each address can be either 16 or 48 bits. If the first bit in the address field is 0, it specifies the address is an individual address and the transmitted frame is intended for a single destination. If the bit is 1, it specifies a group address and the frame is intended either for a logically related group or for all others stations. In this case, the address field is set to all binary 1s.
Length indicator => Specifies the number of octets in the data field.
Pad => If the value of length indicator is less than the minimum frame size, a sequence of octets is added, known as padding.
FCS => Contains CRC value that is used fir error detection.

9. CSMA/CD operation: a) transmit

10. CSMA / CD operation: b) receive

11. Advantages CSMA/CD control software is relatively
simple and produces little overhead.
CSMA/CD network works best on a bus
topology with bursty transmission. Bursty
traffic is characterized by short, sporadic
transmissions. Example: interactive
terminal-host traffic.
This technique is efficient for light to
moderate load.

12. Disadvantages CSMA/CD protocols are probabilistic and depends on the
network (cable) loading. Performance tends to collapse
under heavy load.
Considered unsuitable for channels controlling automated
equipment that must have certain control over channel
access.
We cannot set priorities to give faster access to some
devices.

13. Polling access method
Polling is an access method that designates one device
(called a "controller", "primary", or "master") as a channel
access administrator.
This device (Master) queries each of the other devices
(“secondaries”) in some predetermined order to see
whether they have information to transmit.
If so, they transmit (usually through the master).

14. Polling access method
Secondaries may be linked to the master in many different
configurations.
One of the most common polling topologies is a star, where the
points of the star are secondaries and the master is the hub.
To get data from a secondary, the master addresses a request for
data to the secondary, and then receives the data from the
secondary sends (if secondary sends any).
The primary then polls another secondary and receives the data
from the secondary, and so forth.
System limits how long each secondary can transmit on
each poll.

15. Advantages Polling centralizes channel access control.
Maximum and minimum access times and data rates on the
channel are predictable and fixed.
Priorities can be assigned to ensure faster access from some
secondaries.
When many stations have data to transmit over an extended
period of time, round-robin techniques can be very efficient.
If only a few stations have data to transmit over an extended
period of time, then there is a considerable overhead in
passing the turn from station to station, because most of the
stations will not transmit but simply pass their turns.
Polling is deterministic and is considered suitable for
channels controlling some kinds of automated equipment.

16. Disadvantages
Polling systems often use a lot of bandwidth sending notices
and acknowledgments or listening for messages.
Line turnaround time on a half- duplex line further increases
time overhead.
This overhead reduces both the channel's data rate under low
loads and its throughput.

17. Token passing System
In token-passing systems, a small frame (the token) is
passed in an orderly fashion from one device to another.
A token is a special authorizing message that temporarily
gives control of the channel to the device holding the token.
Passing the token around distributes access control among
the channel's devices.
Each device knows from which device it receives the token
and to which device it passes the token.(see fig.)
Each device periodically gets control of the token, performs
its duties, and then retransmits the token for the next device
to use.
System rules limit how long each device can control the
token.

18. Control token MAC: Token ring

19. Token Ring
Token ring networks are primarily used in technical and office environments.
Whenever a station wishes to send a frame, it first waits for the token. When the station gets the token, it start sending frame.
The intended recipient retains a copy of the frame and indicates by setting the response bits at the tail of the frame.
A station releases the token in one of the two ways depending on the bit rate of the ring.
In slower ring (4 Mbps), the token is released only after the response bits have been received.
In higher speed rings (16 Mbps), the token is released after transmitting the last bit of a frame (early token).

20. Token Ring

21. Token format & Frame format in Token Ring1 1 1
octets SD AC ED
Frame Format:
octets
<5000 1 1 1
2/6
2/6 4 1 1 SD AC FC DA SA
INFO
FCS ED FS
Start of frame
FCS coverage
End of frame

22. Field Descriptions of a Token Ring
Start delimiter (SD)
J K O J K O O O
End delimiter (ED)
J K 1 J K 1 I E
P P P T M R R R
Access control (AC)
FF Z Z Z Z Z Z
Frame control (FC)
Source and destination
Address (SA/DA)
I/G /47 bit address
AC xx ACxx
Frame status (FS)

23. Control token MAC: Token bus

24. Slotted ring principles: bit definitions of each slot

25. Slotted ring principles:Outline topology

26. Advantages
Even though there is more overhead using tokens than using CSMA/CD, performance differences are not noticeable with light traffic and are considerably better with heavy loads because CSMA/CD will spend a lot of time resolving collisions.
A deterministic access method such as Token Ring guarantees that every node will get access to the network within a given length of time. In probabilistic access method (such as CSMA/CD) nodes have to check for network activity when they want to access the network.

27. Disadvantages
Components are more expensive than for Ethernet or ARCnet.
Token Ring architecture is not very easy to extend to wide-area networks (WANs).
Token Ring network is much more expensive than Ethernet. This is due to the complex token passing protocol.