Medium Access Control

Medium Access Control:
Access method is a set of rules that determine the method that followed by the computer to puts its data on the transmission media. The main function of the access method is to organize the access to the transmission media and ensure that all computers on the networks can send and receive the data successfully. In most networks, the computers share a single network cable, however if two computers tries to puts its data on the cable in the same time, the confliction is occurs and it leads to corrupt the transmitted data from both computers. Multiple access protocols can be classified as belonging to one of three categories: channel partitioning protocols, random access protocols, and taking-turns protocols.
• Channel Partitioning
Divide channel into smaller “pieces” (time slots - TDMA,
frequency - FDMA) and allocate part of channel capacity to a node for exclusive use.
• Random Access
Channel not divided, allow collisions and “recover” from collisions
• Taking turns
Nodes take turns, but nodes with more to send can take longer turns.
Goal: efficient, fair, simple, decentralized


1. Channel Partitioning MAC protocols:
1.1. Time Division Multiple Access (TDMA):
The access to channel in "rounds" , each station gets fixed length slot (length = pkt trans time) in each round and unused slots go idle.
Example: 6-station LAN, 1, 3, 4 has pkt, slots 2, 5, 6 are idle.

Figure (35): Time Division Multiple Access protocol.

1.2. Frequency Division Multiple Access (FDMA):
The channel spectrum divided into frequency bands, each station assigned fixed frequency band and unused transmission time in frequency bands go idle.
Example: 6-station LAN, 1, 3, 4 has pkt, frequency bands 2,5,6 idle.


Figure (36): Frequency Division Multiple Access protocol.

2. Random Access Protocols:
2.1. Pure ALOHA Protocol:
The basic idea of an ALOHA system is simple: let users transmit whenever they have data to be sent. There will be collisions, and the colliding frames will be destroyed. however, due to the feedback property of broadcasting, a sender can always find out whether or not its frame was destroyed by listening to the channel, the same way the other users do. With a LAN, the feedback is immediate; with satellite, there is a delay of 270 msec before the sender knows if the transmission was successful. if the frame was destroyed the sender just waits a random amount of time and sends it again. The waiting time must be random or the same frames will collide repeatedly. Systems in which multiple users share a common channel in a way that can lead to conflicts are known as contention systems.
Whenever two frames try to occupy the channel at the same time there will be a collision and both will be garbled. if the first bit of a new frame overlaps with just the last bit of a frame almost finished, both frames will be totally destroyed, and both will have to be retransmitted later.


(a) (b)

Figure (35) :( a): In pure ALOHA, frames are transmitted at completely arbitrary times. (b): Vulnerable period for the shaded frame.

2.2. Slotted ALOHA protocol:
It divide the medium bandwidth into time slots, delimited by common clock, each long enough to accommodate one packet and its subsequent acknowledgement. In slotted ALOHA, the station that has a packet to send must wait the beginning of the next slot before it can begin transmission. If the acknowledgement has not been returned by the end of a slot, the station assumes that the transmission has been unsuccessful, and becomes backlogged for some random number of slots. Otherwise, the stations behave pretty much just as in pure ALOHA.

Figure (36): Slotted ALOHA protocol.


2.3. Carrier-Sense Multiple Access/Collision Detection (CSMA/CD):
In CSMA/CD, if the station want to transmit its data, it must detect whether the medium is idle or not and will only transmit if it finds it so. If it find any signal, it will enter in the defer mode. In the first chance the station, detect that the channel is empty from any signal; it will transmit its data. If the two stations sense that the channel to be idle and begin transmitting simultaneously, they will both detect the collision almost immediately. Therefore, if the adapter detects signal energy from other adapters while transmitting, it stops transmitting its data and instead transmits a jam signal (32-48 bit sequence) to all other stations to announce them about the collision and prevent it from copying the data from the channel because that the data is corrupted. Now, the two stations re-attempt to transmit after a random time, where this reduce the probability of new collision occurring.
Because the attenuation that effect on the transmitted signal in remote distances, the error detection mechanism that used in CSMA/CD work in a limited distance less than 2.5 km. The CSMA/CD is a fast access method but with increasing the network size it become inactive because that the most collision between the data will be occur in the larger networks (larger number of users attempts to access the medium and more data generated and exchanged on the networks). CSMA/CD is suitable for only small networks.
The following relation explains the efficiency of CSMA/CD protocol:
,

Figure (37): CSMA/CD can be in one of three states: contention, transmission, or idle.
Example: what is the efficient of the CSMA/CD protocol when the coaxial cable length is 2.5 km, the transmission rate is 10 Mbps, the packet length is 620 bits and the propagation speed is .
Sol:





2.4. Carrier-Sense Multiple Access/Collision Avoidance (CSMA/CA):
This method tries to prevent the collision occurrence. each station in the LAN that want to transmit a data must sends a signal refer to its wishing to transmitting data before it actually sending the data. it work this by sending a reservation signal before the transmitting, where this signal tells the other stations on the LAN that it will start in transmitting the data in order to no other station sends its data at the same time. This will reduce the probability of collision occurrence but not prevent it completely because there is a likelihood that two stations send a reservation signal simultaneously, and this leads to collision between the two signals and the two stations must retransmit again later. This method is slow because that each station need to sends a signal before the actual data transmission. it is less using from other methods.

2.5. Reservation protocol:
Each contention period consists of exactly N slots. The advance decisions are made for stations that wish to transmit, during a preliminary reservation phase. All stations that have acquired a right to transmit during the reservation phase then do so, in precisely controlled order. Once this is complete, a new reservation phase is entered.
The reservation phase consists of a series of short minislots, which stations can use to reserve a right to transmit. One way of doing this is to provide a 1-bit minislot for every station. If station k wants to transmit a packet, it simply sends a ‘1’ during its minislot (minislot k). At the end of reservation phase those station that have booked in this way will each sends a packet in the same order as their minislots.


Figure (38): The Reservation (bit-map) protocol.
This requires a reservation minislot for every station, which in a large network may become quite an overhead. A variation limits the number of reservation minislots, does not allocate any one to a particular station, and uses a contention protocol like slotted ALOHA to decide between competitors during the reservation phase. Each station that succeeds in acquiring a minislot then transmits a packet in the appropriate order, once the reservation phase is over. The use of a single bit for a minislot is no longer adequate since a successful station must identify itself explicitly to the others.

3. Taking turns Protocols:
3.1 Polling protocol:
One of oldest and most primitive avoidance technique is polling. it requires that one of the stations, the primary, plays the roll of a central controller, initiating the polling sequence, while the other stations, the secondaries, are asked in some predefined polling sequence whether or not they have a packet to transmit.






Figure (39): The Polling Protocol.

The primary polls each secondary in turn, using a special poll packet or signal directed to that secondary. A secondary station wishing to transmit may do so when polled although it is often permitted only to send directly to the primary. This technique is called roll-call polling and is straightforward but inefficient and can be slow. A station that wishes to transmit must wait until it is polled, even if no other station wants to use the medium. In addition, a poll operation needs two propagation delays since the poll request must be sent to the secondary and its response returned.
An enhancement is Hub polling. Here, the primary polls the farthest secondary. If this does not wish to transmit, or when it finishes, it returns the poll to the next secondary along and so on. This is more complex than roll-call method, in that the secondary must be able to pass the poll request on to its neighbor. It is also more efficient, since a poll no longer needs to propagate across the medium twice, but it still requires central control. Also in roll-call polling, if the primary itself wants to send a packet, it has an opportunity to do so after each secondary is finished with the medium, where as in Hub polling, it gets an opportunity only each time all secondaries are finished. If the primary needs more access than this, a second data path must be included for primary to secondary transfers. This can be done either by splitting a single medium using FDM, or by introducing a second physical pathway.
3.2. Token Passing Protocol:
In this method, the stations are assigned positions in a logical circular sequence, which may or may not correspond to their physical positions, and a control packet or token controls access to the medium. A station that has the possession of the token may transmit a packet, or in some cases, require another station to transmit a packet. Packet sizes may be fixed or variable, and there will be some rule as to how long a station can retain possession of the token before passing it on to the next in sequence.
Token passing avoids the requirement for a central polling controller, but one station is required to take responsibility for token management such as regenerating the token if it is lost. To avoid dependence on any particular station, all must be capable of assuming this management role.
Its main drawback, at light loading, a token may need to be passed around many idle stations before reaching one that is waiting to send. We can compute an average time, ATT, for transfer of the token from one station to the next as:

Where: APD is the average propagation delay between two stations.
T is the length of the token in bits.
R is the transmission rate.
r is the time taken for a token to be detected.
Obviously, APD depends not only on the physical spacing of the stations, but also the order in which they are placed in the logical token passing sequence. In some stations, this is the same as the physical order, but in other stations, can be rearranged to make different sequences.