Medium Access Control (MAC)
October 28, 2016
Categorised in: Computer Forensic & Cyber Applications
Types of network:
Switched communication network
Users are interconnected by means of some transmission lines, multiplexers and switches
Eg: Telephone and SONET network (circuit switching)
Eg: X.25,Frame relay,ATM(packet switching)
A single transmission media is shared by all the users and information is broadcasted by an user into the medium.
- Multi-taped Bus
- Ring networks sharing medium
- Satellite communication using sharing of uplink and downlink frequency bands
- Packet radio network
- Wireless communication stations sharing a frequency band
Broadcast networks require a protocol to orchestrate the transmission from the users.
ISSUES IN MAC
The question is “who goes next”?
The protocols used for this purpose are known as Medium Access Control(MAC) techniques.
The key issues involved here are- Where and How the control is exercised.
Centralized: a designated station has an authority to grant access to the network.
Simple logic at each station.
Greater control to provide features like priority, overrides and guaranteed bandwidth.
Distributed: stations can dynamically determine transmission order.
Complex, reliable and scalable
Synchronous: dedicated specific capacity to a connection.(eg: synchronous time division multiplexing(STDM), Frequency division multiplexing(FDM)).
Asynchronous: allocates capacity dynamically.
(data bursty in nature) (e. ATDM)
GOALS OF MAC
- Initialization (stable state)
- Fairness (equal opportunity, access & delay should be same)
- Limitation to one station
- Error limitation
RANDOM MAC TEHNIQUES
Suitable for Bursty nature of traffic
Transmits frame in fixed time slots
Vulnerable period reduces to 2Tf from Tf of pure ALOHA
Pure ALOHA: G=0.5, S=1/2e= .184= 18%
Slotted ALOHA: G=1, S=1/e= .368= 36.8%
When a station sends a packet, others know about it within a fraction of packet transmission time. This led to the development of Carrier-sense-Multiple-Access (CSMA) protocol. Station listens to the medium before transmitting; listen before talking(LBT).
- If medium is idle, Transmit
- If medium is busy, wait random period and then resense medium
P-persistent: if medium is idle, transmit with a probability p.
Carrier-Sense-Multiple-Access with collision Detection (CSMA/CD)
Station listens to the medium while transmitting; Listen while talking(LWT).
- If channel is idle:
- Packet is transmitted if nonpersistent or 1-persistent.
- For p-persistent, the packet is sent with probability p or delayed by the end-to end propagation delay with probability (1-p)
- If channel is busy:
- The packet is backed off and the algorithm is repeated for nonpersistent case.
- The station defers transmission until the channel is sensed idle and then immediately transmits in 1-persistent case.
- For p-persistent CSMA/CD the station defers until the channel is idle, then follow the channel idle procedure.
Need for CSMA/CA
Proliferations of portable computers need to use radio signals for communication.
All radio transmitters have some fixed range
When a receiver is within the range of two active transmitters, the resulting signal will generally get garbled
CSMA/CD is unsuitable in such an environment.
There are so called hidden station and exposed station problems.
Suitable techniques needs to be developed for this kind of environment.
THE HIDDEN STATION PROBLEM
Both A and C tries to communicate with B simultaneously.
Both the signals will reach B and collision will occur, but A and C will not detect the collision.
EXPOSED STATION PROBLEM
Stations B wants to send a frame to A, which is also listened by C, which as assumes that medium is not free.
But, in practice C can send to D.
Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA).
Sender sends a short frame called request to send RTS(20 bytes) to the destination. RTS also contains the length of the data frame.
Destination station responds with a short( 14 bytes) clear to send (CTS) frame.
After receiving the CTS, the sender starts sending the data frame.
If collision occurs, CTS frame is not received within a certain period of time.
A multiple access method in which the available bandwidth of a link is shared in time, frequency or using code by a number of stations.
FDMA: the bandwidth is divided into separate frequency bands
TDMA: the bandwidth is timeshred
CDMA: Data from all stations are transmitted simulteneiously and are seperated based on coding theory.
COCKTAIL PARTY THEORY
- FDMA: when all the people group in widely separated areas and talk within each group.
- TDMA: When all the people are in the middle of the room, but they take turn in speaking.
- CDMA: When all the people are in the middle of the room, but different pairs speak in different languages.