Media access control (MAC) is a sublayer of the data link layer that provides addressing and channel access mechanisms for nodes on a network to communicate. There are two types of channel allocation: static and dynamic. Static allocation preassigns channels using techniques like frequency division multiplexing, while dynamic allocation adapts to network conditions. Common MAC protocols include ALOHA, carrier sense multiple access, and collision-free protocols that avoid packet collisions. Wavelength division multiple access divides the spectrum into channels, and wireless LAN protocols like IEEE 802.11 are commonly used to connect wireless devices within a local area.

1. MEDIA ACCESS CONTROL
Data Communication And Networking

2. MEDIA ACCESS CONTROL
• Media access control (MAC) is a sublayer of the data link layer (DLL) in the seven-
layer OSI network reference model. MAC is responsible for the transmission of data
packets to and from the network-interface card, and to and from another remotely
shared channel.
• The basic function of MAC is to provide an addressing mechanism and channel
access so that each node available on a network can communicate with other nodes
available on the same or other networks. Sometimes people refer to this as the MAC
layer.

3. CHANNELS ALLOCATION
• There are two types of channel allocation
• Static Channel Allocation
• Dynamic Channel Allocation

4. STATIC CHANNEL ALLOCATION
• In static channel allocation there is FDM
• Frequency Division Multiplexing (FDM) is a networking technique in which multiple
data signals are combined for simultaneous transmission via a shared
communication medium. FDM uses a carrier signal at a discrete frequency for each
data stream and then combines many modulated signals .

5. FDMA
• When FDM is used to allow multiple users to share a single physical
communications medium (i.e. not broadcast through the air), the technology is
called frequency-division multiple access (FDMA).

6. WHAT’S THE PROBLEM WITH FDM
• If fewer than N users are currently interested in communication, some portions of
spectrum will be wasted.
• If more than N users want to communicates, some of them will be denied
permission even if some users with allocated frequency hardly ever transmit
anything.
• Even the number of users is N and constant, when some users are quiescent, no
one else can use their bandwidth so it is simply wasted.

7. DYNAMIC CHANNEL ALLOCATION
• Station Model.
• Independent stations for generating frames.
• Once a frame has been generated, the station is blocked until
• the frame has been transmitted.
• Single Channel Assumption
• A single channel for all communication (send and receive), and all stations
are equivalent.
Collision Assumption.
• If the transmission of two frames overlap in time, a collision occurs. All
stations can detect collisions. A collided frame must be retransmitted.

8. MULTIPLE ACCESS CONTROL
• ALOHA
• Carrier Sense Multiple Access Protocols
• Collision-Free Protocols
• Limited-Contention Protocols
• Wavelength Division Multiple Access Protocols
• Wireless LAN Protocols

9. ALOHA
• ALOHA is a system for coordinating and arbitrating access to a shared
communication Networks channel. It was developed in the 1970s by Norman
Abramson and his colleagues at the University of Hawaii. The original system used
for ground based radio broadcasting, but the system has been implemented in
satellite communication systems.
• A shared communication system like ALOHA requires a method of handling
collisions that occur when two or more systems attempt to transmit on the channel
at the same time. In the ALOHA system, a node transmits whenever data is available
to send. If another node transmits at the same time, a collision occurs, and the
frames that were transmitted are lost. However, a node can listen to broadcasts on
the medium, even its own, and determine whether the frames were transmitted.

10. TYPES OF ALOHA

11. PURE ALOHA
• In pure ALOHA, the stations transmit frames whenever they have data to send.
• When two or more stations transmit simultaneously, there is collision and the
frames are destroyed.
• In pure ALOHA, whenever any station transmits a frame, it expects the
acknowledgement from the receiver.
• If acknowledgement is not received within specified time, the station assumes that
the frame (or acknowledgement) has been destroyed.
• If the frame is destroyed because of collision the station waits for a random amount
of time and sends it again. This waiting time must be random otherwise same
frames will collide again and again.

12. SLOTTED ALOHA
• Slotted ALOHA was invented to improve the efficiency of pure ALOHA as chances of
collision in pure ALOHA are very high.
• In slotted ALOHA, the time of the shared channel is divided into discrete intervals
called slots.
• The stations can send a frame only at the beginning of the slot and only one frame
is sent in each slot.
• In slotted ALOHA, if any station is not able to place the frame onto the channel at
the beginning of the slot i.e. it misses the time slot then the station has to wait until
the beginning of the next time slot.

13. CARRIER SENSE MULTIPLE ACCESS
(CSMA)
• Carrier Sense Multiple Access (CSMA) is a network protocol that listens to or senses
network signals on the carrier/medium before transmitting any data. CSMA is
implemented in Ethernet networks with more than one computer or network device
attached to it. CSMA is part of the Media Access Control (MAC) protocol.

14. WORKING PRINCIPLE
• CSMA works on the principle that only one device can transmit signals on the
network, otherwise a collision will occur resulting in the loss of data packets or
frames. CSMA works when a device needs to initiate or transfer data over the
network. Before transferring, each CSMA must check or listen to the network for any
other transmissions that may be in progress. If it senses a transmission, the device
will wait for it to end. Once the transmission is completed, the waiting device can
transmit its data/signals. However, if multiple devices access it simultaneously and a
collision occurs, they both have to wait for a specific time before reinitiating the
transmission process.

15. COLLISION FREE PROTOCOL
• Almost collisions can be avoided in CSMA/CD.they can still occur during the
contention period.the collision during contention period adversely affects the
system performance, this happens when the cable is long and length of packet are
short. This problem becomes serious as fiber optics network come into use. Here we
shall discuss some protocols that resolve the collision during the contention period.
• Bit-map Protocol
• Binary Countdown

16. BITMAP PROTOCOL
• Protocols like this in which the desire to transmit is broadcast before the actual
transmission are called reservation protocols.
Low load situation: the bit map repeats over and over.

17. BINARY COUNTDOWN PROTOCOL
• A problem with the basic bit-map protocol
• is that overhead is 1 contention bit slot per
• station. We can do better than that by using
• binary station addresses.
1. Each station has a binary address. All
• addresses are the same length.
2. To transmit, a station broadcasts its address
• as a binary bit string, starting with highorder
• bit.
3. The bits in each address position from
• different stations are BOOLEAN ORed
• together (so called Binary countdown).

18. LIMITED CONTENTION PROTOCOL
• Limited Contention Protocols are the media access control (MAC) protocols that
combines the advantages of collision based protocols and collision free protocols.
They behave like slotted ALOHA under light loads and bitmap protocols under
heavy loads.
Working Principle
• Limited contention protocols divide the contending stations into groups, which may
or not be disjoint. At slot 0, only stations in group 0 can compete for channel access.
At slot 1, only stations in group 1 can compete for channel access and so on. In this
process, if a station successfully acquires the channel, then it transmits its data
frame. If there is a collision or there are no stations competing for a given slot in a
group, the stations of the next group can compete for the slot.

19. ADAPTIVE TREE WALK PROTOCOL
The algorithm devised by US Army for testing soldiers for
syphilis during WW II:
1. The Army took a blood sample from N soldiers.
2. A portion of each sample was poured into a single test
tube. The mixed sample was then tested.
3. If no antibodies were found, all the solders in the
group were declared healthy.
4. Otherwise, two new mixed samples were prepared,
one for soldiers 1 through N/2 and one from the rest.
5. The process was repeated recursively until the
infected soldiers were determined.
The computer version of this algorithm organizes the
stations in a binary tree, as illustrated below.

20. WAVELENGTH DIVISION MULTIPLE
ACCESS PROTOCOLS
• Wave length division multiple access protocol allows multiple transmission at the
same time . Here the spectrum is divided into channels. In this protocol, WDMA
each station is assigned two channels . A narrow channel is provided as a control
channel to signal the station and a wide channel is provided so the station can
output data frames.
• A different approach to channel allocation is to divide the channel into
subchannels using FDM, TDM, or both, and dynamically allocate them as
needed. This is commonly used on fiber optic LANs

21. WAVELENGTH DIVISION MULTIPLE
ACCESS PROTOCOLS

22. WIRELESS LAN PROTOCOL
• Wireless LANs refer to LANs (Local Area Networks) that use high frequency radio
waves instead of cables for connecting the devices. It can be conceived as a set of
laptops and other wireless devices communicating by radio signals. Users connected
by WLANs can move around within the area of network coverage. Most WLANs are
based upon the standard IEEE 802.11 or WiFi.

23. CONFIGURATION OF WIRELESS LANS
• Each station in a Wireless LAN has a wireless network interface controller. A station
can be of two categories −
• Wireless Access Point (WAP) − WAPs or simply access points (AP) are generally
wireless routers that form the base stations or access points. The APs are wired
together using fiber or copper wires, through the distribution system.
• Client − Clients are workstations, computers, laptops, printers, smart phones etc.
They are around tens of metres within the range of an AP.