Media Access Control | Computer Networks - Computer Science Engineering (CSE) PDF Download

MEDIUM ACCESS CONTROL
This algorithm is commonly called the Ethernet‟s media access control (MAC). It is typically implemented in hardware on the network adaptor.

FRAME FORMAT:
Media Access Control | Computer Networks - Computer Science Engineering (CSE)

 

Preamble allows the receiver to synchronize with the signal. Both the source and destination hosts are identified with a 48-bit address. Each frame contains up to 1,500 bytes of data. A frame must contain at least 46 bytes of data, even if this means the host has to pad the frame before transmitting it. Each frame includes a 32-bit CRC.

ADDRESSES:
It is usually burned into ROM. Ethernet addresses are typically printed in a form humans can read as a sequence of six numbers separated by colons.

Each number corresponds to 1 byte of the 6-byte address and is given by a pair of hexadecimal digits, one for each of the 4-bit nibbles in the byte; leading 0s are dropped.

To ensure that every adaptor gets a unique address, each manufacturer of Ethernet devices is allocated a different prefix that must be prep-ended to the address on every adaptor they build.
UNICAST
MULTICAST
BROADCAST

TRANSMITTER ALGORITHM:
The receiver side of the Ethernet protocol is simple; the real smarts are implemented at the sender‟s side. The transmitter algorithm is defined as follows:
When the adaptor has a frame to send and the line is busy, it waits for the line to go idle and then transmits immediately.
The Ethernet is said to be a 1-persistent protocol because an adaptor with a frame to send transmits with probability 0<=p<=1 after a line becomes idle, and defers with probability q=1-p. Because there is no centralized control it is possible for two (or more) adaptors to begin transmitting at the same time, either because both found the line to be idle or because both had been waiting for a busy line to become idle.
When this happens, the two (or more) frames are said to collide on the network. Each sender, because the Ethernet supports collision detection, is able to determine that a collision is in progress. At the moment an adaptor detects that is frame is colliding with another, it first makes sure to transmit a sure to transmit a 32-bit jamming sequence and then stops the transmission.
Thus, a transmitter will minimally send 96 bits in the case of a collision: 64-bit preamble plus 32-bit jamming sequence. One way that an adaptor will send only 96-bits which is sometimes called a runt frame is if the two hosts are close to each other. Had the two hosts been farther apart, they would have had to transmit longer, and thus send more bits, before detecting the collision.
In fact, the worst-case scenario happens when the two hosts are at opposite ends of the Ethernet. To know for sure that the frame it just sent did not collide with another frame, the transmitter may need to send as many as 512 bits.
Not coincidentally, every Ethernet frame must be at least 512 bits (64 bytes)long: 14 bytes of header plus 46 bytes of data plus 4 bytes of CRC.
Where hosts A and B are at opposite ends of the network. Suppose host A begins transmitting a frame at time t, as shown in (a). it takes it one link latency (let‟s denote the latency as d) for the frame to reach host B.
Thus, the first bit of A‟s frame arrives at B at time t+d, as shown in (b). Suppose an instant before host A‟s frame arrives (i.e., B still sees and idle line), host B begins to transmit its own frame.
B‟s frame will immediately collide with A‟s frame, and this collision will be detected by host B(c). host B will send the 32-bit jamming sequence, as described above.(B‟s frame will be a runt).
Unfortunately, host A will not know that the collision occurred until B‟s frame reaches it, which will happen one link latency later, at time t+2xd, as shown in (d). Host A must continue to transmit until this time in order to detect the collision. In other words, host A must transmit for 2xd should be sure that it detects all possible collisions.
Considering that a maximally configured Ethernet is 2,500 m long, and that there may be up to four repeaters between any two hosts, the round-trip delay has been determined to be 51.2 microseconds, which on a 10-Mbps Ethernet corresponds to 512 bits.
The other way to look at this situation is that we need to limit the Ethernet‟s maximum latency to a fairly small value (e.g., 512micro seconds) for the access algorithm to work; hence, an Ethernet‟s maximum length must be something on the order of 2,500m.
Once an adaptor has detected a collision and stopped its transmission, it waits certain amount of time and tries again. Each time it tries to transmit but fails, the adaptor doubles the amount of time it waits before trying again.
This strategy of doubling the delay interval between each retransmission attempt is a general technique known as exponential back off. More precisely, the adaptor first delays either 0 or 51.2 microseconds, selected at random. If this effort fails, it then waits 0, 51.2, 102.4, or 153.6 microseconds (selected randomly) before trying again; this is kx51.2 for k=0...2^3-1, again selected at random.
In general, the algorithm randomly selects a k between 0 and 2^n-1 and waits kx51.2 microseconds, where n is the number of collisions experienced so far. The adaptor gives up after a given number of tries and reports a transmit error to the host. Adaptor typically retry up to 16 times, although the back off algorithm caps n in the above formula at 10.

Media Access Control | Computer Networks - Computer Science Engineering (CSE)

The document Media Access Control | Computer Networks - Computer Science Engineering (CSE) is a part of the Computer Science Engineering (CSE) Course Computer Networks.
All you need of Computer Science Engineering (CSE) at this link: Computer Science Engineering (CSE)
21 videos|107 docs|66 tests

Up next

FAQs on Media Access Control - Computer Networks - Computer Science Engineering (CSE)

1. What is Media Access Control (MAC) in computer science engineering?
Ans. Media Access Control (MAC) is a sublayer of the Data Link Layer in computer networking. It is responsible for controlling access to the physical medium and managing the communication between devices connected to a network. MAC addresses are unique identifiers assigned to network interface controllers (NICs) by the manufacturer.
2. How does Media Access Control ensure fair and efficient access to the network?
Ans. Media Access Control uses various protocols and techniques to ensure fair and efficient access to the network. One commonly used protocol is Carrier Sense Multiple Access with Collision Detection (CSMA/CD), where devices listen to the medium before transmitting to avoid collisions. Additionally, MAC protocols may use techniques like token passing or polling to allocate access fairly among devices.
3. Can two devices have the same Media Access Control address?
Ans. No, two devices cannot have the same Media Access Control (MAC) address. MAC addresses are globally unique identifiers assigned to network interface controllers (NICs) by the manufacturer. Each MAC address consists of six groups of two hexadecimal digits, and the probability of two devices having the same MAC address is extremely low.
4. How is Media Access Control different from Internet Protocol (IP) address?
Ans. Media Access Control (MAC) address and Internet Protocol (IP) address serve different purposes in computer networking. MAC address is a hardware address assigned to a network interface controller (NIC) and is used for communication within a local network. On the other hand, IP address is a logical address assigned to a device and is used for communication over the internet or across different networks.
5. Can a Media Access Control address be changed?
Ans. Yes, a Media Access Control (MAC) address can be changed, but it requires specific tools and configuration changes. Changing the MAC address is often done for privacy or security reasons, as it can help prevent tracking or bypass network restrictions. However, it is important to note that changing the MAC address of a device may violate certain network policies or regulations.
21 videos|107 docs|66 tests
Download as PDF

Up next

Explore Courses for Computer Science Engineering (CSE) exam
Signup for Free!
Signup to see your scores go up within 7 days! Learn & Practice with 1000+ FREE Notes, Videos & Tests.
10M+ students study on EduRev
Related Searches

Media Access Control | Computer Networks - Computer Science Engineering (CSE)

,

Important questions

,

Summary

,

Sample Paper

,

shortcuts and tricks

,

Semester Notes

,

past year papers

,

video lectures

,

Free

,

practice quizzes

,

MCQs

,

Media Access Control | Computer Networks - Computer Science Engineering (CSE)

,

Previous Year Questions with Solutions

,

Extra Questions

,

Media Access Control | Computer Networks - Computer Science Engineering (CSE)

,

Objective type Questions

,

Viva Questions

,

ppt

,

pdf

,

mock tests for examination

,

study material

,

Exam

;