Access Control (CSMA CD)

If you have faced following questions anytime this tutorial is for you. :)

• How exactly packets are transmitted by the computers?
• If multiple computers are connected over a LAN (Local Area Network) then is there any controlling device which decides who will transmit at what instance of time.
• Is it really possible to have such central access control device?
• Can packet collision occur over LAN?
• How collisions are handled by the devices?
• What is access control, CSMA, CSMA/CD etc.

This tutorial will explains why access control is required and what are the requirements expected from the Access Control system. Then it explains more about the CSMA (Carrier Sense Multiple Access) technology, and how exactly the CSMA is modified to fit for IEEE 802.3 ethernet type of networks.

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Carrier Sense Multiple Access (CSMA) is a probabilistic Media Access Control (MAC) protocol in which a node verifies the absence of other traffic before transmitting on a shared physical medium, such as an electrical bus, or a band of electromagnetic spectrum. The carrier sense part ensures that every transmitter will listen for the carrier wave before sending any packet on the medium. If carrier is sensed on the medium, the transmitting node should wait for the on-going transmission to finish and only then starts its own transmission. If two or more nodes sense the medium at the same time, then there are good chances that both will start transmission at same time, and they will encounter collision.

The CSMA/CD (CSMA with collision detection) takes care of the above case with actively detecting collision and then sending jamming sequence.

Amit Vartak, 27 is working in wired and wireless security fields since last 3-4 years. His current area of interest includes IEEE 802.11 (Wi-Fi) suite of protocols, vulnerabilities in these protocols and countermeasure for those vulnerabilities. Working on cutting edge tools and technology always keeps him busy. He has contributed from concept level to final prototyping for the presentations in Defcon 2007 (The Emperor Has No Cloak - WEP Cloaking Exposed) and Toorcon 2007 (Caffe latte attack). He holds 2 patents with USPTO (current status: Patent Pending) and a few papers in IEEE journals on wireless protocol vulnerabilities. Prior to this, he was working on MEMS (Micro Electro Mechanical Systems) and has published a few papers in SPIE and ICMAT. (Yeah… kindda orthogonal fields… but technology really doesn’t limit the talent :) He did his masters in Electrical Engineering from one of the premier institutes in India, Indian Institute of Technology, Bombay (IIT-Bombay) and his under graduation, from University of Mumbai in Electronics and Telecommunication Engineering. He is currently working with AirTight Networks Inc. as a team lead in technology group since last 3 years.You can get in touch with him at amitcv[at]gmail[dot]com