8.7 CARRIER SENSE MULTIPLE ACCESS (CSMA)

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In local area networks, the medium is either twisted copper pair or coaxial cable. This medium is shared by a number of computers (nodes). While sharing the medium, only one node can transmit at a time. If multiple nodes transmit simultaneously, the data is garbled. Sharing the medium is achieved through a protocol known as Medium Access Control (MAC) protocol. The MAC protocol works as follows. When a node has to transmit data, first it will sense the medium. If some other node is transmitting, it will keep sensing the medium until the medium is free, and then it will start the transmission. This process of checking whether the medium is free or not is known as carrier sense. Since multiple nodes access the medium through carrier sensing, the multiple access is known as carrier sense multiple access.

Now consider the two nodes A and C in Figure 8.10. Node A sensed the carrier, found it free, and started the transmission. For the data to reach the point where node C is connected, it will take some finite time. Before the data on the medium reaches that point, node C senses the carrier, finds the carrier to be free, and then it also sends its data. As a result, the data of A and C collide and get garbled. The garbling of data can be detected because there will be a sudden rise in the voltage level on the cable. It is not enough if the nodes find the carrier to be free, they also need to detect collisions. When a collision is detected by a node, that node has to retransmit its data, waiting for some more time after the carrier is free. If more collisions are detected, the waiting time has to be increased. This mechanism is known as carrier sense multiple access/collision detection (CSMA/CD). CSMA/CD is the protocol used in Ethernet LANs. A variation of CSMA/CD is CSMA/CA, in which CA stands for collision avoidance. In CSMA/CA, collisions are avoided by reserving the medium for a specific duration. We will study the details of these protocols in Chapter 17, "Local Area Networks," where we will study wired and wireless LANs.

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Figure 8.10: CSMA/CD.

In local area networks, the medium (cable) is shared by a number of nodes using CSMA/ CD. Before transmitting its data, a node senses the carrier to check whether the medium is free. If it is free, the node sends its data. If the medium is not free, the node waits for a random amount of time and again senses the carrier. If two or more nodes send their data simultaneously, it causes collision, and the nodes would need to retransmit their data.

Note

CSMA with collision avoidance is used in wireless local area networks. To avoid collisions, time slots have to be reserved for the nodes.

Summary

Multiple access techniques provide the mechanism to share the bandwidth efficiently by multiple terminals. Particularly in radio systems where the spectrum bandwidth is very limited, multiple access techniques are very important. In frequency division multiple access (FDMA), a pool of frequencies is assigned to a base station, and the terminals are allocated a specific frequency for communication. In time division multiple access (TDMA), one frequency will be shared by different terminals in different time slots. Each terminal is assigned a specific time slot during which the terminal has to send the data. In TDMA frequency division duplex (FDD), a pair of frequencies is used for communication uplink and downlink. In TDMA time division duplex (TDD), a single frequency is used for communication in both directions—the time slots will be divided into two portions—downlink slots and uplink slots. The code division multiple access technique requires large bandwidth, but the advantage is that it provides secure communication. In CDMA, there are two mechanisms: frequency hopping (FH) and direct sequence. In FH, the frequency of transmission changes very fast in a pseudo-random fashion. Only those terminals that know the sequence of hopping will be able to decode the data. In direct sequence CDMA, instead of sending the bit stream directly, chipping codes are used to transmit ones and zeros. In local area networks (LANs), the medium is shared by multiple nodes using carrier sense multiple access (CSMA) protocols.

References

R.O. LaMarive et al. "Wireless LANs and Mobile Networking: Standards and Future Directions." IEEE Communications Magazine, Vol. 34, No. 8, August 1996.
J. Karaoguz. "High Rate Personal Area Networks." IEEE Communications Magazine, Vol. 39, No. 11, December 2001.
http://www.3gpp.org The official site of the 3G partnership program. You can get the details of the CDMA-and TDMA-based wireless networks from this site.
http://www.cdg.org Web site of CDMA development group.
http://www.iec.org IEC web site, which has excellent online tutorials.
http://www.qualcomm.com Web site of Qualcomm Corporation. You can get information on CDMA technologies and products from this site.
http://www.standards.ieee.org The IEEE Standards web site. The details of various multiple access techniques used in local area networks can be obtained from this site.

Questions

Explain FDMA with examples.
What is the difference between TDMA-FDD and TDMA-TDD?
Explain how SDMA is used in mobile communication systems.
Explain frequency hopping with an example.
What is direct sequence CDMA?
Explain the variations of CSMA protocol.

Exercises

1.	Explore the details of the multiple access techniques used in the following systems: (a) mobile communication systems based on GSM standards, (b) Bluetooth radio system, (c) 802.11 wireless local area network, (d) HiperLAN, and (e) digital enhanced cordless telecommunications (DECT) system.
2.	Prepare a technical paper on the various multiple access techniques used in satellite communication systems.
3.	Explain the multiple access technique used in mobile communication system based on the Global System for Mobile Communications (GSM) standard.
4.	In TDMA systems, the channel data rate is higher than the data rate of the information source. Explain with an example.

Answers

1.	(a) Mobile communication systems based on GSM standards use FDMA/ TDMA. (b) A Bluetooth radio system uses frequency hopping. (c) 802.11 wireless local area network uses frequency hopping. (d) HiperLAN uses TDMA-TDD. (e) A digital enhanced cordless telecommunications (DECT) system uses TDMA-TDD.
2.	In satellite communication systems, TDM-TDM, TDMA, and CDMA technologies are used. The details are given in Chapter 13.
3.	In GSM, the service area is divided into small regions called cells. Each cell is assigned a number of channels that are shared by the mobile phones in that cell. Hence, FDMA is used. In addition, each channel is time-shared by eight mobile phones. Hence, TDMA is also used. So, the multiple access technique is referred to as FDMA/TDMA. Each radio channel has a pair of frequencies—one for uplink and one for downlink. Hence, the TDMA scheme is TDMA-FDD.
4.	In a TDMA system, each station will give a small time slot during which it has to pump its data. However, the source will generate the data continuously. Hence, the source has to buffer the data and send it fast over the channel when it gets its time slot. For instance, the user talks into his mobile phone continuously. But the mobile phone gets its time slot only at regular intervals. Hence, the mobile device has to buffer the digitized voice and send it in its time slot.

Projects

Make a paper design of a radio communication system that has one base station and 32 remote stations. The base station communicates with the remotes using broadcast mode. All the remotes listen to the base station transmission and, if the address transmitted matches with its own address, the data is decoded, otherwise discarded. The remotes transmit using TDMA. Design the TDMA time frame. Hint: Access is TDMA/FDD. Because there are 32 remotes, the address length should be minimum 5 bits. After carrying out the design, look for details in Chapter 12.
Simulate a direct sequence CDMA system. Replace 1 with 11-bit code and replace 0 with another 11-bit code. Study how these codes have to be chosen.

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