Variations Of The Csma Protocol Computer Science Essay

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CSMA is a widely used medium access protocol. It stands for carrier sense multiple access. Carrier sense means that a station before transmitting a packet first "listens" to the channel so as to be sure that no other station is transmitting that time. Multiple access means that we have a broadcast link where we have multiple sending or receiving stations that are connected to the same broadcast channel. So all the protocols that belong to the "family" of the CSMA protocol, have the above characteristics. Another protocol belonging to the family of CSMA protocols is CSMA/CD. This means CSMA with collision detection. Collision detection means that when a station detects that another station is transmitting, then stops its transmission and tries again later. The time that will try again depends on a protocol. CSMA/CD protocol is used by the widely known LAN, the Ethernet. In the next chapters, is presented a description of the variations of the CSMA protocol.


2.1 1-persistent CSMA

When a station has data to transmit, first "listens" the channel so as to be sure that no one else station transmit at this moment. If the channel at that moment is used, the station waits until the channel becomes idle. When this happens, the station starts sending one frame. If a collision happens then the station has to wait for random time and then repeats the same action. This CSMA protocol is called 1-persistent because the station transmits with the probability of 1, when it understands that the channel is idle. A serious factor for avoiding collisions is the propagation delay of the channel. Propagation delay is the time needed for a signal to propagate from one station to another. If the signal of a station that has began transmitting has not reached the station which is ready to starts transmitting, then the second station will detect that the channel is idle so it will start its transmission. So the probability of collision increases when there is big propagation delay. Another scenario of collision using this protocol is when two or more stations wait for transmission while another station uses that time the channel. When this station ends its transmission and the channel becomes idle, the stations that are ready for transmission, will start transmission so there, a collision will occur. (Tanenbaum,2003)

2.2 Non-persistent CSMA

In this protocol, before a station starts transmitting, first "listens" to the channel to see if it is used by another station. If the channel is idle starts transmitting. If the channel is used, the station does not continue to detect the channel until it becomes idle, but waits for random time and then repeats the algorithm. With this protocol we have better exploitation but more delay than 1-persistent CSMA.

2.3 P-persistent CSMA

This protocol is used in a channel with slots. When a station is ready to transmit, first checks if the channel is used by another station. If the channel is not used, the station transmits with a probability of p. With a probability of q=1-p delays its transmission for the next slot. If this slot is idle too, then either transmits or delays the transmission again with probabilities of p and q. This action continues until a frame is transmitted or another station has started its transmission. (Tobagi and Hunt, 1980)

2.4 PR- CSMA

Ferrari and Tonguz (2003) proposed a modification of non persistent CSMA protocol called PR-CSMA. PR stands for per-route. The basic principles of this protocol are the following: "The source node of a communication route activates the route if the channel is idle. The remaining relay nodes of that route do not sense the channel, once the route has been activated for the remainder of the transmission" (2003, p2828). This protocol is described analytically in the same paper.


This protocol is the base of the Ethernet. In this protocol when a station has a packet for transmission first detects if the channel is idle. Then it continues with the transmission. If the channel is used then the station reschedules its transmission for another time. If two or more stations try to transmit the same time, then a collision will occur. These collisions are detected by comparing the power, or the width of the pulse that is received, with that of the transmitting signal.

When a station detects a collision then it delays its transmission, waits random time and then tries again, assuming that no other station has started its transmission (Tanenbaum, 2003).

The main drawbacks of this protocol are the following: The first is that the stations have to compete for the channel so there are many possibilities of collision. The second is that there is no priority and reservation mechanism, and the third is that the utilization of the network is inversely proportional to the bandwidth of the network. This means that this protocol is not suitable for LANS with high speeds. (Peng and Makki,2006)


RI stands for Reservations by Interruptions. CSMA/RI is an enhancement for the CSMA/CD proposed by Foh and Zucerman (2000). CSMA/RI adds reservations by interruptions to CSMA/CD. The reservation is done during the transmission of a packet. The transmission of the packet is interrupted by a pseudo noise that is broadcasted to all the stations. The transmission stopped by the interruption is resumed after the end of the interruption recovering the interrupted slot. After the successful transmission of the packet, the stations that are allowed to access the channel are those that have performed the reservation (RI stations). By the reservations, the probabilities of collision are reduced significantly. Foh and Zucerman based CSMA/RI protocol on the 1-persistent CSMA because it is more efficient than p-persistent CSMA or non persistent CSMA. The following figure and a further analysis of the above procedure can be found in the same paper (2000, p1574).


Fig.1. The broadcast channel of the CSMA/RI protocol


PRI stands for Priority Reservation by Interruptions. Guo and Kuo (2003) propose en enhancement on CSMA/CD protocol. This protocol categorizes the packets that are ready for transmission into grades. The packet with the highest grade is able to interrupt a transmission so as to reserve bandwidth. In CSMA/PRI protocol two channel reservations are done so as to reserve bandwidth by interruptions. The first selects the stations that are ready to transmit the packets with the highest grade. The second turns the stations that have passed the first reservation, into reservation stations. The problem that occurs when more than one station are reservation stations can be exceeded by using the BEB (Binary Exponential Backoff) policy. BEB assign new times for the stations to retransmit avoiding with this way a collision. More analytically, consider a system with slots. The interval is a length of . The propagation delay is. So as to avoid faults in collision detection the duration of the slot is a minimum of times. Guo and Kuo categorize the priority of the packets in three grades. Grade 0 is for the urgent packets such as real time-traffic and signaling information. Grade 1 is for the packets with service packets that are guaranteed and grade 2 is for the best effort packets. Grade is for the packets with the highest priority. When a station creates a packet, it gives the packet a label with a value that depends on the transmission requirements of the packet. If a station detects that the channel is idle then it transmits the packet. If the stations detect a successful transmission, the stations that are ready for transmission will transmit a pseudo noise for an interval of time in the slot , where is the great value of the packet. If a station that is ready for transmission hears the pseudo noise before doing the first reservation, then stop its attempt for transmission, else the station "thinks" that it can make the second reservation. But the stations that have packets with low priority do not have to make the priority reservation. If that stations did not hear the pseudo noise before the slot , then they continue with the second reservation ( is the grade with the smallest priority). The second reservation starts after the end of the first reservation. The stations that can go for the second reservation have to wait for a random number of slots. The number of the slots depends on the size of the packet that is going to be transmitted.


Peng and Makki in a resent paper (2006) propose an extension of the CSMA/CD protocol called CSMA/DM. DM stands for Dual Mode. This protocol works in two modes: In light mode and in heavy mode. The light mode is used when the LAN has a light load. This mode works nearly in the same way as the classic CSMA/CD works. The heavy mode has no collisions and is used when the LAN has a heavy load. This protocol has the ability to automatically switch between the two modes depending on the LAN load. An advantage of this protocol is that it does not need extra hubs or switches while having the same performance. The operations in the heavy mode are controlled by a station called monitor station. This station keeps a record of the busy stations. A busy station is one that has data frames ready for transmission while its MAC address is in the records of the monitor station. In CSMA/DM the stations that have no data frames ready for transmitting are called passive while the stations that have data frames ready for transmitting and whose MAC addresses are not in the records of the monitor station are called active. In the light mode we have only active and passive stations. The basic operation is the following: When a station has a packet ready for transmission first has to detect the operation mode of the LAN (light or heavy mode). If the LAN works in heavy mode the station has to wait for a frame called INSBF (inviting new busy stations frame). The station has to continue detecting the mode of the LAN because the LAN can switch to light mode while it waits for the INSBF frame. If the station receives an INSBF frame it will try to access the LAN in the interval of slots. This interval is specified in the INSBF frame. In the next figure is presented more analytically the operation of the CSMA/DM protocol (Peng and Makki, 2006, p904).


Fig.2. The basic operation of the CSMA/DM protocol


This protocol was introduced by Colbourn in a paper in 2007. PB stands for Power Backoff. CSMA/PB resolves the collisions, using the transmission power control by backing off in space. In this protocol there is used a four way handshake so as to transmit a data packet. Colbourn considers the following so as to analyze the protocol: a node that is transmitting a sequence of packets with data. is the power level needed by the node so as to transmit the data packet. If there are n transmission levels assume that and. At the beginning the power level of the transmission of the node is. So as to transmit the packet, node has to check if the channel is idle. If the channel is used then the node updates the NAV (network allocation vector). If the channel is idle then it transmits an RTS packet (request to send). The RTS packet includes the pi power level that has been used so as to transmit the packet. After the transmission there are two scenarios (Colbourn, 1237). First scenario: "If subsequently receives the corresponding CTS, then the data packet is transmitted at power level. If the transmission is successful, then receives an ACK". Second scenario: "If does not receive a CTS, then the RTS may have been involved in a collision. In this case, the current transmission power level () of is reduced and sends a new RTS at a reduced power level if the channel is free". In the previous paragraph ACK stands for acknowledgement and CTS stands for clear to send. In the following schemes you can see in more details the CSMA/PB protocol used by the transmitter and the receiver respectively (Colbourn, 2007, p1238).


Fig.3. Basic CSMA/PB transmitter protocol


Fig.4. Basic CSMA/PB receiver protocol

4. Conclusion

In the above paragraphs were described the classic CSMA and CSMA/CD protocols and their variations. These variations can help improve many drawbacks of the classic protocols. There are many factors that have to take under serious consideration so as to use them.

Some of these factors are the following: the speed of the network, the bandwidth, the number of the stations, the quality of status, the traffic, the power consumption or the hardware requirements. Summarizing CSMA/CD has great performance at networks with light load while having three main drawbacks mentioned in paragraph 3. CSMA/DM is efficient in high speed wireless LANS and does not need any extra hubs or switches which leads to hardware savings. It can also eliminate the drawbacks of the classic CSMA/CD. CSMA/RI has better performance than CSMA/CD especially for large packets. CSMA/PB improves the end to end throughput and consumes less energy than the CSMA protocol. CSMA/PRI improves the efficiency and can provide different quality of status for different services. Concluding none of the protocols is ideal for all the networks. The effectiveness of these protocols depends on the factors mentioned at the beginning of this paragraph.


Colbourn, Cui, Lloyd, Syrotiuk. A carrier sense multiple access protocol with power backoff (CSMA/PB). Science Direct AdHoc Networks, Vol.5, 2007, pp.1233-1250.

Ferrari, Tonguz. Performance of Ad Hoc Wireless Networks with Aloha and PR-CSMA MAC Protocols. IEEE Global Telecommunications Conference, Vol.5, 1-5 December, 2003, pp.2824-2829.

Foh, Zukerman. CSMA with Reservations by Interruptions (CSMA/RI): A novel Approach to Reduce Collisions in CSMA/CD. IEEE Journal on selected areas in communications, Vol.18(9), September 2000,pp.1572-1579.

Guo,Kuo. CSMA with Priority Reservation by Interruptions for Efficiency Improvement and QoS Support. IEEE Consumer Communications and Networking Conference, 3-6 January, 2005, pp.456-460.

Peng, Makki. The CSMA/DM LAN protocol. Wiley international journal of communication systems, Vol.19, 2006, pp.897-914.

Tanenbaum.(2003). Computer Networks, 4th Edition. Pearson Education.

Tobagi, Hunt. Performance Analysis of Carrier Sense Multiple Access with Collision Detection. Sciense direct Computer Networks, Vol.4(5), October-November, 1980, pp. 245-259.