carrier sensing multiple access - collision detection

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Polytechnic University of Puerto Rico

Electrical Engineering Department

CECS 3302 Data and Computer Communications

Data Communication Simulations

The sensitivity of delay to the maximum allowed packet size

Julin Durn 48732

October 27 2009

Professor Paul Bartus


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Introduction

The type of protocol used in IEEE 802.3 standard for Ethernet includes the CSMA/CD, (Carrier Sensing

Multiple Access/Collision Detection). This is is a multiplexing scheme of Ethernet used to control the traffic in a

full duplex transmission network. A broadcast network is a single communication channel that is shared by all

machines on the network. Ethernet can be a broadcasting network, which broadcasts packets. Packets are short

messages sent by any machine. They contain the address to whom the message is intended. Besides

broadcasting, a machine may multicast, which means that it may send a message to some of the other

machines, not all of them.

Since Ethernet is a shared medium, packets collide because machines are sending data simultaneously

using the same line. When packets are sent to all machines (broadcast) the sent packets compare their

destination address with the recipient machine address. Only the machine intended will retain the packet, if the

packet was not meant for a machine that still received it, it will discard it. As you can see this is a rather

ineffective and inefficient way of transmission, since we are wasting valuable time on the line. In CSMA, we

broadcast and sense collisions, or listen to the line to check if someone is transmitting. Collisions still occur with

CD, we obtain less collisions.

We can improve the performance of our simple network greatly if we introduce carrier sensing (CS).

This way each host listens to data being transmitted over the channel. A host will only transmit its own frames

when it cannot hear any data being transmitted by other hosts. It listens for traffic on the link, when the frame

is finished, an inter-frame gap of 9.6us is allowed to pass before another host starts transmission.


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CSMA collisions

Because of propagation delay, collisions still occur. This means that 2 nodes may not hear each others

transmission. This collision means that the entire packet is wasted.

CSMA/CD

collisions are detected withing short time

colliding transmission aborted, reducing channel wastage.

Easy in wired LAN; we measure the signal strength, compare with trasnmitter and received signal.

Difficult in WLAN, because the receiver shuts off while transmitting

Ethernet:

the dominant LAN technology

very cheap

first widely used LAN technology

simple and cheaper than token ring LAN and ATM

has been able to keep up with speeds race (10, 100, 1000) Mbps

Characteristics of IEEE 802.3 standard

1. Uses Carrier Sensing Multiple Access/Collision Detection to provide a more efficient use of the link

2. Main topology used is the Bus

3. Each station has equal access to the network, but can only broadcast when the network is idle.

Before transmitting, station listens to the network to sense if another workstation is

transmitting(carrier sense). If network is still idle then it proceeds to transmit.

IEEE 802.3 is a collection of IEEE standards defining the Physical Layer and Data Link Layer's media

access control (MAC) sublayer of wired Ethernet. This is generally a LAN technology with some WAN


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Collision Detection

1. While transmitting, a station must perform CD, to detect if its message was destroyed

2. If collision is detected, the detecting station broadcast a collision or jam signal to alert the other

stations that a collision has occurred. Each transmitting station then waits a random amount of time

(10-90)ms before attempting retransmission.

Below is the algorithm to CSMA/CD


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Minimum Frame Length

To ensure that no node may completely receive a frame before the transmission node has finished

sending it , Ethernet defines a minimum frame size. The minimum frame size is related to the distance which

the network spans, the type of media being used and number of repeaters which the signal may have to pass in

order to reach the LAN. Together, these define a value known as Ethernet slot time, corresponding to 512 bit

times at 10Mbps.

The longest time between starting to transmitt a frame and receiving the first bit of a jam sequence is twice (2)

the propagation delay from one end to the other. This means that a frame must have enough bits to last twice

the propagation delay.

.

Example

for a cable of 400m @ Rb=10Mbps; with v = 2*108 m/s

td propagation delay

td = d/v = 400m/ 2*108 m/s = 2us

2td = 4us

for a bit time (tbit) = 1/Rb = 0.1 us

nb = 2(td/tb) = 4/.1 = 40 bits = 5bytes

the minimum frame length for the example would be 5bytes. A margin of error is usually added (we make it a

power of 2) so we use 64 bits = 8bytes.

The standard frame length is at least 512 bits (64 bytes). This does not present a problem with LANs of less than

2.5km.


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Simulations

Topic: The sensitivity of delay to the maximum allowed packet size.

For this research assignment we will investigate how the packet size affects the delay of the link transmission.

To test the sensitivity of our network we will compare graphically packet size vs. propagation delay of the

network. The software Opnet IT Guru will provide us with the simulations.

Our network will consist of an Ethernet server which provides communications to two networks connected via

a router. The networks consists of 1: 15 workstations linked by a switch and 2: 30 workstations linked by

another switch. The total area covered is 2 x 3 km wide.


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Our dependent variable will be the delay and the independent variable will be the packet size.

We performed various simulations using different packet size values ranging from below the standard to well

above the standard. These include 128, 512, 1028, 2048, 8192 bits.


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carrier sensing multiple access - collision detection

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