Transmission Times

There are several factors that determine how long it will take to send a quantity of data from one point to another. In general the time for data to be sent from one point to another is the sum of the transmission time (the time to send the data out of the source) and the propagation time (the time for the signal to travel to the destination). Assume:

 

X = number of bits to be sent (bits). This is the number of bytes of data multiplied by 8.

B = transmission line speed (bits/second). This is the rate at which the network interface can send bits down the line. Traditional Ethernet can send data at 10,000,000 bits/second while home modems often send data at 28,800 bits/second.

L = distance between source and destination (meters). This is the physical length of the communications channel or wire between source and destination.

C = propagation speed (meters/sec). The speed at which a signal travels down the wire. For both electrical and optical media signals propagate at about 2/3 the speed of light or 2.0x108 m/s

The transmission time can be calculated as:

There are three general transmission methods:

Circuit switching - An electrical connection is made between the source and the destination. The telephone system uses circuit switching when connecting local calls. Circuit switching generally requires some initial setup time. This is analogous to dialing the phone. After the connection is made, the data can be sent with no delay.

Message switching - All of the digital data is sent from the source to the destination as a unit. When there are intermediate nodes between the source and destination, each intermediate node must receive the entire message before sending it on to the next intermediate or final destination. This is called "store and forward" transmission. The intermediate nodes may have to make a decision as to which route the message will be sent. A header is attached to the beginning of the message to identify the destination.

Packet switching - This is similar to message switching except the data is divided into packets. The packets can be variable sized or (more often) fixed sized. The size of a packet is usually much smaller than the total data size. Packet sizes range from 48 bytes for ATM to 1500 bytes for Ethernet to 8K bytes for frame relay. Intermediate nodes must receive an entire packet before sending on towards the destination, but they do not have to receive the entire message. Each packet needs a header to identify its destination.

Consider an arbitrary network that has at least K nodes

Assume we want to send X bits from node A to node D. To simplify equations, we will use an average propagation delay in time instead of the distance divided by the speed of propagation. Some additional values we will need are:

D = propagation delay (seconds) = L/C is the average time it takes a signal to propagate from one node to the next intermediate.

P = packet size (bits). In packet switching systems this is the amount of data in the average packet.

R = circuit setup request size (bits). The amount of information sent to establish a circuit, which would normally include the destination address.

H = header size (bits). This is the size of an identifying header for message switching or packet switching systems.

S = switching time (seconds). The time required to find the proper path towards the destination.

K = number of hops or data lines used.

Circuit Switched

Circuit switched systems require an initial setup period. After the circuit is established, the data can be sent without any switching delays at the intermediate nodes.

The setup time can be further defined as the time to send a small circuit request to the destination plus a reply back to the destination.

Message Switching

Message switching does not require any initial setup, although header information must be appended to the beginning of the data. Once the entire message and header have been received, an intermediate node can determine the proper routing and then send the message to the next node.

Packet Switching

Packet switching allows the transmission of the packets to overlap. When the first intermediate node is transmitting packet 1, the source node can be sending packet 2. Each packet has a header, which adds to the overhead. Each packet must also be routed. In most real systems the route determined for the first packet is cached allowing the following packets to be routed quickly.

 

last modified January 02, 2004