Speed, Rates, Times, Delays: Data Link Parameters for CSE 461

Propagation Speed

• Propagation speed: At best the signals propagate (radiate) through free space at the speed of lignt (3*10**8 m/s). In contrast, the speed of propagation through twisted pair wire or coax is 2/3 of this value. The speed at which electrical signals can travel and limited as a percentage of the speed of light. As such, it is typically at 2 * 10**8 m/s. Not also that the speed of 3*10**8 m/s applies to the unguided media in general, except for saltwater.

 

Throughput

• Throughput: Rate of information arriving at, and possible passing through, a particular point in a network system

 

Time to Transmit a Bit

• Time to transmit a bit: Time for transmitter to emit a single bit. The time it takes to physically place a bit on the wire. It is the difference between when the bit starts to be placed on the wire until it is finished. For example, if a certain device can transmit at a rate of 10 Mbps, the time it takes for a single bit to be transmitted is 1*10**(-7) seconds. This is used to determine the Bit Length. (See Homeworks for calculations!)

 

Bit Length

• Bit Length: The length of the signal on the wire. Following the above example, since it takes 1*10**-7 seconds to put a single bit on the line and bit speed is 2*10**8 m/s, it follows the Bit Length is (2*10**8)/(1*10**7) meters == 20 meters. (See Homework for calculations)

 

Propagation Delay

• Propagation Delay: There is always a short but finite time delay for a signal (electrical, optical, or radio) to propagate (or travel) from one end of the transmission medium to the other. This is known as the propagation delay. Although this may seem insignificant, in some situations this delay is important! It can be thought of as the time it takes for a single bit to travel from one point to another, where the two points are the two "ends" of the medium. (See Homeworks for how to calculate the propagation delay!)

• Propagation Delay (Alternate definition): Time requiired for data to travel over a network, from its source to its ultimate destination.

• Propagation Delay (One more definition): Time taken by a signal to propagate from transmitter to receiver across a communications medium.

 

Transmission Time

• Transmission Time of a Frame (Packet, ...): Time for transmitter to emit a single frame. This is the time it takes to physically place a frame on the wire. It is the difference between when the first bit of the frame starts to be placed on the wire until the last bit of the frame is finished. Both Transmission time of a frame and packet are analoguous to the Time to Transmit a bit. (See Lecture notes on Bandwidth Utilization for the illustration, example, and calculations!)Here is a illustration.

• Packet Time: Same as Transmission time of a packet.

• Frame Time: Same as Tranmission time of a frame.

   

Round-trip Delay

• Round-trip delay: Round-trip delay is associated with a data link. It is the time delay between the first bit of a block being transmitted by the sender and the last bit of its associated acknowledgement being received. (Clearly, this will be a function not only of the time taken to transmit the frame at the link bit rate but also on the propagation delay of the link.)

Bit Rate

• Bit Rate: The number of bits per second sent or received.

 

Data Rate

• Data Rate: The same as Bit Rate

 

Bit Error Rate

• Bit Error rate (BER): The measure of the probability of a bit being received in error. A BER of 10^(-5) means that, on average, one bit in 100,000 will be corrupted.

 

Prcessing Delay

• Processing delay: The delay a packet (frame, message) suffers while being processed in an intermediate or end node, according to the protocol rules for that packet (frame, message). Some examples are the time to do error checking (for the data link layers), mapping of the destination addresses from the packet onto outgoing circuits and then ports, generating acknowledgements if there are such defined for the protocol, transmitting the packet onto an outgoing circuit/port if this is not the end destination for the packed.