1. Calculating “a”

2. a Used as a term in analyzing many network characteristics Has a physical interpretation Represents the number of frames on a link.

3. Link Workstation A Workstation B Connection between workstations/computers

4. Multiple frames on a link Workstation A Workstation B About 6 2/3 frames on a link

5. Less than ONE frame on a link Workstation A Workstation B

6. How do you determine “a”? First we have to do a little background Find the transmission time Find the propagation time Divide the propagation time by transmission time But… consider that it is possible to have the head of the message arrive at the destination before the tail leaves from the sender (last overhead -> a 1).

7. Process of sending Workstation A Workstation A Workstation A Workstation A 1. Begin transmission 2. Finish transmission Workstation A Workstation A 3. In transit Workstation A Workstation A 4. Finish propagation Transmission time = Time 2. – Time 1. Propagation time = Time 4. – Time 2.

8. Transmission Time (Delay) Network “speed” is really transmission rate. Faster rate means shorter bit width (time). Example: 1000 bps means each bit is 1 millisecond 1Mbps means each bit is 1 microsecond Divide frame size by rate to get transmission time Example: 1000 bit frame and 10Mbps network rate 1k / 1M = 10 3 /10 7 = 10 -4 =.1 millisecond

9. Propagation time Time to travel from a to b. time = distance/speed Dependent on the medium: optical -> speed of light (3x10 8 m/s) electrical -> 2/3 speed of light (2x10 8 m/s) Larger frames have longer transmission times but identical propagation times. Example: signal propagation on electrical cable 2Km 2000/2x10 8 = 10 3 /10 8 = 10 -5 = 10x10 -6 = 10 microseconds

10. Back to “a” a = propagation time / transmission time Example: 100 milliseconds to propagate and 10 milliseconds to transmit means 10 frames will be transmitted before first one begins to arrive -> a=10 Example: 10 milliseconds to propagate and 100 milliseconds to transmit means 1/10 of the frame will be transmitted before the first head begins to arrive -> a=0.1