Packet Dynamics

1
Packet Dynamics

• Jean Walrand
• U.C. Berkeley
• www.eecs.berkeley.edu/wlr

2
Outline

• Packet Delay
• Queuing Delay

3
Packet Delay

• Packet Delay TRANS PROP QD
• TRANS Transmission Time Number of bits/Rate
  P/R
• PROP Propagation Time Distance/Speed
  D/SSpeed 5ms/km in fiber 4ms/km in
  copper 3.3ms/km in space
• QD Queuing Delay This delay depends on the
  load of the router and burstiness of the
  traffic.

4
Packet Delay - continued

• Illustration

5
Packet Delay - continued

• Examples

6
Queuing Delay

• Isolated Packets

X(t) number of bits at time t
R bps
X(t)
P
P/R
Time t
QD 0
7
Queuing Delay (continued)
X(t) number of bits at time t

• Packet Bursts

R bps
Average value of QD (0 1 2)TRANS/3
TRANS For an isolated burst of size N
(0 1 N - 1 )TRANS/N (N 1)TRANS/2
P
QD3
P
X(t)
QD2
QD (N 1)TRANS/2
P
P/R
P/R
P/R
Time t
8
Queuing Delay (continued)

• Rule of thumb .
• 80 utilization gt QD ? 4TRANS
• Example

P 1,000 bits 80 utilization on each link
QD ? 410ms (n 1)0.01ms 10ms ? 80ms PROP
5?1500?s 7.5ms TRANS 10ms (n 1)0.01ms
10ms ? 20ms gt Delay ? 108ms