1. CS 5565 Network Architecture and Protocols
Lecture 3
Godmar Back

2. Announcements Assignment: Created CS5565 Forum on Piazza
Do Wireshark Lab 1
Don’t need to hand it in.
Created CS5565 Forum on Piazza
Use this to find a project partner
All 4 projects will be done in groups of up to 2.
CS 5565 Spring 2012

3. Summary
Terminology: hosts (end systems), communication links, routers, transmission rates, packets, internet vs. intranet vs. the Internet
Protocols: protocols define format, order of messages sent and received among network entities, and actions taken on msg transmission, receipt
View from network edge:
Client/server, peer2peer, other models
Service view
Communication infrastructure provide connection-oriented + connectionless service
View from network core:
Circuit-switching vs packet-switching
Datagram network vs. virtual-circuit networks
CS 5565 Spring 2012

4. How do loss and delay occur?
packets queue in router buffers
packet arrival rate to link exceeds output link capacity
packets queue, wait for turn B A
packet being transmitted (delay)
packets queueing (delay)
free (available) buffers: arriving packets
dropped (loss) if no free buffers
CS 5565 Spring 2012

5. Four sources of packet delay
1. Nodal processing delay:
check bit errors
determine output link
2. Queuing delay
time waiting at output link for transmission
depends on congestion level of router A B
propagation
transmission
nodal
processing
queueing
CS 5565 Spring 2012

6. Queuing Delay Show Applet here Queuing delay depends on
Queuing delay depends on
traffic intensity
nature of packet arrival process (bursts, periodic, periodic bursts, random intervals)
CS 5565 Spring 2012

7. Queueing Delay traffic intensity = La/R
R=link bandwidth (bps)
L=packet length (bits)
a=average packet arrival rate
traffic intensity = La/R
La/R ~ 0: average queueing delay small
La/R  1: delays become large
La/R > 1: more “work” arriving than can be serviced, average delay infinite!
CS 5565 Spring 2012

8. Queuing Analysis Source: Ts : mean service time
Stallings
Ts : mean service time
Coefficient of variation determines increase in delay
CS 5565 Spring 2012

9. Queuing Analysis (II)
Depending on estimated coefficient of variance, pick appropriate server model
M/M/1: both arrival rate & service time is “M”, Poisson process vs. negative exponentially distributed services
In general: X/Y/n, where n number of servers & X, Y one of
G: general independent
M: negative exponential distribution
D: deterministic (fixed length of service)
Q.: what’s the expected queue length for a D/D/1 queue if arrival rate < service rate?
CS 5565 Spring 2012

10. Delay in packet-switched networks
3. Transmission delay:
R=link bandwidth (bps)
L=packet length (bits)
time to send bits into link = L/R
4. Propagation delay:
d = length of physical link
s = propagation speed in medium (~2x108 m/sec)
propagation delay = d/s A B
propagation
transmission
nodal
processing
queueing
Note: s and R are very different quantities!
CS 5565 Spring 2012

11. Transmission vs. Propagation Delay
Show Applet here
Transmission delay depends on speed of link (10Mbps, 1000Mbps, …)
Propagation delay depends on distance (and speed of light in medium)
CS 5565 Spring 2012

12. Nodal delay dproc = processing delay dqueue = queuing delay
typically a few microsecs or less
dqueue = queuing delay
depends on congestion
dtrans = transmission delay
= L/R, significant for low-speed links
dprop = propagation delay
a few microsecs to hundreds of msecs
CS 5565 Spring 2012

13. Packet-switching: store-and-forwardL R R R
Takes L/R seconds to transmit (push out) packet of L bits on to link or R bps
Entire packet must arrive at router before it can be transmitted on next link: store and forward
Example:
L = 7.5 Mbits
R = 1.5 Mbps
delay = ?
(assume no propagation/processing/queuing delay)
15 seconds
CS 5565 Spring 2012

14. End-to-end vs. nodal delay
Question: in store-and-forward model, does end-to-end delay for a message of length L depend on the number/size of packets the message is split into?
Let’s look at:
CS 5565 Spring 2012

15. End-to-end vs. nodal delay
Question: in store-and-forward model, end-to-end delay for a message of length L depends on the number/size of packets the message is split into:
Transmission times of packets can be overlayed if multiple packets are part of a message
“Store-and-forward” model applies to packet, not message
Consequence: packetization reduces transmission delay
But you pay a price for header overhead
CS 5565 Spring 2012

16. Circuit vs. Packet Switching (2)
Circuit Switching
Dedicated link bandwidth
Dedicated switch capacity
Low link utilization
Low overall utilization
Bounded delay variance
Packet Switching
Better Link utilization
Better overall utilization
Need for congestion control
Need to identify to which “call” a packet belongs
Smaller average delay than TDM
High variance in delay
CS 5565 Spring 2012

17. Latency vs Bandwidth Latency (Delay) lags Bandwidth [Patterson 2004]
Similar pattern in many areas
CS 5565 Spring 2012

18. Bandwidth Delay Product
Delay d
Bandwidth b
Aka “size of pipe”
Important in protocol design
b in bps
d in ms
bxd in bytes
Gigabit Ethernet 1G
.150
18,750
100Mbit Ethernet
100M .7
8,750
Dialup Modem+Internet
56K
180
1,260
Cable Modem+Internet 4M
67,500
CS 5565 Spring 2012

19. Summary Transmission & Propagation Delay
End-to-end delay in packet-switched networks
Bandwidth-delay product
CS 5565 Spring 2012