1. CS 457 - Lecture 2 Network Performance
Fall 2011

2. How Is the Link Shared? Circuit Switched or Packet Switched
Circuit switching
dedicate link bandwidth & switch capacity to each “call”
Requires call setup
Guaranteed performance
Packet switching
Packet: small chunks of data
Send packets as soon as link available
Switch receives a full packet then forwards it towards the destination
How to share the same physical network
Guaranteed performance means exactly the defined performance, not the best performance
Some hosts may want to send faster than some others
Packet switching buffers data -> delay

3. Circuit Switching: FDM and TDM
Example:
2 users
FDM
frequency
time
network resources (e.g., bandwidth) divided into “pieces”
pieces allocated to calls
Notice that one must send at constant rate to fully use the allocated resource; otherwise it is wasted. May not matter to you, but may matter to others who want to go faster than the allocated rate.
Another problem: bursty behavior of most data sources
resource piece idle if not used by owning call (no sharing)
dividing link bandwidth into “pieces”
frequency division
time division Two simple multiple access control techniques.
Circuit switching net has no memory
Problems:
- Idle channels
- Reject further users when the system full
TDM
frequency
time

4. Packet Switching: Statistical Multiplexing
Store-and-forward
Packet switch can temporarily buffer up packets
Introduces queueing delay
Packets get dropped when the queue is full

5. How Many Users Can Share?
Given 1 megabits/sec (1 Mbps) link
Assume each user:
User send 100,000 bits/sec when “active”
User is active 10% of time
100% of capacity used if active users.
Circuit-switching:
Link can support 10 users
Packet Switching:
Link can support 35 users
Prob.(n > 10) 
Great for bursty data
resource sharing
no call setup
Excessive congestion: packet delay and loss
protocols needed for reliable data transfer, congestion control
Q: How to provide circuit-like behavior to less flexible applications?
performance (delay/thruput) bounds needed for audio/video apps
still a developing area….sometimes it is network problem and sometimes application (chapter 6)
1 Mbps link

6. Circuit Switching Performance
Given 1 megabits/sec (1 Mbps) link
Divided into 10 distinct slices (TDM or FDM)
No Interaction between users
Potential bandwidth for my connection is Mbps/10 = 100,000 bps
How long to send 4 Mb file?
4 Mb / 100,000 bps = 40 sec
Plus some delay to setup connection
Assume 1 seconds
Total of 41 seconds
Throughput = 97,560 bps
How long to send 100,000b file?
100,000b / 100,000bps = 1 sec
Total of 2 seconds
Throughput = 50,000 bps
Great for bursty data
resource sharing
no call setup
Excessive congestion: packet delay and loss
protocols needed for reliable data transfer, congestion control
Q: How to provide circuit-like behavior to less flexible applications?
performance (delay/thruput) bounds needed for audio/video apps
still a developing area….sometimes it is network problem and sometimes application (chapter 6)
1 Mbps link

7. Packet Switching Performance
= L / R
Transmission
R = link bandwidth (bps)
L = packet length (bits)
Propagation
d = length of physical link
s = propagation speed in medium (~2x108 m/sec)
Queueing = #packets in queue X transmission time of each packet
= d/s
transmission A
propagation C B
queueing

8. Delay on A Single Link Relevant Specifications Assumptions
Bandwidth: R = 1 Mbps
Packet Size: L = 1000 bits
Link length: d = 100 km
Propagation Speed: s = 2.0 x 10^8 m/sec (typical fiber)
Assumptions
qlength = 2 Packets in queue when our packet arrives
Total Delay = transmit + prop + queue
= (L/R) + (d/s) + (qlength * L/R)
= (1000/ ) + (100000/2 x 10^8) + (2*1000/ )
= 3.5 ms

9. Some Comments about Units
Bits or Bytes
Bits denoted by “b”
Bytes denoted by “B”
Mb = megabits while MB = megabytes
Kb = kilobits while KB = kilobytes
How big is K and M? It depends….
Mega = 2^20, Kilo = 2^10
Mega = 10^6, Kilo = 10^3
Bandwidth uses powers of 10
Tied to MHz which is 10^6 hertz
So bandwidth of 1 Mbps = 10^ bits per second
Messages use powers of 2
Tied to computer memory measures in powers of 2
So packet/file/message of 1 Mb = 2^20 bits

10. Example Problem Find total time to transfer a file assuming
File size is 1.5 MB, RTT is 80 ms, Packet size is 1KB
Initial 2*RTT “handshake” before sending data
Bandwidth is 10 Mbps and packets sent continuously
Total Time to get all bytes to receiver is: Handshake + transmit prop 2*RTT FileSize/R + ½ RTT 2 * 80 ms MB/ 10 Mbps + ½ 80 ms 160 ms , b/ 10,000,000 b/s + 40 ms approx seconds

11. Network latency Time to send a packet from point A to point B2 1 3
A: my laptop B:
Time to send a packet from point A to point B
sum of delays across each hop along the path
RTT: round-trip-time

12. Bandwidth and Latency Bandwidth matters L large => L/R dominates
Increasing R reduces latency
Prop delay matters
L small => L/R also small
Increasing R has no impact

13. Examples Link Type Bandwidth Distance RTT BW x Delay Dial-up 56Kb/s
10Km
87 ms
5 bits
Wireless Lan
54Mb/s
50m
0.33ms
18 bits
Satellite
45 Mb/s
33,000 Km
230 ms
10 Mb
Cross-country fiber
10Gb/s
4,000 Km
40 ms
400 Mb

14. Store and Forward (1/2) Example: L = 8000 bits (1000bytes) R = 2 Mbps
Delay(A-B) = 3L/R = 12 msec
Store and Forward: Entire packet must arrive at router before it can be transmitted on next link:
Let dqueue = dprop = 0
Takes dtrans = L/R seconds to transmit (push out) packet of L bits on to link of R bps

15. Application Performance
Application input to network
Traffic data rate
Traffic pattern (bursty or constant bit rate)
Traffic target (multipoint or single destination, mobile or fixed)
Network service delivered to application
Delay sensitivity
Loss sensitivity
What the application delivers to the network? What the application needs from the network: a target data rate, target latencies etc. Talk about each one of these and how it affects network design. What kinds of tradeoff can we do? How traffic burstiness affects the aggregate characteristics; in turn, how are applications affected by delay and loss. Explain burstiness.

16. Email, Reliable File Transfer
Loss sensitive
Not delay sensitive relative to round trip times
Point-to-point
Bursty
Eg. FTP. Cannot tolerate loss, can tolerate modest amounts of delay, not inherently bursty, but because of some control algorithms… Analogy of humans transferring (taking dictation). Other characteristics: large packet sizes etc.

17. Remote Login Loss sensitive Delay sensitive Bursty Point to point
Subject to interactive constraints
Can tolerate up to several hundreds of milliseconds
Bursty
Point to point

18. Network Audio Relatively low bandwidth Delay variance sensitive
Digitized samples, packetized
Delay variance sensitive
Loss tolerant
Possibly multipoint, long duration sessions
Natural limit to number of simultaneous senders

19. Network Video High bandwidth Compressed video, bursty
Loss tolerance function of compression
Delay tolerance a function of interactivity
Possibly multipoint
Larger number of simultaneous sources

20. Web Transactional traffic Loss (bug?) tolerant Delay sensitive
Short requests, possibly large responses
Loss (bug?) tolerant
Delay sensitive
Human interactivity
Point-to-point (multipoint is asynchronous)