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EE122 Communication Networks
I. Administrivia
TA: Ling Huang
Email: [email protected]
[email protected]
Office Hours: Tu 2-3pm, 382 Cory
Today:

Homework due on Friday

admin

questions about the class content, homework, projects and exams

overview

switching

performance and queueing

examples
Overview:

Last week:
o definition and components of computer network
o Goal of network: carry data from one part to another
o Components of network:

Hardware: links, routers/switches, end hosts

Software: protocols, applications
o the history of Internet
o Taxonomy of network:

circuit switching vs. packet switching

virtual circuit vs. datagram
o Services:

Routing with best-effort services

DNS: translation between IPs and names.
o Applications
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File transfer, email, web

P2P overlay network
o Performance and Queueing of the network
2. Switching Review
 Circuit switching:
o Characteristics: dedicated path, continuous transmission,


messages not stored
o 3-phase operation: circuit establishment, data transfer, circuit
disconnect.
o Channel sharing: TDM, FDM
o Pros: guaranteed service
o Cons: lower utilization, circuit setup time
Packet switching
o Characteristics: no dedicated path, transmission of packets, storeand-forward
o Operation: packet routing, packet queuing
o Pros: higher utilization
o Cons: no guaranteed service
Virtual circuit switching
o Characteristics: dedicated path, transmission of packets, store-andforward
o
Operation: forward packets along established circuit
o
Pros/Cons: tradeoff between packet and circuit switching
3. Performance and Queueing
3.1 Metrics of a network
Bandwidth (capacity): The rate (bits/second) of a communication channel, which is
the amount of data that can be passed along a communication channel in a given
period of time.
Utilization: the fraction of capacity in actual use measured over some interval of time
Throughput: the data rate (bits/ second) available to a particular application over an
interval of time.
Delay/Latency: period of time a packet is held by the network before being delivered
to the destination. It can be calculated as arrival time of last bit at receiver- arrival
time of first bit at transmitter.
Loss rate: we are not talk about this in detail here.
Bandwidth-Delay Product: product of bw and delay, indicates “storage” capacity of
network. Draw a picture, bandwidth is the width of a pipe, and delay is the length of
the pipe. Show them that B-D product is the volume of the pipe.
3.2 Latency in detail
Draw the first part of the picture and ask students what is the big formula:
Latency = Propagation Time + Transmission Time + Queuing Delay (+ Setup Time)
Draw the second part of the picture, interpret the picture
Propagation Time: Time for signal to travel length of network
Propagation Time = Distance/Speed of light
Transmission Time = Size/Bandwidth
QueuingDelay: time the packet need to wait before being transmitted because the
queue was not empty when it arrived. Queueing happens everywhere, switch, router,
even in your network card!
Throughput= Size / Latency
3.3 Simple Example
Consider a 100-Mbps optical link with a length of 10km. Assume the link transmits
packets of 10000 bits. (The speed of light is 2x108 m/sec). From the start of a packet
transmission until the last bit of the packet arrives at the receiver, it takes how much
time?
Latency
= TransmitTime + PropagationTime + QueuingDelay + SetupTime
= 10000/100x106 + 10/2x108s + 0 + 0
= 100s
3.4 Example (from "Data and computer communications," William Stallings)
Define the following parameters for a switching network:
N= number of hops between two given end systems
L= message length in bits
B= data rate in bits per second (bps), on all links
P= packet size
H= overhead (header) bits per packet
S= call setup time (circuit switching or virtual circuit) in seconds
D= propagation delay per hop in seconds
For N=4, L=3200, B=9600, p=1024, H=16, S=0.2, D=0.001, compute the end-toend delay for circuit, virtual-circuit, and packet switching. Assume there are no
acknowledgements, and no queuing delay.
circuit:
Latency = SetupTime + TransmitTime + PropagationTime + QueuingDelay
= (S) + (Size/Bandwidth) + (N*D) + 0
= S + L/B + N*D= 0.2 + 3200/9600 + 4*0.001 =537ms
packet:
Latency = TransmitTime + PropagationTime + QueuingDelay
= (Size/Bandwidth) + (N*D) + 0, where Size=(number-of-packets)*(overhead+size-ofpacket)
= (L/P)*(H+P)/B+N*D=(3200/1024)*(16+1024)/9600+4*0.001=343 ms
virtual circuit:
Latency = SetupTime + TransmitTime + PropagationTime + QueuingDelay
= (S) + (Size/Bandwidth) + (N*D) + 0, where Size=(number-of-packets)*(overhead+sizeof-packet)
= S + (L/P)*(H+P)/B + N*D =0.2 + (3200/1024)*(16+1024)/9600 + 4*0.001=543 ms
4. Extra example
a. Design a TDM multiplexer (time-division multiplexer) to multiplex these two flows. The
objective is for the delay of both flows A and B through the multiplexer to be at most 1ms
and have the smallest possible value for R. (Delay is measured from the arrival time of a
bit to the departure time of the same bit.) What is the value of R? Explain how the
multiplexer operates.
b. Statistical multiplexing. Now assume that each burst corresponds to one packet.
Consider a statistical multiplexer and assume that R=6Mbps. Find the worst delay per
packet through the multiplexer. (Packet delay is measured from the arrival time of the
last bit to the departure time of the last bit).