EE122 Communication Networks



EE122 Communication Networks

I. Administrivia

TA: Ling Huang

Email: ee122-tc@imail.eecs.berkeley.edu

hling@cs.berkeley.edu

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

▪ 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, store-and-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-and-forward

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

[pic]

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

= 100μs

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-to-end 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-of-packet)

= (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+size-of-packet)

= S + (L/P)*(H+P)/B + N*D =0.2 + (3200/1024)*(16+1024)/9600 + 4*0.001=543 ms

4. Extra example

[pic]

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).

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