Network Design: Multiplexing, Inter-Process Communication, and Performance Metrics, Slides of Computer Networks

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• Lecture No.

3

Multiplexing

•^

Physical links/switches must be shared among users– (synchronous) Time-Division Multiplexing (TDM)– Frequency-Division Multiplexing (FDM)

L1 L2 L

R1 R2 R

Switch 1

Switch 2

Multiple flowson a single link

Do you see any problem with TDM / FDM?

5

Statistical Multiplexing

-^

An application needs to break-up its message in packets,and re-assemble at the receiver

-^

Fair allocation of link capacity: FIFO, round-robin or QoS

-^

If

congestion

occurs at a switch - buffer may overflow,

packets may be lost

…

6

Enough …?

•^

A network is delivering packets among acollection of computers

-^

How application processes communicate ina^

meaningful

way?

•^

Hide network complexity by implementingthe common services once

8

IPC Abstractions

•^

Semantics and interface depend on applications

-^

Request/Reply– distributed file systems

• file servers (FTP)
  • digital libraries / HTTP
    • information retrieval

•^

Message stream– video on-demand– video conferencing

• delay sensitive• two-way frame flow• 1/4 NTSC = 352

x240 pixels

• (

x

240

x

24)/8=247.5KB

• 30 fps = 7500KBps = 60Mbps• 10fps + compression < 10Mbps -^

reliable?

-^

prioritized?

-^

delay/bandwidth guarantees?

9

Abstract Channel Functionality

•^

What functionality does a channel provide ?– Smallest set of abstract channel types adequate

for largest number of applications

•^

Where the functionality is implemented ?– Network as a simple

bit-pipe

with all high-level

communication semantics at the hosts

• More intelligent switches allowing hosts to be

“dumb” devices (telephone network)

11

What Goes Undesirable in the Network?

Required performance at stake

•^

Messages are delayed

-^

Messages are delivered out-of-order

-^

Third party’s eavesdrop

-^

The challenge is to fill the gap betweenapplication expectations and hardwarecapabilities

12

Performance Metrics

• … and to do so while delivering

“good” performance

• Bandwidth (throughput)
  • data transmitted per unit time, e.g. 10 Mbps– link bandwidth versus

end-to-end

bandwidth

• notation
  • KB = 2

10

bytes

• Kbps = 10

3

bits per second

14

Performance Metrics

•^

Latency / delay–

time to send message from point A to point B– one-way versus round-trip time (

RTT

)

-^

components^ Latency = Propagation + Transmit + QueuePropagation = Distance / cTransmit = Size / Bandwidth

-^

Note:

• No queuing delay in direct (point-to-point) link• Bandwidth irrelevant if size = 1 bit• Process-to-process latency includes software processing overhead

(dominates over shorter distances)

15

Bandwidth versus Latency

•^

Relative importance, depends on application

-^

1-byte character:– Choice of 1ms vs 100ms dominates 1Mbps vs

100Mbps

•^

25MB file:– Choice of 1Mbps vs 100Mbps dominates 1ms vs

100ms

•^

Large data (file transfer) is bandwidth critical

-^

Small data (HTTP) is latency critical

17

Delay x Bandwidth Product

•^

Amount of data “in flight” or “in the pipe”

-^

Example: 100ms RTT

x

45Mbps BW = 560KB

•^

This much data must be buffered before the senderresponds to slowdown the request

Bandwidth

Delay

18

Infinite Bandwidth

•^

Latency (RTT) dominates instead of throughput^ – Throughput = TransferSize / TransferTime– TransferTime = RTT + 1/Bandwidth x TransferSize

-^

1 MB file over a 1 Mbps network takes around 8 sec– With RTT of 100ms, it corresponds to 80 RTTs– Effective throughput is 1MB/8.1s = 0.987Mbps

-^

1 MB file over a 1 Gbps network takes 100ms + 8ms– Effective throughput is 1MB/108ms = 74.1 Mbps

-^

1-MB

file

to 1-Gbps link apears like a 1-KB

packet

to 1-Mbps link

20

Wrap-up

•^

Established a comprehensive set of requirementsfor network design

-^

Networks evolve to accommodate changes inunderlying technologies and user demands

-^

However, hardware and user expectations are moving targets …

21

Network Architecture

•^

The challenge is to

fill the gap

between hardware

capabilities and application expectations, and to doso while delivering “good” performance

-^

Designers cope with this complex task bydeveloping a

network architecture

as a guideline

• Layering, protocols, standards