Communication Networks

Sanjay K. Bose

1

Lecture Set I

Introduction

Recommended Texts

Texts:

1. A. L.eon-Garcia and I. Widjaja: Communication Networks

2. A. S. Tanenbaum: Computer Networks

3. D. Bertsekas and R. Gallagar: Data Networks

References:

1. Kurose and Ross: Computer Networking

2. W. Stallings: Data and Computer Communication; Prentice-Hall, 1997.

2

Introduction

• An overview of networks, the Internet, services and protocols

• Circuit Switching, Packet Switching

• Store-&-Forward Operation in Packet Switching, Pipelining

• Layered Network Architecture, Summary overview of the TCP/IP Network Layers

3

4

• Network Devices

– Routers, servers, switches, multiplexers, hubs, modems, …

• Communication links

– fiber, copper, radio, satellite, ..

• Protocols control sending, receiving of messages

– e.g., TCP, IP, HTTP, FTP, PPP

Elements of the Internet

Local ISP

Campus LAN

Regional ISP

router workstation

server mobile

The Internet is a loosely hierarchical,

network of networks

5

Circuit Switching Direct connection through

interconnected wires

Circuit

Switched

Communication

Network

Packet

Switched

Packet switching preferred in computer communications

Packet Switching Data divided into packets and

sent through a network – no direct connection between the end nodes

6

Communication Through a Public Circuit Switched Network

Modem

Modem

Typical way of setting up a Circuit-Switched connection between computers using a Public Switched Network such as the PSTN

(Public Switched Telephone Network)

Circuit Switching

• Dedicated communication path between the end nodes set up through circuit switches of the network

• Three phases

– Circuit establishment set up circuit

– Data transfer use circuit to send/receive data

– Circuit disconnect disconnect the circuit

Must have sufficient switching capacity and channel capacity to establish connection

8

Circuit Switching

• Inefficient

– Channel capacity dedicated for duration of connection

– If no data is being transmitted, capacity is wasted

• Set up (connection) takes time

• Once connected, transfer is transparent

– Data is not examined by network, no delay

– Data rate is fixed for circuit-switching network

– Both ends must operate at the same rate

9

Communication Through a Packet Switching Network

• Network consists of interconnected routers or switches

• Data transmitted in small packets

– Data is organized into packets with size typically up to 1000 octets

– Each packet contains a portion of user data (Data) plus some control information (Header)

• Packets are received, stored briefly (buffered) and passed on to the next node

– Store and forward causes packet delay

Note that the Header is actually “overhead” as what we really want to send is just the Data. However, in packet switching the header is required to make sure that the packet reaches the destination

packets

Destination

packets

packets

packets

Routers or Switches

Source

10

Using Packet Switching

Intermediate nodes(routers) store the packet, decide the route, and forward the packet: processing delay

packet header contains routing information

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Packet Switching Implementations

• Station breaks long message into packets

• Packets sent one at a time to the network

• Packets handled in two approaches

– Datagram Service • For each packet, each node makes its own decision as to

how to forward it so that it eventually reaches its destination

– Virtual circuit • A path for the packet is pre-determined for the sender-

receiver pair and the packet always follows this path

12

Datagram Packet Switching

• Each packet contains a full destination address

• Packets can take any practical route

• Each packet treated independently

• Packets may arrive out of order (receiver must re-order the packets)

• Packets may go missing (end-nodes handle recovery of missing packets)

3

2

1 1 3 2

Store-&-Forward Operation required at

each node for each packet

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Virtual Circuit Packet Switching

• Preplanned route established before any packets sent

– Call setup establishes the route for data connection

– All packets in a connection follow the established route

• Each packet contains a virtual circuit identifier instead of destination address

• No routing decisions required for each packet

• Virtual circuit is not a dedicated path (may be shared)

3

2 1

3 2 1

Store-&-Forward Operation required at

each node for each packet

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Virtual Circuit vs Datagram Switching

• Virtual circuit

– Network can provide sequencing and error control

– Packets are forwarded more quickly

• No routing decisions to make

– Less reliable

• Loss of a node loses all circuits through that node

• Datagram

– No call setup phase

• More efficient for short message or fewer packets

– More flexible

• Routing can be used to avoid congested parts of the network

Intermediate nodes will need to do Store-&-Forward for both kinds of packet switching

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Advantages of Packet Switching

• Line efficiency

– Single node to node link can be shared by many packets over time

– Packets queued and transmitted as fast as possible

• Data rate conversion

– Each station connects to the local node at its own speed

– Nodes buffer data if required to equalize rates

• Packets are accepted even when network is busy

– Delivery may slow down

• Priorities can be used

• Robust against link and node failure

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Store-&-Forward Operation in Packet Switching

A B Switch

R bits/second R bits/second

• A transmits a message of F bits to B in one packet • Header of h bits attached to make a packet of F +h bits • Link Speed R bits per second (bps) • No link congestion so no queueing delay

Example

Time to transfer message

from A to the Switch

secondsF h

R

Time to transfer message

from the Switch to B seconds

F h

R

Total Time to Transfer message from A to B

2 secondsF h

R

Note that having more

intermediate switches

will cause more delays!

Can we do better than this?

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Store-&-Forward Operation in Packet Switching

A B Switch

R bits/second R bits/second

• Example (continued)

Time to transfer a packet

from A to the Switch 2 seconds

F h

R

Time to transfer a packet

from the Switch to B

Total Time to Transfer message from A to B

23 seconds

F h

R

2 seconds

F h

R

Pipelining (illustrated in the next slide)

•A transmits message of F bits as two segments of F/2 bits each • Header of h bits attached to each segment to form packet of F/2 +h bits • After receiving packet 1, the Switch starts forwarding it to B while it receives packet 2. After packet 2 is received (and packet 1 is transmitted), the Switch starts forwarding packet 2 to B.

Message may be divided into more than two segments

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“Pipelining” Pipelining packets (of a multi-packet message) may improve overall delay performance when packets go through store-&-forward nodes in between

A B

Store-&-Forward Switch/Node

tim

e one p

acket

two p

ackets

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Some Simple Thumb-rules for Pipelining in Store-&-Packet Transmissions

• Generally better to divide a long message into smaller segments even though the header bits have to be added to each segment to form each packet (i.e. more overhead bits)

• This is because Pipelining can then be used to reduce the overall message transfer delay

• This would be even more effective when there are multiple switches between the source and the destination nodes

• Avoid making the segments too small as then the added overhead of h bits per packet will begin to have a detrimental effect (i.e. higher delays)

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What is a protocol?

• Communications between computers requires very specific unambiguous rules

• A protocol is a set of rules that governs how two or more communicating parties are to interact

• The purpose of a protocol is to provide some type of communications services, e.g.

• Internet Protocol (IP) – packet transfer • Transmission Control Protocol (TCP) – transfer reliability • Hyper Text Transfer Protocol (HTTP) – web service • Simple Mail Transfer Protocol (SMTP) – email service

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Example of a Protocol (Calling Directory Assistance)

“Do you have an address?”

Caller

“What name?”

Give Name

Caller

replies

System replies

with number

System

replies

System

replies

System

replies

Operator

replies

“Thank you, please hold”

Give Address

Caller

replies

Give City Name

Dials Directory Assistance

“What city”?

Caller

replies Operator

replies

Caller

waits

“Thank you, please hold”

Caller

waits

Caller

dials

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Protocol “Layers” Networks are complex!

• many “pieces”:

– hosts

– routers

– links of various media

– applications

– protocols

– hardware, software

Question: Is there any hope of organizing

the structure of network?

This is where “layering” helps!

Each layer does a small bit of the work required and the overall work is divided between the layers

Basic Philosophy: Layer n at one node talks to Layer n at another node using the Layer n Protocol

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Internet protocol stack

• Application: supporting network applications – FTP, SMTP, HTTP

• Transport: host-host data transfer – TCP, UDP

• Network: routing of datagrams from source to destination – IP, routing protocols

• Link: data transfer between neighboring network elements – PPP, Ethernet

• Physical: bits “on the wire”

Application Transport Network Link Physical

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Layering: Physical Communication

application

transport

network

link

physical

application

transport

network

link

physical application

transport

network

link

physical

application

transport

network

link

physical

network

link

physical

data

data

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Protocol layering and data

Each layer takes data from above

• adds header information to create new data unit

• passes new data unit to layer below

application

transport

network

link

physical

application

transport

network

link

physical

source destination

M

M

M

M

H t

H t H

n H

t H n H

l

M

M

M

M

H t

H t H

n H

t H n H

l

message

segment

datagram

frame

Internet Protocol (IP)

• This is the protocol most commonly used for the Network Layer

• Routers interconnect different networks

• Host computers prepare IP packets and transmit them over their attached network (complexity at edge)

• Routers forward IP packets across networks

• Best-effort IP transfer service, no retransmission

Net 1 Net 2

Router

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Addressing & Routing

• Hierarchical address: Net ID + Host ID (e.g. 128.100.11.1)

• IP packets routed according to Net ID

• Routers compute routing tables using distributed algorithm

G

G

G

G G

G

Net 1

Net 5

Net 3

Net 4 Net 2

H

H H

H

Transport Protocols

• Host computers run two transport protocols on top of IP to enable process-to-process communications

• User Datagram Protocol (UDP) enables best-effort transfer of individual blocks of information

• Transmission Control Protocol (TCP) enables reliable transfer of a stream of bytes

Internet

Transport Protocol

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Internet Applications

• Internet applications run on either TCP or UDP

• TCP: HTTP (web); SMTP (e-mail); FTP (file transfer); telnet (remote terminal)

• UDP: DNS, RTP (voice & multimedia)

• TCP & UDP incorporated into computer operating systems

• Any application designed to operate over TCP or UDP will run over the Internet!!!