328

14 COMMUNICATION AND INTERNET TECHNOLOGIES

Communication and int ernet

technologies

In this chapter, you will learn about

+ the need for protocols during communication

+ the implementation of protocols such as a stack

+ TCP/IP protocols, including the four layers (Application, Transport,

Internet and Link), the purpose and function of the four layers, and

application when a message is sent from one host to another on

the internet

+ HTTP, FTP, POP3/4, IMAP, SMTP and BitTorrent protocols (such as

BitTorrent provides peer-to-peer file sharing)

+ circuit switching (including benefits and drawbacks)

+ the benefits and drawbacks of packet switching

+ the function of a router in packet switching

+ the use of packet switching to pass messages across the network

(including the internet).

14.1 Protocols

WHAT YOU SHOULD ALREADY KNOW

In Chapter 2, you learned about networks. Try

these five questions before you read the first

part of this chapter.

1 a) Explain the terms IP and TCP.

b) What are protocols and why are they used?

2 a) Explain how peer-to-peer networks

operate.

b) What are the pros and cons of peer-to-

peer networks?

3 a) Explain the term stack.

b) Explain the term queue.

c) Give examples of the use of a stack and the

use of a queue.

4 a) Explain what is meant by Ethernet.

b) What is meant by IP conflicts when using

Ethernet?

Explain how they can be overcome.

5 a) What is a DNS?

b) What is meant by HTTP?

c) Explain the role of status flags.

329

14.1 Protocols

Key terms

Protocol – a set of rules governing communication

across a network: the rules are agreed by both sender

and recipient.

HTTP – hypertext transfer protocol.

Packet – a message/data is split up into smaller groups

of bits for transmission over a network.

Segment (transport layer) – this is a unit of data

(packet) associated with the transport layer protocols.

FTP – file transfer protocol.

SMTP – simple mail transfer protocol.

Push protocol – protocol used when sending emails, in

which the client opens the connection to the server and

keeps the connection active all the time, then uploads

new emails to the server.

Binary file – a file that does not contain text only. The

file is machine-readable but not human-readable.

MIME – multi-purpose internet mail extension. A

protocol that allows email attachments containing

media files as well as text to be sent.

POP – post office protocol.

IMAP – internet message access protocol.

TCP – transmission control protocol.

Pull protocol – protocol used when receiving emails,

in which the client periodically connects to a server,

checks for and downloads new emails from a server

and then closes the connection.

Host-to-host – a protocol used by TCP when

communicating between two devices.

Host – a computer or device that can communicate with

other computers or devices on a network.

BitTorrent – protocol used in peer-to-peer networks

when sharing files between peers.

Peer – a client who is part of a peer-to-peer

network/file sharing community.

Metadata – a set of data that describes and gives

information about other data.

Pieces – splitting up of a file when using peer-to-peer

file sharing.

Tracker – central server that stores details of all other

computers in the swarm.

Swarm – connected peers (clients) that share a

torrent/tracker.

Seed – a peer that has downloaded a file (or pieces of a

file) and has then made it available to other peers in the

swarm.

Leech – a peer with negative feedback from swarm

members.

Lurker – user/client that downloads files but does not

supply any new content to the community.

14.1.1 The need for protocols

When communicating over networks, it is essential that some form of protocol

is used by the sender and receiver of the data. Both parties need to agree

the protocol being used to ensure successful communication takes place. In

Chapter6, we discussed parity checking as a way of determining whether

data was transmitted correctly. With this method, it was essential to agree

the protocol: even or odd parity. Without agreeing this protocol, it would be

impossible to use parity checking. Many different protocols exist since there

are several activities taking place over the internet.

The next section considers one of the most common sets of protocols, which

are implemented by using a stack structure with several layers.

14.1.2 TCP/IP protocols

This is the four-layer structure for TCP/IP protocols:

DARPA (Defense Advanced Research

Projects Agency) Layers

4 APPLICATION LAYER

3 TRANSPORT LAYER

2 INTERNET (NETWORK) LAYER

1 LINK NETWORK

V Figure 14.1 Four layer structure for TCP/IP

Sending Receiving

330

14 COMMUNICATION AND INTERNET TECHNOLOGIES

Using layers breaks the process down into manageable self-contained modules

(this process is known as decomposition), making it easier to develop and

easier to make software and hardware compatible.

When sending data across the internet (network), the layers are used in the

order layer 4 to layer 1; when receiving data across the internet (network), the

layers are used in the order layer 1 to layer 4. Each of the layers is implemented

using software.

Application layer

The application layer contains all the programs that exchange data, such as

web browsers or server software; it sends files to the transport layer. This layer

allows applications to access the services used in other layers and also defines

the protocols that any app uses to allow the exchange of data.

There are several protocols associated with the application layer:

HTTP hypertext transfer protocol; this is a protocol responsible for correct

transfer of files that make up web pages on the world wide web

SMTP simple mail transfer protocol; this handles the sending of emails

POP3/4 post office protocol; this handles the receiving of emails

IMAP internet message access protocol; this handles the receiving of emails

DNS domain name service; protocol used to find the IP address, for example,

when sending emails

FTP file transfer protocol; this is a protocol used when transferring messages

and attachments

RIP routing information protocol; this is the protocol routers use to exchange

routing information over an IP network

SNMP simple network management protocol; protocol used when exchanging

network management information between network management and

network devices (such as routers, servers and other network devices)

V Table 14.1 Protocols associated with the application layer

It is worth re-visiting the terms packet and rout er.

Messages are split up into small groups of bits called packets (for example, a

web page would be split up into a number of packets before sending over the

network).

A router is used to transmit packets of data; routers contain connections to

many other routers; when packets arrive at a router it decides where next to

send them.

Important terminology: packets are known as frames at the data-link layer, datagrams

at the internet layer and segments at the transport layer. Different names are used as

each layer adds its own header to the packet.

Do not confuse ‘frames’ in this context with ‘frames’ when discussing paging memory

management in Chapter 16.

Hypertext transfer protocol (HTTP)

HTTP

is probably the most important application layer protocol. Essentially,

this protocol underpins the world wide web. It is used when, for example,

fetching an HTML document from a web server (Figure 14.2).

331

14.1 Protocols

video server

adverts server

web server

Travel to Brazil with

Watson Travel Group

… visit our website

The region is very Germanic in nature with many

towns and villages resembling the Bavaria region.

Click on the video link below to find out more.

Check out this video:

https://www.youtube.com/watch?MHsl34P90plm

This is one of the most

beautiful mountain

ranges in the southern

part of Brazil.

page text

images

video links

advert links

layout of pages

Internet

V Figure 14.2 Fetching an HTML document from a web server

This makes use of hyperlinks (rules for the transferring of data over the

internet). HTTP is a client/server protocol: request messages are sent out to the

web servers which then respond.

HTTP protocols define the format of the messages sent and received. The web

browser (which is part of the application layer) initiates the web page request

and also converts HTML into a format which can be displayed on the user’s

screen or can be played through their media player.

The following summarises what happens when a user requests a web page from

a website.

Q The user keys the URL into their browser.

Q HTTP(s) transmits the request from the application layer to the transport layer (TCP).

Q The TCP creates data packets and sends them (via port 80) to the destination port(s).

Q The DNS server stores a database of URLs and matching IP addresses.

Q The DNS server uses the domain name typed into the browser to look up the IP

address of the appropriate website.

Q The server TCP sends back an acknowledgement (see the section on host-to-host

communication on page 333).

Q Once communication has been established, the web server sends the web page back

in HTML format to the browser.

Q The browser interprets the page and displays it or sends the data in the correct

format to the media player.

File transfer protocol (FTP)

The file transfer protocol (FTP) is a network protocol used when transferring

files from one computer/device to another via the internet or other networks.

It is similar to HTTP and SMTP, but FTP’s only task is the application protocol

for the transfer of files over a network. Web browsers can be used to connect

to an FTP address in a way similar to HTTP, for example,

ftp://username@ftp.example.gov/

332

14 COMMUNICATION AND INTERNET TECHNOLOGIES

Additional features of FTP include

» anonymous ftp – this allows a user to access files without the need to

identify who they are to the ftp server; for example, ‘331 Anonymous access

allowed’ would be a message received to confirm anonymous access

» ftp commands – a user is able to carry out actions that can change files

stored on the ftp server; for example, delete, close, rename, cd (change

directory on a remote machine), lcd (change directory on a local machine)

» ftp server – this is where the files, which can be downloaded as required by

a user, are stored.

A session would be started by typing in the ftp host_name (of remote system),

followed by a user id and password. The user would then be able to use ftp

commands to carry out a number of actions.

Simple mail transfer protocol (SMTP)

is a text-based (and connection-based)

protocol used when sending emails. It is sometimes referred to as a push protocol

(in other words, a client opens a connection to a server and keeps the connection

active all the time; the client then uploads a new email to the server).

Since SMTP is text-based only, it doesn’t handle

binary files (a binary file is a

file containing media/images as well as text and is regarded as being computer-

readable only). If an email contains attachments made up of, for example,

images, video, music then it is necessary to use the

multi-purpose internet

mail extension (MIME) protocol instead. A MIME header is used at the

beginning of the transmission; clients use this header to select which media

player is needed when the attachment is opened.

POP3/4 and IMAP (post office protocol and internet message access protocol)

Post office protocol (POP3/4)

and internet message access protocol (IMAP)

are protocols used when receiving emails from the email server. These are

known as pull protocols (the client periodically connects to a server; checks

for and downloads new emails from the server – the connection is then closed;

this process is repeated to ensure the client is updated). IMAP is a more a

recent protocol than POP3/4, but both have really been superseded by the

increasing use of HTTP protocols. However, SMTP is still used when transferring

emails between email servers.

Figures 14.3 and 14.4 give an overall view and a more detailed view of the

email protocol set up.

POP/IMAP

(receive email)

email server

SMTP

(send email)

V Figure 14.3 Overview of email protocol set up

333

14.1 Protocols

client’s ISP

email server

uses SMTP/MIME

protocol

client

internet

recipient’s domain

email server

uses POP/IMAP

protocol

recipient

V Figure 14.4 Detailed view of email protocol set up

The main difference between POP3/4 and IMAP is synchronisation:

POP3/4 IMAP

POP3/4 does not keep the server and

client in synchronisation; when emails are

downloaded by the client, they are then

deleted from the server which means it is

not further updated.

IMAP keeps the server and client in

synchronisation; only a copy of the email

is downloaded with the original remaining

on the server until the client manually

deletes it.

V Table 14.2

Transport layer

The transport layer regulates the network connections; this is where data is

broken up into packets which are then sent to the internet/network layer (IP

protocol). The transport layer ensures that packets arrive in sequence, without

errors, by swapping acknowledgements and retransmitting packets if they

become lost or corrupted. The main protocols associated with the transport

layer are

transmission control protocol (TCP), user datagram protocol (UDP)

and SCTP. We will only consider TCP.

Transmission control protocol (TCP)

TCP is responsible for the safe delivery of a message by creating sufficient

packets for transmission. It uses positive acknowledgement with re-

transmission (PAR) which means it automatically re-sends a data packet if

it has not received a positive acknowledgement. TCP is also connection-

orientated since it establishes an end-to-end connection between two host

computers using handshakes. For this last reason, TCP is often referred to as a

host-to-host transmission protocol.

The term

host has been used previously; this refers to a computer or device

that can communicate with another computer/device (host). Hosts can include

clients and servers that send/receive data, provide services or apps.

These are the steps taken when host computer ‘X’ communicates with another

host ‘Y’ (this is an expansion of what happens during TCP involvement shown in

the HTTP algorithm earlier on):

334

14 COMMUNICATION AND INTERNET TECHNOLOGIES

» Host ‘X’ will first of all send host ‘Y’ a segment (packet) which will include

synchronisation sequence bits so that segments will be received in the

correct order.

» Host ‘Y’ will now respond by sending back its own segment (containing an

acknowledgement together with its own synchronisation sequence bits).

» Host ‘X’ now sends out its own acknowledgement that the segment from ‘Y’

was received.

» Transmission of data between ‘X’ and ‘Y’ can now take place.

Internet/network layer and network/data-link layer

The internet layer identifies the intended network and host. The common

protocol is IP (internet protocol). The concept of IPv4 and IPv6 was covered in

depth in Chapter 2.

The network/data-link layer identifies and moves traffic across local segments,

encapsulates IP packets into frames for transmission, maps IP addresses to MAC

(physical) addresses and ensures correct protocols are followed. The physical

network layer specifies requirements of the hardware to be used for the

network. The data-link layer identifies network protocols in the packet header

(TCP/IP in the case here) and delivers packets to the network.

This is a summary of the IP functions:

» Ensure correct routing of packets of data over the internet/network.

» Responsible for protocols when communicating between networks.

» Take a packet from the transport layer and add its own header which will

include the IP addresses of both sender and recipient.

» The IP packet (datagram) is sent to the data-link layer where it is assembles

the datagrams into frames for transmission.

Ethernet protocols

Ethernet is a system that connects a number of computers or devices together

to form a LAN. It uses protocols to control the movement of frames between

computers or devices and to avoid simultaneous transmission by two or more

devices. It is a local protocol and does not provide any means to communicate

with external devices; this requires the use of IP which sits on top of the

Ethernet protocol.

Figure 14.5 shows the make-up of a typical frame used by the Ethernet protocol.

Pre-amble

(8 bytes)

Start frame

(1 byte)

Interpacket gap

(12 bytes)

Destination

(6 bytes)

Source

(6 bytes)

Ethernet type/

length (2 bytes)

Actual message

(46–1500 bytes)

Frame check

sequence (4 bytes)

Ethernet data

(64–1518 bytes)

Ethernet data

(64–1518 bytes)

V Figure 14.5 A typical frame used by the Ethernet protocol

If VLAN is used, the Ethernet data size increases from 1539 bytes to around

9000 bytes per frame.

The components that make up Ethernet data are

» destination – this is the MAC address of the destination computer or device

(it is possible to use the value FF:FF:FF:FF:FF:FF as the MAC address if the

335

14.1 Protocols

sender wishes to target every device (for example, to advertise services) or

if they do not know the MAC address of the destination device)

» source – this is the MAC address of the source computer (using the usual

MAC address format of 6 bytes)

» Ethernet type or length – if the frame length ൑ 1539 then the value here is

the length of the Ethernet frame; if the frame length

> 1539 then the value

here is the Ethernet type (IPv4 or IPv6 in our example)

» frame check – this will include a checksum to provide a method of checking

data integrity following transmission of the frame.

Wireless (WiFi) protocols

Wireless LANs (standard IEEE 802.11 protocol) use a MAC protocol called carrier

sense multiple access with collision avoidance (CSMA/CA) (not to be confused

with CSMA/CD considered in Chapter 2, since this is a totally different concept).

CSMA/CA uses distributed control function (DCF) to ensure a WiFi device can

only transmit when there is a free channel available. Since all transmissions are

acknowledged when using DCF, if a device does not receive an acknowledgement

it will assume a collision will occur and waits for a random time interval before

trying again. This is an important protocol to ensure the security and integrity

of data being sent over a wireless network (such as WLAN).

Bluetooth protocols

Bluetooth was considered in Chapter 2; it uses the standard IEEE 802.15

protocol for short-range data transmission/communication. There are numerous

additional Bluetooth protocols due to the many applications that may use this

wireless connectivity; this is outside the scope of this textbook.

WiMax

Worldwide interoperability for microwave access (WiMax) runs under IEEE 802.16

protocol. This connectivity was designed originally for wireless MANs (WMAN).

Fixed WiMax networks are based on the IEEE 802.16-2004 protocol, whereas

mobile WiMax is based on IEEE 802.16-2005 protocol.

Peer-to-peer file sharing/BitTorrent protocol

The BitTorrent is a protocol which is based on the peer-to-peer networking

concept (this was covered in Chapter 2). This allows for very fast sharing of

files between computers (known as peers). While peer-to-peer networks only

work well with very small numbers of computers, the concept of sharing files

using BitTorrent can be used by thousands of users who connect together over

the internet. Because user computers are sharing files directly with each other

(rather than using a web server) they are sharing files in a way similar to that

used in a peer-to-peer network; the main difference is that the BitTorrent

protocol allows many computers (acting as peers) to share files.

Suppose computer ‘A’ wishes to share a file with a number of other interested

peers. How can we use the BitTorrent protocol to allow this file sharing?

Initially, to share a file, the peer (computer ‘A’) creates a small file called a

torrent (for example, MyVideoFile.torrent). The torrent contains

metadata

about the file about to be shared.

The actual file is broken up into equal segments known as

pieces (typically a

20MiB file may be broken up into 20 × 1 MiB pieces).

336

14 COMMUNICATION AND INTERNET TECHNOLOGIES

Other peers who wish to download this file must first obtain the torrent and

connect to the appropriate

tracker – a central server that contains data about

all of the computers connected to it.

As each peer receives a piece of file they then become a source for that piece

of file. Other peers connected to the tracker will, therefore, know where to find

the piece of file they need.

Once a peer has downloaded a file completely and they make the file (or

required pieces of the file) available to other peers in the

swarm (a group of

peers connected together), they become a

seed. The more seeds in the swarm,

the faster the file downloading process between peers.

Logging off once the full file download has been completed is frowned upon by

the swarm community; such a peer is termed a

leech.

Usually, once a file is fully downloaded, a peer is requested to remain online so

they can become part of the seeding process until all peers have received the

whole file. Note that file pieces may not be downloaded sequentially and have

to be rearranged in the correct order by the BitTorrent protocol to produce the

final file (quite important if the file is a video!).

At the time of writing, BitTorrent was responsible for about 12% of the video file

sharing, for example, being carried out over the internet. This is only a fraction

of the video file activity which uses YouTube (which is about 50% of all of the

video file sharing over the internet), but is still a considerable amount of data.

Here is a summary of some of the terms used when discussing BitTorrent:

» Swarm – a group of peers connected together is known as a swarm; one

of the most important facts when considering whether or not a swarm can

continue to allow peers to complete a torrent is its availability; availability

refers to the number of complete copies of torrent contents that are

distributed amongst a swarm. Note: a torrent is simply the name given to a

file being shared on the peer-to-peer network.

» Seed – a peer that has downloaded a file (or pieces of a file) and has then

made it available to other peers in the swarm.

» Tracker – this is a central server that stores details about other computers

that make up the swarm; it will store details about all the peers

downloading or uploading the file, allowing the peers to locate each other

using the stored IP addresses.

» Leech – a peer that has a negative impact on the swarm by having a poor

share ratio, that is, they are downloading much more data than they are

uploading to the others; the ratio is determined using the formula:

amount of data the peer has uploaded

ratio =

amount of data the peer has downloaded

If the ratio > 1 then the peer has a positive impact on the swarm; if the

ratio < 1 then the peer has a negative effect on the swarm.

» Lurker – a peer that downloads many files but does not make available any

new content for the community as a whole.

In Figure 14.6, we will assume 12 peers have connected to the tracker. One peer

has begun to upload a video file and six peers are acting as seeds. Two peers

are behaving as leeches and three peers have just joined and have requested a

download of the video file. The arrangement would look something like this:

337

14.2 Circuit switching and packet switching

tracker

Key

new peers

requesting

download

original peer

uploading file

peers acting

as seeds

peers acting

as leeches

upload/down

load pieces

download file

only

upload file

only

request for file

download

V Figure 14.6 The arrangement of peers during the download and upload of file (pieces)

14.2 Circuit switching and packet switching

WHAT YOU SHOULD ALREADY KNOW

In Chapter 2, you learnt about communications. Try these two questions

before you read the second part of this chapter.

1 a) Explain how PSTN is used when making a phone call.

b) Explain how it is possible to use VoIP to make a video call over the

internet.

2 Draw a diagram to show how a message containing three packets of

data could be routed from computer ‘A’ to computer ‘B’.

Key terms

Circuit switching – method of transmission in which a dedicated circuit/channel lasts

throughout the duration of the communication.

Packet switching – method of transmission where a message is broken into packets

which can be sent along paths independently from each other.

Hop number/hopping – number in the packet header used to stop packets which

never reach their destination from ‘clogging up’ routes.

Header (data packet) – part of a data packet containing key data such as destination

IP address, sequence number, and so on.

Routing table – a data table that contains the information necessary to forward a

package along the shortest or best route to allow it to reach its destination.

338

14 COMMUNICATION AND INTERNET TECHNOLOGIES

14.2.1 Circuit switching

The concept of circuit switching was introduced in Chapter 2 during the

description of how a public switched telephone network (PSTN) was used to

make a phone call. Circuit switching uses a dedicated channel/circuit which

lasts throughout the connection: the communication line is effectively ‘tied

up’. When sending data across a network, there are three stages:

First, a circuit/channel between sender and receiver must be established.

2 Data transfer then takes place (which can be analogue or digital);

transmission is usually bi-directional.

3 After the data transfer is complete, the connection is terminated.

The pros and cons of circuit switching are summarised in this table. Figure 14.7

shows an example of circuit switching.

Pros Cons

the circuit used is dedicated to the single

transmission only

it is not very flexible (for example, it will

send empty frames and it has to use a

single, dedicated line)

the whole of the bandwidth is available nobody else can use the circuit/channel

even when it is idle

the data transfer rate is faster than with

packet switching

the circuit is always there whether or not

it is used

the packets of data (frames) arrive at the

destination in the same order as they were

sent

if there is a failure/fault on the dedicated

line, there is no alternative routing

available

a packet of data cannot get lost since all

packets follow on in sequence along the

same single route

dedicated channels require a greater

bandwidth

it works better than packet switching in

real-time applications

prior to actual transmission, the time

required to establish a link can be long

V Table 14.3 Pros and cons of circuit switching

device

‘A’

device

‘B’

router

‘A’

router

‘B’

R10

V Figure 14.7 An example of circuit switching

The dedicated route from ‘A’ to ‘B’ is first of all established (shown in orange on

the diagram). The following connections are then partially implemented: A–R2,

R2–R5, R5–R8, R8–R7, R7–R10 and finally R10–B. All packets (frames) follow

this single route and communication will take place, provided ‘B’ is not busy.

The main uses of circuit switching include public telephone networks, private

telephone networks and private data networks.

339

14.2 Circuit switching and packet switching

14.2.2 Packet switching

Packet switching was introduced in Chapter 2 when describing VoIP, together

with a diagram to show how the individual packets are routed from client to

client.

Packet switching is a method of transmission in which a message is broken up

into a number of packets that can be sent independently to each other from

start point to end point. The data packets will need to be reassembled into

their correct order at the destination. Figure 14.8 shows an example of packet

switching.

Note that

» each packet follows its own path

» routing selection depends on the number of datagram packets waiting to be

processed at each node (router)

» the shortest path available is selected

» packets can reach the destination in a different order to that in which they

are sent.

computer

‘A’

router

‘A’

R10

router

‘B’

computer

‘B’

V Figure 14.8 An example of packet switching

As Figure 14.8 shows, the message sent by computer ‘A’ was split into four

packets. The original packet order was:

and they arrived in the order:

which means they need to be reassembled in the correct order at the

destination.

340

14 COMMUNICATION AND INTERNET TECHNOLOGIES

The pros and cons of packet switching are summarised in this table.

Pros Cons

no need to tie up a communication line the protocols for packet switching can

be more complex than those for circuit

switching

it is possible to overcome failed or faulty

lines by simply re-routing packages

if a packet is lost, the sender must re-send

the packet (which wastes time)

it is easy to expand the traffic usage does not work well with real-time data

streams

circuit switching charges the user on the

distance and duration of a connection, but

packet switching charges users only for the

duration of the connectivity

the circuit/channel has to share its

bandwidth with other packets

high data transmission is possible with

packet switching

there is a delay at the destination while

packets are reassembled

packet switching always uses digital

networks which means digital data is

transmitted directly to the destination

needs large amounts of RAM to handle the

large amounts of data

V Table 14.4 Pros and cons of packet switching

Comparison of circuit switching and packet switching

Feature Circuit

switching

Packet

switching

actual route used needs to be set up before transmission can begin



a dedicated transmission path is required



each packet uses the same route



packets arrive at destination in the correct order



all the bandwidth of the channel is required



is bandwidth wasted?



V Table 14.5 Comparison of circuit switching and packet switching

Sometimes it is possible for packets to get lost and keep ‘bouncing’ around

from router to router and never actually get to their destination. Eventually,

the network could grind to a halt as the number of ‘lost’ packets mounts up and

clogs up the system. To overcome this, a method called

hopping is used. A hop

number is added to the header of each packet. Each packet is only allowed to

hop a finite number of times (this number is determined by the network protocol

and routing table being used). Each time a packet passes through a router, the

hop number is decreased by 1. If the packet has not reached its destination and

the hop number = 0, then it will be deleted when it reaches the next router.

Each packet also contains an error checking technique such as a checksum or

parity check. If a checksum is used, this value is calculated for each packet

and is added to the header. The checksum for each package is recalculated at

the destination to ensure no errors have occurred. If the checksum values are

different, then a request is made to re-send the packet. A priority value is

sometimes also added to a header. A high priority value indicates which packet

queue should be used.

341

14.2 Circuit switching and packet switching

This is the make-up of a packet header (together with the data in the message

being sent) if TCP/IP protocol is being used when sending packets:

IP address

of source

computer

address of

destination

computer

current hop

number of

data packet

length of

packet in

bytes

number of

packets

in the

message

sequence

number

to allow

reassembly

of packets

checksum

value

V Figure 14.9

More generally, packet headers contain the following information (the

information used with TCP/IP protocol headers is highlighted in

green)

» 4 bits to identify protocol version (such as IPv4, IPv6 – in the example

above we assume IP)

» 4 bits to identify header length (in multiples of four; for example, a value of

six implies 6 × 4 = 24 bytes)

» 8 bits to represent packet priority

» 16bits to identify the length of the packet in bytes

» 3 bits are used for fragmentation; the DF flag indicates whether a packet

can be fragmented or not (DF = do not fragment) and the MF flag indicates

whether there are more fragments of a packet to follow (MF = more fragments)

0DFMF

» 13 bits to show fragmentation offset to identify the position of the

fragments within the original packet

» 8 bits that show the current hop number of the packet

» 16bits to show the number of packets in the message

» 16bits to represent the sequence number of the packet

» 8 bits that contain the transmission protocol being used (TCP, UDP)

» 16bits that contain the header checksum value

» 32bits that contain the source IP address

» 32bits that contain the destination IP address.

Routing tables

Routing tables contain the information necessary to forward a package along

the shortest/best route to allow it to reach its destination. As soon as the

packet reaches a router, the packet header is examined and compared with

the routing table. The table supplies the router with instructions to send the

packet (hop) to the next available router.

Routing tables include

» number of hops

» MAC address of the next router where the packet is to be forwarded to

(hopped)

» metrics (a cost is assigned to each available route so that the most efficient

route/path is found)

» network destination (network ID) or pathway

» gateway (the same information as the next hop; it points to the gateway

through which target network can be reached)

» netmask (used to generate network ID)

» interface (indicates which locally available interface is responsible for

reaching the gateway).

342

14 COMMUNICATION AND INTERNET TECHNOLOGIES

routing table

router

packet header

the packet header is compared to the routing

table; the next router in the path is

determined; once established, a new MAC

address is added to the packet header so

that the packet knows which router is next; if

no route can be found or hop number = 0,

then the router deletes the data package

alternative

routes

V Figure 14.10

Suppose we are carrying out video conferencing using packet switching as the

method of routing data. The performance of the communication is not very good.

a) Describe some of the poor performance you might expect.

Give a reason for this poor performance.

b) What features of circuit switching could potentially improve the performance?

Example 14.1

Solution

a) Answers might include: picture and sound may not be in synchronisation

(packets arriving at different times); video not continuous – pauses (time delay

in reassembling the packets of data); degraded quality of sound and video

(possibly caused by competing traffic in communication lines); possible drop

out (packets take different routes, so it is possible for packets to be lost).

Answers might include: only one route used in circuit switching; therefore,

all packets arrive in correct order; dedicated communication channel with

circuit switching; therefore, full bandwidth available; there is no loss of

synchronisation with circuit switching.

343

14.2 Circuit switching and packet switching

How would packet switching be used to download a page from a website?

Example 14.2

Solution

Answers might include

O the web page is divided up into data packets

O each packet has a header, which includes the IP address of the destination

O the router checks the header against values stored in the routing table …

O … to determine which router the packet needs to be sent to next (hopped) …

O … the MAC address of the next router is added to the package header

O the hop value is checked to see if it equals zero

O each packet may go by a different route

O the destination computer reassembles the packets building up the final

web page.

ACTIVITY 14A

1 a) Name the four layers which show the TCP/IP protocols.

b) Name one protocol associated with each layer.

c) i) Describe the use of protocols when sending and receiving emails.

ii) What is the difference between SMTP and MIME when sending

emails?

2 a) What is an Ethernet?

b) Describe the contents of an Ethernet frame.

c) Ethernet protocols do not provide a means to communicate with

devices outside a LAN.

How can external devices be communicated with when using an

Ethernet?

3 a) Explain the following terms used in peer-to-peer BitTorrent.

i) peer

ii) swarm

iii) tracker

iv) leech

v) seed

b) Explain how it could be possible to deal with peers acting as leeches.

4 a) Describe the difference between a packet header and a routing table.

b) How are the packet header and the routing table used to route a

package?

5 A person is making a video call using VoIP software.

Explain how packet switching could be used and describe any problems

that might occur.

344

14 COMMUNICATION AND INTERNET TECHNOLOGIES

1a)Copy the diagram below and connect each peer-to-peer term to its

correct description. [5]

Peer-to-peer term Description

lurker

central server that contains

details of other computers

in a peer-to-peer swarm

leech

peer in a peer-to-peer system

uploads files for other

peers to download

seed

peer with a negative feedback

from other peers in a

peer-to-peer system

tracker

protocol used in peer-to-peer

when sharing files

between peers

BitTorrent

peer that downloads files but

does not supply new content to

the peer-to-peer community

b) Copy and complete the diagram to show the layers in a TCP/IP protocol. [3]

internet (network) layer

c) Describe the protocols used when sending and receiving emails. [4]

2a)An Ethernet frame contains a section called Ethernet data.

Copy and complete this diagram to show the other four items missing from

the Ethernet data section. [4]

Ethernet type

b) State what is meant by the term metadata.[1]

c) Describe how files can be shared using the BitTorrent protocol. [4]

3a)Explain what is meant by circuit switching. [2]

b) There are many applications in which digital data are transferred across a

network. Video conferencing is one of these.

For this application, circuit switching is preferable to the use of packet

switching. Explain why this is so. [6]

c) A web page is transferred from a web server to a home computer using the

Internet.

Explain how the web page is transferred using packet switching. [3]

Cambridge International AS & A Level Computer Science 9608

Paper 32 Q3 November 2015

End of chapter

questions

345

14.2 Circuit switching and packet switching

4a)This table shows some statements about circuit switching and packet

switching.

Copy the table and indicate which statements are true (

) and which are

false (

). [5]

statements circuit

switching

packet

switching

a dedicated circuit/path is needed at all times

the same route/circuit is used for every packet in

the message

bandwidth is shared with other packets of data

none of the bandwidth available is wasted during

transmission

packets arrive at the destination in the correct

order

b) Explain the following terms and why they are used when sending packets

across a network.

i) hop number/hopping [2]

ii) checksum [2]

c) Describe how headers and routing tables are used to route packets efficiently

from a sender to recipient. [5]