Wide Area Networks Lecture 10

Wide Area Networks - Lecture 10

Presentation Layer & Mobile Telephony Overview

These Notes are divided into hyperlinked sections

Introduction
Data Structures and Transfer Syntaxes
Data Compression
Cryptography, public key, digital signatures, key distribution
Symmetrical Algorithms
Asymmetrical Algorithms
Authentication
Mobile Telephony Operators and Systems
Structure of Cells
Cell size
Base Station Aerials
Connection of the Base Stations
GSM Operation
Frequency Hopping
Paging
Location Updating
Conclusion
Further Reading

Introduction

This lecture will look at the reason for having a presentation layer.
We will also briefly examine how mobile telephony is structured and its basis of operation.

Data Structures and Transfer Syntaxes

There are several data structures commonly used for storing data within computers. To store text, two commonly used schemes are ASCII and EBCDIC. When data is sent from one computer to another, there must be some common ground for establishing the language of data presentation, both at the application level and at the machine level e.g. big and little endian storage methods.

In the OSI model, a separate layer known as the Presentation layer is used for this. In the TCP/ IP model, this operation is carried out within the application layer.

FTP applications are able to carry out the complex translation of data representation and storage formats so that files may be transferred seamlessly between totally different platforms.

Data compression

To speed transmission of data, it is wise to compress it first before transmission. Compression is reducing the size of the file to be sent by looking for repetitions and saving them in a more efficient manner. For example a bitmap is described pixel by pixel. If many of the pixels are the same colour, they may be described in an implicit manner rather than explicitly. Common compression methods are JPEG (Joint Photographic Experts Group) and GIF (Graphics Interchange Format).

The smaller size file may then be sent more quickly than the uncompressed version.

Cryptography, public key, digital signatures, key distribution

Cryptography is the art of hiding the contents of a file using a suitable algorithm so that it may be sent (or stored) without eavesdroppers being able to discover the contents of the message.

The encryption process may be thought of as a process that produces cyphertext C from plaintext P such that:

P = D(E(C))

Where D is the decryption key and E is the encryption key

A symmetric encryption algorithm uses the same key for both encryption and decryption i.e. E = D.

An asymmetric encryption algorithm uses a different key for encryption and decryption, i.e. E D

There are two types of encryption in common use and they use two different types of algorithms. The algorithm is secured by an alphanumerical string known as a key.

Symmetrical Algorithms

The symmetrical algorithm uses one key for both encryption and decryption. Therefore both sender and recipient need to use the same key. This can pose security problems in distributing the key and it may be safer to send it by post. Even so, it is unwise to assume that the postal service itself is secure. A face to face meeting with the other party to be involved in the encryption process is the safest (but not necessarily the most practical) solution.

Asymmetrical Algorithms

Asymmetrical algorithms generate two separate keys. One key is used to encrypt the data to be transmitted and the other to decrypt the data. The two keys are different and knowledge of one plus the encryption algorithm is insufficient to work out the other key.

If a user needs to be sent sensitive data, he generates the two keys and keeps the decryption key. He then may publish freely the encryption algorithm for others to use and encrypt data to be sent to that user. Providing the decryption key is never disclosed, all data will remain secure.

The keys are given names:

The encryption key is known as the public key
The decryption key is known as the private key

Authentication

Sometimes, it is not the contents of the message that needs to be secured so much as that it must be ensured that the message itself has not been tampered with in transit and also that it is from the person that it purports to have come from.

Here a process known as authentication is used. The goals of authentication are to ensure that:

· The user is known and is a legitimate message source and/ or recipient.

· The recipient can tell whether the message has come from a legitimate source.

· The message has not been intercepted and modified or corrupted.

The message to be sent is passed through an application known as a hash function. The sender uses the intended recipient's public key and this generates a tag (128 bits using MD5 - a common encryption scheme).

The tag is appended to the message and it is sent to the recipient. The tag may be thought of as an encrypted checksum. Upon receipt of the message, the receiver separates the tag from the message and passes the message (minus tag) through his own hash function with his private key.

A comparison is then performed between the tag thus produced and the received tag. If they match, the message is authenticated and can be safely assumed to have come from the source specified and has not been tampered with en route.

This is often used for communication between people agreeing deals so that there can be no doubt as to whether the message has been changed maliciously by a third party.

Mobile Telephony Operators and Systems

In the UK there are 4 operators of mobile cellular radio systems. These are Cellnet, Vodaphone, Orange and One2one. There are three types of service offered by the companies, TACS (Total Access Communication System) and GSM (Global System for Mobiles). Both of theses systems operate in the 900 MHz band. A third type of service known as PCN (Personal Communication Network) is offered by Orange and One2one.

Table 10.1 Comparison between TACS and GSM

The service operators offer these services as shown in table 10.2 below.

Table 10.2 UK services offered.

TACS is the older of the three systems and is an analogue service. GSM and PCN are digital systems.

Structure of Cells

An area that is served by cellular radio is divided into small areas known as macrocells. Each macrocell has a range of channels that it can use. The adjacent cells to this macrocell will use a different set of channels. This avoids interference from adjacent macrocells.

At the centre of each cell is a base station whose coverage is limited to that particular cell. The advantage of this is to allow re-use of channels in more distant macrocells where co-channel interference will be so low as to be within acceptable limits.

This allows frequencies (channels) to be re-used in other macrocells around the entire area of coverage and is known as frequency re-use.

Fig 10.1 Illustrating frequency re-use

In the above figure it can be seen that the cells are grouped into sets of 7. It can be seen that it is possible to re-use frequencies as there is a large enough distance between similar cells to avoid interference. The 7-cell grouping is the most commonly used configuration. Other cell groupings are possible using 4, 7, 12 or 21 cells.

Within each cell there are two sets of frequencies in use. One is from the base station to the mobile and the other is from transmissions from the mobile to the base station. This is illustrated for GSM below in figs 10.2 and 10.3.

Fig 10.2 The use of GSM frequencies from mobile to base

Fig 10.3 The use of GSM frequencies from base to mobile

The carrier frequencies used within each cell are assigned to allow re-use of the same carrier frequency only a few cells distant. The distance between cells using the same frequencies is known as the repeat or re-use distance.

Cell size

Macrocells can vary in size from about 1 km radius to about 8 km radius. These are found in rural and less busy urban areas where the number of simultaneous calls likely to be made is low. In certain busy areas, cells may be subdivided into microcells having a radius of 200 - 300 metres. This could be used in a railway station or a shopping centre. In more busy areas, microcells may be divided into picocells having a few tens of metres radius which would cover one floor of a building or an airport lounge etc.

Picocells offer the best usage of frequencies per unit area but the hardware costs are higher as there must be more base transceivers per unit area.

Base Station Aerials

The aerials for macrocells are mounted on the top of towers or high buildings. For microcells and picocells, the aerials will be located at a lower height and will have a lower transmission power than that of a macrocell. The base station is in the centre of the hexagon.

To help reduce co-channel interference, sectored aerials are used at each base station. A three-sectored aerial gives 3 X 120 degree coverage. This divides each cell into 3 parts known as sectors.

Fig 10.4 A 3-sectored base station aerial

Connection of the Base Stations

The base stations are themselves connected by high bandwidth links using either fibre optic links or microwaves to a mobile switching centre (MSC). The MSCs are themselves interconnected and also connected to the Public Switched Telephone Network (PSTN).

GSM Operation

As mobile telephones move around within the area of coverage, the precise location of each mobile set is monitored by the system to allow communication at all times. The location of each mobile set is achieved by a combination of paging and location update procedures.

When a mobile moves from one cell to another, there must be a transparent hand-over. This means that the user must not realise that a change-over is taking place. The procedure is known as hand-over. This takes place when the signal level from one cell begins to drop. The mobile set constantly monitors the received power from its current base station and adjacent base stations from other cells. When the signal power from the current base station falls to a level below the power from the cell adjacent for a period of time, the mobile informs the current base station that another cell has higher power level. Hand-over then takes place.

GSM has a main function as a telephony service but also offers SMS, a text messaging service, data services, FAX and access to the packet data system. This means that information from any site on the Internet may be accessed providing that it is in the correct format.

Frequency Hopping

While a mobile call is in progress, the frequency that is being used to carry the signal is constantly changed. This improves the performance of the system when the signal power drops in bad reception areas. It also shares bad frequencies between a number of users so that the bad frequency will hopefully not be noticed.

Paging

To page a mobile telephone, a message is broadcast by the base station to inform the mobile set that a call is waiting to be received. If the system has knowledge of the location of the phone, only one cell needs to be paged. If the location were not known, many cells would have to be paged and this is a waste of bandwidth. To keep track of a mobile, a technique known as location updating is used.

Location Updating

The GSM system is divided into a number of location areas and this is typically a larger area than one cell. When a mobile moves from one location area to another, it informs the network of its new location. To keep these records up to date, mobiles are requested periodically to perform a location update.

All users have a permanent (home) location to which they are attached. The area within which the user is registered has a home agent that keeps track of the users registered to its area.

Each area has one or more foreign agents (software driven) whose job is to keep track of visiting (mobile) hosts who are based in another area. When a mobile host arrives in a new area it informs the foreign agent for that area that it is now in the foreign agent's area. The foreign agent then contacts the home agent and informs it of the location of the mobile host. After suitable security checks have been made, the home agent now knows how to route packets to the mobile host.

When an attempt is made to contact the mobile host by a third party, the packet is forwarded by the home agent in the mobile user's home location to the mobile user via the foreign agent for the area that the mobile user is currently in.

The home agent now informs the third party of the location of the mobile host and henceforth all packets are delivered directly to the mobile host from the third party.

Fig 10.5 Locating a Mobile User within a Cellular Telephone Network

Conclusion

The Presentation Layer deals with such issues as data representation so that different platforms may communicate despite different storage languages and formats.

Compression is carried out at this layer and is a vital technique for increasing bandwidth without upgrading the physical layer.

Encryption is carried out at this layer, a technique for hiding the meaning of messages from eavesdroppers. Symmetrical and asymmetrical encryption algorithms exist for this purpose. Asymmetrical algorithms produce two different keys, one of which can be freely published for senders to use – the public key.

Authentication uses the public key to generate a tag that may be used to verify the sender and contents of a communication have not been tampered with.

Analogue and digital cellular radio services are available in the UK. Served areas are divided into macrocells at the centre of which are the base stations. Busy areas can be divided into microcells or even picocells.