Wireless Networks: Summary
By- Vasuki M. Basavanahalli
April 5, 2002
The objective of this paper is to present very briefly the summary of the chapter as
covered by Stallings-chapter 13. It by no means replaces the student’s efforts to go
through this chapter and go through Dr Lin’s slides in the class. In this chapter we
are going to give a very brief overview of the Mobile Telephonic and Satellite
Communication solutions. The author does not claim to present any new ideas, but
has only tried to present the facts provided in the chapter in a more simplistic
manner covering the “basic fundamentals” and expecting the students to cover the
detailed stuffs by themselves.
The first important thing to understand is the advantage (or condition) as to when do we
need a wireless solution and what are the disadvantages (or limitations) of such a
solution. Some of the situations where mobile solutions are a very interesting solution
proposition are when we face challenging and hostile geographic terrains like lakes,
mountains, deserts or when penetrating into the remote rural areas of a country is a very
costly proposition. (Don’t you think countries like Nepal with very high mountainous
terrain can offer its citizens the mobile solutions rather than offering land line solutionsthe other fact is some of these far-flung areas may be very scarcely populated.?. But at
the same time we must understand the limitations of the technology available today, the
cost features etc. while considering such a solution.
Mobile Telephonic Solutions:
We generally divide the mobile telephonic solutions according to the different
generations. We usually divide them into 3 Generations (Though one can divide into
more stages in the actual evolution process).
a) The First Generation ---Analog Solutions----AMPS (Common in North &
South America)
b) The Second Generation ----Digital Solutions----GSM (Common in Europe
and the fastest growing mobile technology solution.)
c) The Third Generation ---Multimedia solutions---Closest is I-mode of NTT
DoCoMo, Japan
In order to understand the AMPS or GSM in detail, which is beyond the scope of this
chapter and the scope of this book) one has to understand the fundamental building
blocks (cellular concepts) of such a structure. These are:
1. The mobile Device or mobile unit or mobile phone (in lay man’s terms)
2. The Base Transceiver or Base Stations or Base Station (in lay man’s
terms).
3. The Mobile Switching Center (MSC)
The Mobile Switching Center (MSC) coordinates the activities of number of base
stations and also connects all the calls to the public wire telephone Network. This is like
the “Brain” coordinating all the different activities of such a set-up. (See Fig 13.1 in the
book for more clarification.- Try to also realize the importance of MSC by looking at
the functions it provides in Page 345 of the book)
Designing a cellular Network:
This is a very interesting subject and in order to fully understand the topic requires a
detailed Network Engineering Analysis. Although keep the following thing in mind:
1. It is only suffice to say that it has been found that the cellular structure would be
more effective in a Hexagonal grid shape (i.e. when we make a cell in the Beehive
structure). But appreciate the fact that all the cellular structures would not be
exactly hexagonal because of the actual geographic terrain but we try to build such
a cell as close to the hexagonal structure as possible.
2. The mobile phone (or mobile unit or mobile device) talk or communicate to the
Base Station (or Base transceiver) communicate with each other in two different
frequencies: a) Frequency linking the mobile phone to the base station---Upward
Frequency or upward link b) Frequency linking the Base station with the mobile
phone is called Downward Frequency or Downward Link. These two
frequencies are separate (to avoid interference)
3. Frequency is a precious commodity! (For which auctions were created in some
countries and in some countries these frequencies were allocated on the basis of
beauty contest.). Telephone companies had to pay huge money (or debt) to get
these frequencies and this was one of the reasons, which led to the crash in the
telecom stocks because of the unmanageable debts. Various Technologies are
used to make the most use of this precious commodity---like Code Division
Multiple Access (CDMA), Frequency division Multiple Access (FDMA), Time
Division Multiple Access (TDMA) or Space division Multiple Access (SDMA) etc.
4.
As the mobile unit passes from one cell to another cell in the hexagonal grid, it is
important that the call is transferred from the leaving base station to the entering
base station. Such a transfer of call process is called as a Handoff.
5. The usage of the precious frequency has prompted cellular companies to come out
with innovative solutions regarding the usage of it. One such solution is dividing
the area into smaller cells or “borrowing” the frequencies from the adjacent
cells. The actual technicalities involved in such processes are beyond the scope of
this chapter.
SOME INTERESTING FACTS about mobiles: (About Mobile Numbers)
One can associate a mobile unit (or a mobile device or a mobile phone) with a
number of numbers. For example an AMPS mobile unit can be associated with 3
numbers (a GSM unit can be associated with more numbers—but this should give
some understanding of the ideas involved!)
a) ESN---Electronic Serial Number- This is a unique number given to every
mobile unit by the manufacturer.
b) System Identification Number - It identifies the mobile operator with which
the mobile phone has been subscribed for operation.
c) Mobile Identification Number - It is the 10 digit telephone number of the
mobile (i.e. like one’s telephone Number)
Please understand the operation as to how does a mobile phone operates, as
explaining in words will require a lot of pages.
b) GSM (Global System for Mobile)
GSM is catching the world by rage! And that is why it is called as “Global”
solution. The thing which makes its interesting is that it is a “digital” solution which has
its advantages over analog solutions. It was possible to send packets over the network and
thereby solutions like SMS became a commercially hit solution in Europe.
The other interesting feature (there are plenty and one would require a course in
GSM alone to appreciate all those aspects) is its SIM card feature (Subscriber Identity
Module)—akin the smart cards! .
The other thing which differentiates GSM from AMPS is its base station
subsystem with different kinds of registers namely Home Location Register (HLR),
Visitor Location Register (VLR), Authentication Database (AuC) and Equipment
Identity Register (EIR). All these are databases.
d) The Third Generation systems promise to provide rich context multimedia
possible and make the dream of wireless web possible.
e) Go through the concept of WAP in the book and some of the slides presented in
the class, which is pretty much self-explanatory in nature.
f)
Some of the satellites, which are launched into space, are only for the
communication purposes and therefore called as the “communication satellites”
and may be these are Geo stationary (orbiting at over 35,000 km) or some of the
others may be low and medium Earth Orbiting satellites (LEOS / MEOS.)