Mumbai University > Information Technology > Sem4 > Computer Networks

Marks: 5M

Year: May 2014

The mobile phone system is used for wide area voice and data communication. Mobile phones (sometimes called cell phones) have gone through three distinct generations, widely called 1G, 2G, and 3G. The generations are:

First-Generation (1G) Mobile Phones: Analog Voice

• AMPS (Advanced Mobile Phone System), invented by Bell Labs and first installed in the United States in 1982. It was also used in England, where it was called TACS, and in Japan, where it was called MCS-L1.

• In all mobile phone systems, a geographic region is divided up into cells, which is why the devices are sometimes called cell phones. In AMPS, the cells are typically 10 to 20 km across; in digital systems, the cells are smaller. Each cell uses some set of frequencies not used by any of its neighbors.

• The key idea that gives cellular systems far more capacity than previous systems is the use of relatively small cells and the reuse of transmission frequencies in nearby (but not adjacent) cells. Thus, the cellular design increases the system capacity by at least an order of magnitude, more as the cells get smaller which also leads to smaller and cheaper transmitters and handsets.

Second-Generation (2G) Mobile Phones: Digital Voice

• Several different systems were developed, and three have been widely deployed. DAMPS (Digital Advanced Mobile Phone System) is a digital version of AMPS that coexists with AMPS and uses TDM to place multiple calls on the same frequency channel. It is described in International Standard IS-54 and its successor IS-136.

• GSM (Global System for Mobile communications) has emerged as the dominant system, and while it was slow to catch on in the U.S. it is now used virtually everywhere in the world. Like D-AMPS, GSM is based on a mix of FDM and TDM. CDMA (Code Division Multiple Access), described in International Standard IS-95, is a completely different kind of system and is based on neither FDM nor TDM. While CDMA has not become the dominant 2G system, its technology has become the basis for 3G systems.

Third-Generation (3G) Mobile Phones: Digital Voice and Data

• Many operators have taken cautious steps in the direction of 3G by going to what is sometimes called 2.5G, although 2.1G might be more accurate. One such system is EDGE (Enhanced Data rates for GSM Evolution) which is just GSM with more bits per symbol. The trouble is, more bits per symbol also means more errors per symbol, so EDGE has nine different schemes for modulation and error correction, differing in terms of how much of the bandwidth is devoted to fixing the errors introduced by the higher speed. EDGE is one step along an evolutionary path that is defined from GSM to WCDMA (UMTS).

• Similarly, there is an evolutionary path defined for operators to upgrade from IS-95 to CDMA2000 networks.

• WCDMA uses 5-MHz channels and CDMA2000 uses 1.25- MHz channels.