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Wire and Wireless Communication Applications Chap. 8
While a call is in progress, the cellular subscriber may be moving from one cell area to
another, so the MSC does the following:
• Monitors the signal strength from the cellular telephone as received at the cell base
station. If the signal drops below some designated level, the MSC initiates a “hand off”
sequence.
• For “hand off,” the MSC inquires about the signal strength received at adjacent cell sites.
• When the signal level becomes sufficiently large at an adjacent cell site, the MSC
instructs the cellular radio to switch over to an appropriate channel for communication
with that new cell site. This switching process takes less than 250 ms and usually is
unnoticed by the subscriber.
Table 8–10 shows three major analog standards that are used around the world.
Moreover, since cellular telephones are so popular, it became clear that these wideband FM
analog systems did not have the capacity to accommodate the new subscribers. For larger
capacity, new digital cellular systems, shown in Table 8–11, are used. The GSM digital system
has been used in Europe to provide one uniform standard that replaces the numerous analog
systems that have been phased out. As of 2011, most of the 1G systems have been replaced by
higher-generation systems.
Second Generation (2G)—The Digital
Circuit-Switched Systems
Table 8–11 shows standards for the popular second-generation (2G) cellular telephone
systems. They are all digital. The analog VF signal is converted to a compressed digital bit
stream, which is modulated onto a carrier. The bit rate of the compressed data is designed to
be relatively small so that a large number of users can be accommodated.
The digital approach has many advantages. It provides private conversations, resistance to
cellular fraud, and allows special features, such as caller ID, that can be easily implemented. The
digital approach provides noise-free telephone conversations out to the edge of the cell fringe
area because of the sharp threshold effect of digital systems. That is, the data are received either
error-free for recovery of noise-free audio or with so many errors that the audio is turned off.
Compare this with the noisy audio of analog FM systems when fringe-area signals are received.
The Group Special Mobile (GSM) system (see Table 8–11) is the 2G system that
replaces many different 1G analog systems in Europe. Now, one can travel around Europe and
remain in communication via the use of a GSM phone, instead of needing up to 15 different
types of 1G cell phones. GSM uses TDMA that provides time slots to accommodate up to
eight users on each 200-kHz-wide channel. The GSM phone is programmed for a particular
user’s account by an attached sim card, which contains the user’s phone number and other
account information. The sim card allows the subscriber to clone any GSM phone, whether it
is a handset or a fixed mobile phone, to hisher account by attaching the card. If a user travels
to a country where a different frequency band is used, a rented GSM phone can be instantly
programmed to the user’s account by attaching the card. GSM has been adopted by many
countries around the world. Details of GSM system design are published in a book by Garg
and Wilkes [Garg and Wilks, 1999].