Mobile Wireless Communications Today (cont.)
by Puneet
Gupta
Global System for Mobile Communication (GSM)
GSM's air interface is based on narrowband TDMA technology,
where available frequency bands are divided into time slots,
with each user having access to one time slot at regular
intervals. Narrow band TDMA allows eight simultaneous
communications on a single 200Khz carrier and is designed to
support 16 half-rate channels. The fundamental unit of time in
this TDMA scheme is called a burst period and it lasts
15/26 ms (or approx. 0.577 ms). Eight burst periods are
grouped into a TDMA frame (120/26 ms, or approx. 4.615
ms), which forms the basic unit for the definition of logical
channels. One physical channel is one burst period per TDMA
frame. A GSM mobile can seamlessly roam nationally and
internationally, which requires that registration,
authentication, call routing and location updating functions
exist and be standardized in GSM networks.
GSM offers a variety of data services. GSM users can send
and receive data, at rates up to 9600 bps, to users on POTS
(Plain Old Telephone Service), ISDN, Packet Switched Public
Data Networks, and Circuit Switched Public Data Networks using
a variety of access methods and protocols, such as X.25 or
X.32. Other data services include Group 3 facsimile, as
described in ITU-T recommendation T.30, which is supported by
use of an appropriate fax adapter. A unique feature of GSM,
not found in older analog systems, is the Short Message
Service (SMS). SMS is a bi-directional service for short
alphanumeric (up to 160 bytes) messages. Messages are
transported in a store-and-forward fashion. For point-to-point
SMS, a message can be sent to another subscriber to the
service, and an acknowledgment of receipt is provided to the
sender. SMS can also be used in a cell-broadcast mode, for
sending messages such as traffic updates or news updates.
Messages can also be stored in the SIM card for later
retrieval.
The European version of GSM operates at the 900 MHz
frequency (and now at the newer 1800 MHz frequency). Since the
North American version of GSM operates at the 1900 MHz
frequency, the phones are not interoperable, but the SIMs are.
Dual-band 900 -1800 and 900 -1900 phones are already released
and in production. Tri-band 900 -1800 -1900 GSM phone are
expected to be manufactured in the next few years, which will
allow interoperability between Europe and North America
A GSM network consists of mobile stations talking to the
base transceiver station, on the Um interface. Many BTS are
connected to a BSC via the Abis interface and the BSC connect
to the MSC (The core switching network) via the A interface.
