2nd Generation Wireless Networks
Second-generation (2G) mobile systems were introduced in the end of
1980s. Second-generation systems have been developed to provide
higher quality signals, higher data rates for support of digital voice
service, and greater capacity. 2G mobile systems also introduced new
network architecture that have reduced the computational burden of
MSC. i.e. GSM introduced Base Station Controller (BSC) which is
inserted between several base stations and MSC. 2G mobile systems
have data bits up to 14.4kbps which allows them to support FAX and
Limited data support.
The most popular 2G wireless standards are Global Systems for Mobile
Communications (GSM), Interim Standard 136 (IS-136), and Interim
Standard 95 (IS-95).
The first GSM systems used a fully digital system with 900MHz
frequency band. FDMA (Frequency Division Multiple Access), which is
a standard that lets multiple users access a group of radio frequency
bands and eliminates interference of message traffic, is used to split the
available 25MHz of bandwidth into 124 carrier frequencies of 200 kHz
each. Each frequency is then divided using a TDMA (Time Division
Multiple Access) scheme into eight timeslots and allows eight
simultaneous calls on the same frequency. This protocol allows large
numbers of users to access one radio frequency by allocating time slots
to multiple voice or data calls. GSM has a data rate of 24.7 kbps.
Subscriber Identity Module (SIM) was also introduced during the
emergence of GSM technology. i.e. stores information such as
subscriber's identification number,
GSM can handle some data capabilities such as short message service
(SMS), roaming across countries, encryption of speech, and parity check
for error correction and detection which it gained quick popularity.
IS-136 was popularly known as North American Digital Cellular
(NADC) system. In this system, there were 3 full-rate TDMA users over
each 30 KHz channel. The need of this system was mainly to increase
the capacity over the earlier analog (AMPS) system. Similar to GSM,
IS-136 uses and parity check for error correction and detection and has a
data rate of 48.6 kbps. IS-136 systems allows three users on the same
frequency.
The IS-95 standard is the most widely used second-generation CDMA
scheme, which is primarily deployed in North America. It is also
popularly known as CDMA-one, uses 64 orthogonally coded users and
code words are transmitted simultaneously on each of 1.25 MHz
channels. Certain services that have been standardized as a part of IS-95
standard are: short messaging service (SMS), slotted paging, over-the-air
activation (meaning the mobile can be activated by the service provider
without any third party intervention), enhanced mobile station identities
etc. The IS-95 standard uses CDMA (Code Division Multiple Access)
scheme to allow multiple users (up to 63 users) and has a data rate of
14.4 kbps.
While GSM and other TDMA-based systems have become the dominant
2G wirelesses technologies, CDMA technology are recognized as
providing clearer voice quality with less background noise, fewer
dropped calls, enhanced security, greater reliability and greater network
capacity.
Differences with 1st Generation Wireless Networks
Digital traffic channels: The most notable difference between the two
generations is that first-generation systems are almost purely analog,
whereas second generation systems are digital. In particular, the first-
generation systems are designed to support voice channels using FM;
digital traffic is supported only by the use of a modem that converts the
digital data into analog form. Second generation systems provide digital
traffic channels. These readily support digital data; voice traffic is first
encoded in digital form before transmitting. Of course, for second-
generation systems, the user traffic (data or digitized voice) must be
converted to an analog signal for transmission between the mobile unit
and the base station.
Encryption: Because all of the user traffic, as well as control traffic, is
digitized in second-generation systems, it is a relatively simple matter to
encrypt all of the traffic to prevent eavesdropping. All second-generation
systems provide this capability, whereas first-generation systems send
user traffic in the clear, providing no security.
Error detection and correction: The digital traffic stream of second-
generation systems also lends itself to the use of error detection and
correction techniques. The result can be very clear voice reception.
Channel access: In first-generation systems, each cell supports a
number of channels. At any given time a channel is allocated to only one
user. Second generation systems also provide multiple channels per cell,
but each channel is dynamically shared by a number of users using time
division multiple access (TDMA) or code division multiple access
(CDMA).
Advantages of 2nd Generation Wireless Networks
Compatible with previous generation technologies such as AMPS.
Roaming was possible (Cross country communication was
possible).
Disadvantages of 2nd Generation Wireless Networks
Unable to support huge data such as videos.
Roaming was only possible for the same technology.
