GPRS Cellular Network
Although designed to run alongside the GSM system, the core
network structure updated for GPRS has several new elements added to enable it
to carry the packet data. The network between the BSC and BTS is similar, but
behind this there is a new infrastructure to support the packet data.
For GPRS, the data from the BSC is routed through what is
termed a Serving GPRS Support Node (SGSN). This forms the gateway to the
services within the network, and then a Gateway GPRS Support Node (GGSN) which
forms the gateway to the outside world.
SGSN
The SGSN serves a number of functions for GPRS mobiles. It
enables authentication to occur, and it then tracks the location of the mobile
within the network, and ensures that the quality of service is to the required
level.
For the network protocols there are two layers that are used
and supported by GPRS, namely X25 and IP. In operation the protocols assign
addresses (Packet Data Protocol or PDP addresses) to the devices in the network
for the purpose of routing the data through the system. Thus the GGSN appears as
a data gateway to the public packet network, and thus the fact that the users
are mobiles cannot be seen.
In operation the mobile must attach itself to the SGSN and
activate its PDP address. This address is supplied by the GGSN which is
associated with the SGSN. As a result a mobile can only attach to one SGSN,
although once assigned its address it can receive data from multiple GGSNs using
multiple PDP addresses.
GPRS mobiles
Not all GPRS mobiles are designed to offer the same levels of
service. As a result they are split into three basic categories according to
their capabilities in terms of the ability to connect to GSM and GPRS
facilities:
- Class A: This class describes mobile phones that can be
connected to both GPRS and GSM services at the same time.
- Class B: These mobiles can be attached to both GPRS and
GSM services but they can be used on only one service at a time. A Class B
mobile can make or receive a voice call, or send and or receive a SMS
message during a GPRS connection. During voice calls or texting the GPRS
service is suspended but it is re-established when the voice call or SMS
session is complete.
- Class C: This classification covers phones that can be
attached to either GPRS or GSM services but user needs to switch manually
between the two different types.
GPRS mobiles are also categorized by the data rates they can
support. Within GSM there are eight time slots that can be used to provide TDMA,
allowing multiple mobiles onto a single RF signal carrier. Within GPRS it is
possible to use more than one slot to enable much higher data rates to be
achieved when these are available. The different speed classes of the mobiles
are dependent upon the number of slots that can be used in either direction.
There are a total of 29 speed classes. Class one mobiles are able to send and
receive in one slot in either direction, i.e. uplink and downlink, and class 29
mobiles are able to send and receive in all eight slots. The classes within
these two limits are able to support sending and receiving in different
combinations of uplink and downlink slots.
|
