General packet radio service
Circuit-switched cellular can be used for data at speeds of about 10 kbps over a dedicated channel. This is not a very good speed, and the bursty nature of most data traffic does not efficiently utilize the channel. To better accommodate data, the GSM developers created the general packet radio service (GPRS) as part of Release 97 of GSM. Note that a version of GPRS is also available on the U.S.-style CDMA networks. Note also that GPRS is different from the short message service (SMS) offering. SMS carries 160-character messages on the control channels whereas GPRS uses the voice channels.
GPRS is implemented by adding a serving GPRS support node (SGSN) at the BSC in the cell site. GPRS works by putting data packets into unused time slots on the voice channels. At the cell site, voice is handled by the standard BSC functions and data is handled by the SGSN functions. Both are backhauled to the MSC/MTSO, which now has packet switching capabilities to handle the data. GPRS supports IP, PPP, and X.25 networking. However, as a practical matter, about the only protocol that you will find is IP. For those that need X.25, it is encapsulated in an IP packet.
The SSGN creates and maintains packet connectivity between UEs and between UEs and gateways. In addition to the basic packet connectivity, the SSGN provides the functions below.
- Mobility management to include location and state of the UE as well as authentication of the subscriber and UE via the AuC.
- Session management deals with connection admission control and any changes that may occur during the session. It supervises the 3G network resources and services and well as any traffic management functions.
- Gateway for user data tunneling between the UE and GGSN at the server side of the connection. The SGSN also ensures that the connections receive the appropriate quality of service and generates charging information.
In short, the SGSN facilitates the connection and the data traffic flow for the UE.
If all eight time slots on a channel were devoted to data, the maximum speed would be about 170 kbps. Given the voice loadings, this is not possible so the actual speeds range from 30 to 70 kbps. To facilitate services like Web browsing, the service is usually implemented asymmetrically to provide for higher downlink speeds. As an aside, the addition of enhanced data rates for GSM/global evolution (EDGE) results in speeds that range from 20 to 200 kbps depending on the number of timeslots allocated for data in a channel.
GPRS adds a gateway to transport traffic to another network. The gateway GPRS support node (GGSN) is used to move data to other GPRS networks or to IP-based networks such as the Internet.
The GGSN provides interworking with external packet networks so that the UE can be connected to the appropriate packet network for the required service. It is the GGSN that dynamically assigns the IP address to the UE. The GGSN also polices the traffic and generates charging information as needed.
Basic GPRS: Packet-Switched
The visual shows the configuration for GPRS without roaming. Note the addition of the SGSN at the cell site, the packet switching function at the MSC/MTSO, the router at the MSC/MTSO, the GGSN for external network connectivity, and the packet-switched core for data transport.
There is no difference between the GPRS operation and the voice operation except that after the power-on sequence, GPRS is an always-on, always-available service. That is, the UE will receive data without any specific intervention by the subscriber.
GPRS with Roaming
GPRS roaming is illustrated on the visual. Please note that from a cellular perspective, we now have two overlaid networks that provide us with both voice and data anywhere provided that the visited network has the same capabilities. It is possible, but highly unlikely, that you will visit a network without GPRS since it has been actively deployed over the past eight years.