A Guide to General Packet Radio Service -1

GPRS Guide

GPRS (short for General Packet Radio Service) is a data service upgrade for GSM networks. This allows GSM Networks to be completely compatible with the Internet. GPRS uses a packet-mode technique to transfer traffic in bursts. These bursts allow higher efficiency, and therefore higher speeds. The packet
bursting technique is also used in DSL modems, and other methods of high-speed internet access. Due to this technique GPRS allows bit rates of 9.6 Kbps to anywhere more than 150 Kbps per user.

There are a couple major benefits of using GPRS. These include better use of  radio/network resources and a completely transparent support of IP. Radio resources are only used when data is being sent and/or received.

GPRS also provides an immediate connection (again like DSL or Cable) and a high throughput. It also allows end user applications to only occupy the network when data is being transferred, and is an almost perfect
design for the short data burst which data applications seem to have these days.

Applications based on standard protocols (data) like IP and X.25 are supported. Four different quality of service levels are supported by GPRS. To supports data apps, GPRS uses several new network nodes in addition to the GSM PLMN network nodes.

They are responsible for traffic routing, and various other internetworking functions with other, external, packet-switched data networks (can anyone say Datapac?), subscriber location, cell selection, roaming and all the other functions which all cellular networks need to operate.

Now that we have the general info on what GPRS is, I will talk about a few other protocols which are linked with GPRS.

NS



NS (Network Service) transfers the NS SDUs between the SGSN (serving GPRS support node) and BSS (Base station system). There are several services which are provided to the NS user. They include:

   Network Congestion Indication - The Sub-Network Service (i.e. Frame Relay) perform congestion recovery control actions. The network service uses various congestion reporting mechanisms which are in the Sub-Network Service implementation.

   Status Indication - Is used to tell the NS user of NS affecting events. An example is a change in the capabilities of transmission.

   Network Service SDU Transfer - Allows network service primitives. This lets transmission and reception of upper layer protocol data unitsbetween the BSS and SGSN. NS SDU's are transferred in order of the
Network Service, but under certain circumstances order might not be maintained.

The NS PDU format is:

                1 byte
              |----------------------------|
              |        PDU Type                 |
              |----------------------------|
              | Other Information Elements |
              |----------------------------|


The PDU Type can be any of the following:
NS-ALIVE
NS-ALIVE-ACK
NS-BLOCK
NS-BLOCK-ACK
NS-RESET
NS-RESET-ACK
NS-STATUS
NS-UNBLOCK
NS-UNBLOCK-ACK
NS-UNITDATA

Next we're onto the Information Elements (IEs) of the PDU. The IEs which are present depend on what the PDU type is. The structure of an IE is as follows:

                1 byte
              |------------------------------|
              | Information Element ID (IEI) |
              |------------------------------|
              |       Length Indicator             |
              |------------------------------|
              |   Information Element Value  |
              |------------------------------|


The first 8th (or octet) of an information element, having the TLV format, contains the IEI of the IE. If the IEI is not known to the PDU, the receiver assumes that the next octet is the first octet of the length indicator.
This rule is used to allow the receiver to skip unknown IEs to analyze any other following elements,

Next up is the length indicator. This varies in length, and can be either one or two octets long. However, the second octet may not be present. This field has the field extension bit, 0/1 ext, and closely following it is
the length of field in octets.

The 8th bit of the first octet is reserved for the field extension bit. If the field extension bit is set to zero, the second octet of the length indicator is present. If it is set to one, then the first octet is the final octet of the length indicator.

Lastly, the IE Value. The following IEs can be present, but are, once again, dependent on the PDU type:

Cause
NS-VCI
NS PDU
BVCI
NSEI

BSSGP
~~~~~

The primary functions of the BSSGP are:
- Provision by an SGSN to a BSS of radio related information used by the   RLC/MAC function (in downlink)

- Provision by a BSS to an SGSN of radio related information from the RLC/MAC   function (in uplink)

- Provision of functionality to allow two physically distinct node, an SGSN   and a BSS, to operate node management control functions.

The BSSGP PDUs format is:

                1 byte
              |----------------------------|
              |        PDU Type                  |
              |----------------------------|
              | Other Information Elements |
              |----------------------------|

Click here for next part of this guide.
Written by: PsychoSpy and The Clone

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