GPRS Protocol Stack White Paper

Some cooperation still exists between elements of the current GSM services and GPRS Protocol Stack. On the physical layer, resources can be reused and some common signaling issues exist. In the same radio carrier, there can be time slots reserved simultaneously for circuit-switched and GPRS Protocol Stack use. The most optimum resource utilization is obtained through dynamic sharing between circuit-switched and GPRS Protocol Stack channels. During the establishment of a circuit-switched call, there is enough time to preempt the GPRS Protocol Stack resources for circuit-switched calls.

One of the main issues in the GPRS Protocol Stack network is the routing of data packets to/from a mobile user. The main functions of the GGSN involve interaction with the external data network. The GGSN updates the location directory using routing information supplied by the SGSNs about the location of a MS and routes the external data network protocol packet encapsulated over the GPRS Protocol Stack backbone to the SGSN currently serving the MS. It also de capsulate and forwards external data network packets to the appropriate data network and collects charging data that is forwarded to a charging gateway.

Three different routing schemes are possible: mobile-originated message, network-initiated message when the MS is in its home network, and network-initiated message when the MS has roamed to another GPRS Protocol Stack operator's network. In these examples, the operator's GPRS Protocol Stack network consists of multiple GSNs (with a gateway and serving functionality) and an intra-operator backbone network.

The GPRS Protocol Stack network encapsulates all data network protocols into its own encapsulation protocol, called the GPRS Protocol Stack Tunneling Protocol (GTP). This is done to ensure security in the backbone network and to simplify the routing mechanism and the delivery of data over the GPRS Protocol Stack network.

The operation of the GPRS Protocol Stack is partly independent of the GSM network. However, some procedures share the network elements with current GSM functions to increase efficiency and to make optimum use of free GSM resources (such as unallocated time slots).

An MS has three states in the GPRS Protocol Stack system: idle, standby, and active. The three-state model represents the nature of packet radio relative to the GSM two-state model (idle or active). Data is transmitted between a MS and the GPRS Protocol Stack network only when the MS is in the active state. In the active state, the SGSN knows the cell location of the MS. However, in the standby state, the location of the MS is known only as to which routing area it is in.

When the SGSN sends a packet to a MS that is in the standby state, the MS must be paged. Because the SGSN knows the routing area in which the MS is located, a packet paging message is sent to that routing area. After receiving the packet paging message, the MS gives its cell location to the SGSN to establish the active state.Packet transmission to an active MS is initiated by packet paging to notify the MS of an incoming data packet. The data transmission proceeds immediately after packet paging through the channel indicated by the paging message. The purpose of the packet paging message is to simplify the process of receiving packets. The MS has to listen to only the packet paging messages, instead of all the data packets in the downlink channels, reducing battery use significantly.

When an MS has a packet to be transmitted, access to the uplink channel is needed. The uplink channel is shared by a number of MSs, and its use is allocated by a BSS. The MS requests use of the channel in a packet random access message. The transmission of the packet random access message follows Slotted Aloha procedures. The BSS allocates an unused channel to the MS and sends a packet access grant message in reply to the packet random access message. The description of the channel (one or multiple time slots) is included in the packet access grant message. The data is transmitted on the reserved channels.

The main reasons for the standby state are to reduce the load in the GPRS Protocol Stack network caused by cell based routing update messages and to conserve the MS battery. When a MS is in the standby state, there is no need to inform the SGSN of every cell change only of every routing area change. The operator can define the size of the routing area and, in this way, adjust the number of routing update messages.

In the idle state, the MS does not have a logical GPRS Protocol Stack context activated or any Packet Switched Public Data Network (PSPDN) addresses allocated. In this state, the MS can receive only those multicast messages that can be received by any GPRS Protocol Stack MS. Because the GPRS Protocol Stack network infrastructure does not know the location of the MS, it is not possible to send messages to the MS from external data networks.

A cell-based routing update procedure is invoked when an active MS enters a new cell. In this case, the MS sends a short message containing information about its move through GPRS Protocol Stack channels to its current SGSN. This procedure is used only when the MS is in the active state. When an MS in an active or a standby state moves from one routing area to another in the service area of one SGSN, it must again perform a routing update. The routing area information in the SGSN is updated and the success of the procedure is indicated in the response message.The inter-SGSN routing update is the most complicated of the three routing updates. In this case, the MS changes from one SGSN area to another, and it must establish a new connection to a new SGSN. This means creating a new logical link context between the MS and the new SGSN, as well as informing the GGSN about the new location of the MS.

GPRS Protocol Stack does impact a network's existing cell capacity. There are only limited radio resources that can be deployed for different uses. Use for one purpose precludes simultaneous use for another. For example, voice and GPRS Protocol Stack calls both use the same network resources. The extent of the impact depends upon the number of timeslots, if any, that are reserved for exclusive use of GPRS Protocol Stack. However, GPRS Protocol Stack does dynamically manage channel allocation and allow a reduction in peak time signaling channel loading by sending short messages over GPRS Protocol Stack channels instead.

Achieving the theoretical maximum GPRS Protocol Stack data transmission speed of 170 kbps would require a single user taking over all eight timeslots without any error protection. Clearly, it is unlikely that a network operator will allow all timeslots to be used by a single GPRS Protocol Stack user. Additionally, the initial GPRS Protocol Stack terminals are expected be severely limited supporting only one, two or three timeslots. The bandwidth available to a GPRS Protocol Stack user will therefore be severely limited. As such, the theoretical maximum GPRS Protocol Stack speeds should be checked against the reality of constraints in the networks and terminals. The reality is that mobile networks are always likely to have lower data transmission speeds than fixed networks, around 120 kbps.

Relatively high mobile data speeds may not be available to individual mobile users until Enhanced Data rates for GSM Evolution (EDGE) or Universal Mobile Telephone System (3GSM) are introduced.

GPRS Protocol Stack is based on a modulation technique known as Gaussian Minimum Shift Keying (GMSK). EDGE is based on a modulation scheme that allows a much higher bit rate across the air interface this is called 8 Phase Shift Keying (8 PSK) modulation. Since 8 PSK will also be used for 3G, network operators will need to incorporate it at some stage to make the transition to third generation mobile phone systems.

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4. GPRS Protocol Stack Milestones