GPRS

GPRS

1. Introduction

It has been forecast that the trend taking place in fixed networks -- whereby data traffic is overtaking voice traffic -- will also spill over into wireless networks. This transition is expected to occur in fixed networks around year 2000. The transition in wireless networks will follow soon thereafter. However, one challenge to this trend is that the current wireless infrastructures must evolve to accommodate the anticipated data traffic while simultaneously carrying voice traffic in an efficient, high-speed manner.

The different approaches to 3rd generation wireless systems (IMT-2000, UMTS, etc.) were intended to address the challenge of voice-to-data crossover and integration. However, the complexities of new and exciting wireless technologies have slowed down progress in their development and widespread deployment. For example, international industry standard bodies have not been able to approve all of the required standards. News regarding feasible prototypes of 3rd generation systems appears, especially from Korea and Japan, but no 3rd generation wireless system or network has been deployed yet.

To lessen the impact of the delay in implementing 3rd generation wireless systems, General Packet Radio Service (GPRS) is being introduced as an intermediate step to efficiently transport high-speed data over the current Global Systems for Mobile Communications (GSM) and TDMA-based wireless network infrastructures. GPRS signalling and data traffic do not travel through the GSM network. The GSM network is only used for table look up, in the Location Register data bases, to obtain GPRS user profile data. GPRS uses 1 to 8 radio channel timeslots which can be shared by multiple users. It packetizes the user data and transports it over Public Land Mobile Networks (PLMN) using an IP backbone. From there, it interfaces to other Public Data Networks (PDNs), including the Internet. As a result, GPRS has the ability to offer speeds of 14,400 bps to 115,000 bps, which allow for comfortable Internet access using wireless devices. Because GPRS has a range of supported bandwidths, it allows for short "bursty" traffic, such as e-mail and web browsing, as well as large volumes of data. In addition, because GPRS supports Quality of Service, service providers can offer selective services to users. Finally, because GPRS has fast connection setup, the user has the perception of being "always on" for continuous operation.

To support GPRS operations, new protocols and new network devices are required. These are described in the next sections.

2. GPRS Protocols and Network Devices
2.1 GPRS Components
To ensure the interworking of the PLMN, PDN and the wireless networks (GSM or TDMA), two new major components are required. These components are called GPRS Support Nodes. There are two types of GPRS Support Nodes, described as follows:

Serving GPRS Support Node (SGSN)

An SGSN delivers packets to mobile stations within its service area. SGSNs send queries to Home Location Registers (HLRs) to obtain profile data of GPRS subscribers. SGSNs detect new GPRS mobile stations in a given service area; and, finally, SGSNs process registration of new mobile subscribers and keep a record of their location inside a given service area.

Gateway GPRS Support Node (GGSN)

GGSNs are used as interfaces to external PDNs. GGSNs maintain routing information that is necessary to tunnel the Protocol Data Units (PDUs) to the SGSNs that service particular mobile stations. Other functions include network and subscriber screening and address mapping. One or more GGSNs may support multiple SGSNs.

2.2 GPRS Network Enhancements
In addition to the new GPRS components, existing GSM and TDMA network elements must also be enhanced in order to support GPRS. The following two pieces of equipment must be enhanced:

Base Station System (BSS): must be enhanced to recognize and send user data to the SGSN that is serving the area.

Home Location Register (HLR): must be enhanced to register GPRS user profiles and respond to queries originating from SGSNs regarding these profiles.

2.3 GPRS Network Protocols
There are several protocols used in the network equipment mentioned above. These protocols operate in both the data and signalling planes. The following is a brief description of each protocol layer:

Sub-Network Dependent Convergence Protocol (SNDCP): protocol that maps a network-level protocol, such as IP or X.25, to the underlying logical link control. SNDCP also provides other functions such as compression, segmentation and multiplexing of network-layer messages to a single virtual connection.

Logical Link Control (LLC): a data link layer protocol for GPRS which functions similar to Link Access Protocol - D (LAPD). This layer assures the reliable transfer of user data across a wireless network.

Base Station System GPRS Protocol (BSSGP): BSSGP processes routing and quality of service (QoS) information for the BSS. BSSGP uses the Frame Relay Q.922 core protocol as its transport mechanism.

GPRS Tunnel Protocol (GTP): protocol that tunnels the protocol data units through the IP backbone by adding routing information. GTP operates on top of TCP/UDP over IP.

GPRS Mobility Management (GMM): protocol that operates in the signalling plane of GPRS and handles mobility issues such as roaming, authentication, and selection of encryption algorithms.

Network Service: protocol that manages the convergence sub-layer that operates between BSSGP and the Frame Relay Q.922 Core by mapping BSSGP's service requests to the appropriate Frame Relay services.

BSSAP+: protocol that manages paging for voice and data connections and optimizes paging for mobile subscribers. BSSAP+ is also responsible for location and routing updates as well as mobile station alerting.

3. GPRS Network Operations

The GPRS user perceives a connectionless network. In fact, however, a network connection must be established for each transaction and released once the transaction is completed. The complexities of a mobile network also must be addressed. Data transmission in a GPRS network requires several steps described below in the context of the protocol layers described in the previous section.

3.1 Network Access
Once a GPRS mobile station has begun operation, i.e., power is on, it "introduces" itself to the network by sending a "GPRS attach" request. Network access can be achieved from either the fixed side or the mobile side of the GPRS network (See network diagram). Point-to-Point, Point-to-Multi-point or anonymous connections are then available. As in cellular networks, several administrative functions are performed to validate a user, including:

User Registration - associates the mobile ID with the user's PDP (packet data protocol) and address within the PLMN. Within the home area of the mobile station, traditional HLRs are enhanced to reference GPRS data. Outside the home area, dynamically allocated records are referenced in VLRs.

Authentication - ensures the validity of the GPRS mobile station and its associated services. Mobility Management functions (GMM protocol stack) are used for this part of signalling.

Call Admission Control (CAC) - determines the required network resources for the quality of service (QoS) that is requested. If these resources are available, they will be reserved.

3.2 Routing and Data Transfer
Once a mobile station begins data transmission, routing is performed by the GSNs on a hop-by-hop basis through the mobile network using the destination address in the message header. Routing tables are maintained by the GSNs utilizing the GTP layer which may carry out Address Translation and Mapping functions to convert the external PDN addresses to an address that is usable for routing within PLMNs. The data itself will go through several transformations as it travels through the network. Depending on the destination PDN, the data can be:

Forwarded, using the relay function, to go from one node to the other in the route,

Tunneled to transfer data from one PLMN to another,

Compressed to use the radio path in an efficient manner (Compression algorithms may be used for manufacturers to differentiate themselves, however, they may face interoperability issues in heterogeneous networks), and/or

Encrypted to protect the mobile station from eavesdropping (Encryption algorithms can also be used as a differentiating factor).

3.3 Mobility Management

As a mobile station moves from one area to another, mobility management functions are used to track its location within each PLMN. Then, SGSNs communicate with each other and update the user location. The mobile station's profiles are preserved in the VLRs that are accessible to SGSNs via the local MSC. A logical link is established and maintained between the mobile station and the SGSN at each PLMN. At the end of transmission or when a mobile station moves out of the area of a specific SGSN, the logical link is released and the resources associated with it can be reallocated.

The following diagram depicts the placement of each protocol layer within each piece of equipment:

4 Market Drivers

Popular wisdom dictates that successful applications and products are customer-driven, not supplier-driven. This axiom does not necessarily always apply to emerging technologies since specific user populations do not always directly create new markets. Sophisticated users often rush to experience the benefits of new applications long before a more widespread demand is apparent. More often than not, technology providers are faced with substantial risks just by introducing new services. GPRS is one such example because the technology requires substantial initial investment in new infrastructure minus an apparent "push" from the end-user. Nevertheless, with the advent of the Internet and the resulting explosive growth of data traffic in fixed networks, it is inevitable that the wireless world will gravitate towards GPRS. GPRS opens many new vertical -- as well as horizontal -- markets. Although GPRS was originally promoted by communications equipment suppliers, it has also generated strong interest among service providers.

GPRS has the potential to benefit various points in the network, as depicted in the table below:

Market Participant Benefits Availability

End-user Mobile high-speed Internet access - micro web browsers; e-mail; Wireless Virtual Private Networks; New IN services, such as "unified messaging"; electronic commerce; education and entertainment services. Q4 2000

Service Providers New sources of revenue with data traffic through existing GSM and TDMA infrastructure; Charge on a packet-volume basis; Expansion of market to accommodate data traffic as well as voice traffic Deployment and testing by Q2 2000

"Inside the network" equipment vendors Opportunities for existing equipment and development of new equipment, such as GGSN, SGSN and test equipment. Immediately

"Outside the network" equipment vendors Opportunities to develop GPRS-enhanced handheld devices and appliances offering a range of capabilities. Q4 1999

5 Trillium's GPRS Solutions

Trillium Digital Systems, Inc. is in a unique position to offer complete solutions for all of the GPRS network components. Trillium's implementation of GPRS protocol layers encompasses every node in a wireless network. The first version of the protocol stack will support the GSM-based wireless networks. Soon thereafter, TDMA-based networks will also be supported. Each layer can simultaneously support features in any GPRS component. For example, a single instance of SNDCP can support both the SGSN side as well as mobile terminal side. This feature is useful for testing purposes during development. When an SGSN is being built, the code for the mobile station part can be compiled out in the final product. The GPRS stack is designed so that multiple copies of every layer can be distributed across multiple processors to handle large volumes of data and for future scaling of GPRS equipment. Trillium's code has been optimized for higher performance and minimum memory requirements. In addition, Trillium delivers its software in standard C programming language.

The following GPRS stacks and layers are available from Trillium Digital Systems, Inc.:

Component

Protocol Stacks Unique Features

Serving GPRS Support Node (SGSN) GPRS - GMM, SNDCP, LLC, Network Service, and GTP

SS7 - MAP, TCAP, BSSAP+, SCCP, MTP Level 3, MTP Level 2 and MTP Level 1

Frame Relay - Q.922 Core and LMI • Each Trillium stack can service thousands of mobile stations (configurable by the operator).

• Provides load balancing among active logical interfaces (virtual channels).

• Traffic distribution can be configured per logical interface and can be used for providing dissimilar services to different users.

• Uses the "Precedent Class" to prioritize messages from various Packet Data Protocols (PDP) contexts when congestion occurs.

Gateway GPRS Support Node (GGSN) GPRS - GTP

X.25 - X.25 and LAPB • Supports socket interfaces with generic TCP/UCP stack; One instance of GTP software can support multiple GSNs at the same time.

• Can transparently pass user-defined messages; private messages and information elements can easily be added.

• Can directly interwork with Trillium's X.25 products.

Base Station System (BSS) GPRS - LLC Relay, BSSGP, and Network Service

SS7 - SCCP, MTP Level 3, MTP Level 2, and MTP Level 1

Frame Relay - Q.922 Core and LMI • Supports point-to-point connections.

• Both static and dynamic mapping of BSS Virtual circuits to Network Service Virtual circuits are provided.

Home Location Register (HLR) GPRS - BSSAP+

SS7 - MAP (GSM Phase II+), TCAP,

SCCP, ISUP, MTP Level 3, MTP Level 2 and MTP Level 1 • Existing equipment can easily be upgraded to accommodate GPRS by porting BSSAP+ whether a Trillium or a non-Trillium SS7 stack is currently in operation.

Mobile Terminal GPRS - GMM, SNDCP, LLC

X.25 - X.25 and LAPB • Small code footprint to accommodate low memory devices.

6 Summary

Until IMT-2000 becomes a reality, and perhaps long afterward, GPRS will thrive in both vertical and horizontal markets where high-speed data transmission over wireless networks is required. The deployment of GPRS networks will enable a plethora of new applications ranging from mobile e-commerce to mobile corporate VPN access. Deployment of GPRS will also have a great impact on the wireless data traffic volume by generating new sources of revenue for the service providers, especially since any current GSM network user can upgrade services to include high-speed data. We have already witnessed deployment of GPRS in several countries in Europe and the Far East, including: Telfort in Holland, One 2 One in England, T-Mobil in Germany, SmarTone Mobile Communications, Ltd. in Hong Kong, and Omnipoint in the USA.

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