GSM Overview

This section gives a quick overview of GPRSClosed General Packet Radio System, a non-voice value-added service for faster data transactions over a mobile telephone network, designed for deployment on GSM and TDMA-based mobile networks. GPRS overlays a packet-based air interface on the existing switched network., UMTSClosed Universal Mobile Telephone System, a third generation service (part of the IMT-2000 vision) that is expected to enable cellular service providers to deliver high-value broadband information, commerce and entertainment services to mobile users via fixed, wireless and satellite networks., and LTEClosed Long Term Evolution - a standard for wireless broadband communication for mobile devices and data terminals, based on the GSM/EDGE and UMTS/HSPA technologies. It increases the capacity and speed using a different radio interface together with core network improvements..

A Global System for Mobile Communications

The most widely deployed wireless networks worldwide are those based on Global System for Mobile Communications, or GSMClosed Global System for Mobile Communications (originally Groupe Speciale Mobile, hence the acronym) - a second generation time-division mobile network standard., technology. Formerly known as "Groupe Special Mobile," GSM is a world-wide standard for digital wireless mobile phones. The standard was originated by the European Conference of Postal and Telecommunications Administrations (CEPT) and further developed by the European Telecommunications Standards Institute (ETSI) as a standard for European mobile phones, with the intention of developing an open, non-proprietary standard for adoption world-wide. It has been remarkably successful, with more than one billion people using GSM phones as of early 2004.

The ubiquity of the GSM standard makes intra-nation roaming very common, with international roaming frequently enabled by "roaming agreements" between operators. GSM differs from its predecessors most significantly in that both signaling and speech channels are digital. It has also been designed for a moderate level of security. GSM employs time division multiple access between stations on a frequency duplex pair of radio channels, with slow frequency hopping between channels.

General Packet Radio Services

General Packet Radio Services, or GPRS, is a GSM extension which allows packet switched data transmission. GPRS has been called 2.5G, as it is viewed as a stepping stone toward pure 3G systems like UMTS/W-CDMA or similar. GPRS is backward compatible with GSM, a fact that eases the migration path for GSM operators, who can gradually upgrade their infrastructure as the GPRS market expands.

From the user's point of view, GPRS is a wireless extension of data networks. It can access multiple types of data networks, such as IP based networks like the public Internet, private intranet, both IPv4 and IPv6 protocols, and X.25 based networks. GPRS upgrades GSM data services providing:

Thus mobile subscribers can receive an array of services, including web browsing, e-mail communications, intranet access and location-based services.

GPRS is basically an addition to GSM that enables packet based communications. Data transmitted by packet switching is faster and more efficient than circuit switching, the method used in 2G networks. Whereas in GSM timeslots are normally allocated to create a circuit-switched connection, in GPRS timeslots are allocated to a packet-connection on an as-needed basis. This means that if no data are sent by a sender, the frequencies involved remain free to be used by others. Users of GPRS networks can stay continuously logged in to email and Internet services, while paying for these services only when sending and receiving data.

Development of GPRS is directed by the 3rd Generation Partnership Project (3GPP), a collaboration agreement established in 1998. 3GPP's original goal was to produce technical specifications for third generation mobile systems, and now is involved in maintaining and developing GSM standards, including GPRS.

Universal Mobile Telecommunications System

Universal Mobile Telecommunications System, or UMTS, is one of the third generation (3G) mobile phone. UMTS further extends the capabilities of GPRS networks, offering much higher air interface bandwidth. UMTS networks provide an average bandwidth of up to 384Kbit/sec, which is more than 26 times the bandwidth obtainable on a single GSM error-corrected circuit switched data channel. This increased bandwidth allows for the development and support of a whole new set of services, mostly multimedia-based, such as video streaming, video conferencing, online games, advanced location services, and more.

IP Multimedia Subsystem

A description of the evolving UMTS network would not be complete without mentioning IP Multimedia Subsystem, or IMS. The IP Multimedia Subsystem (IMS) is a common architecture that allows cellular operators to provide multimedia services. Promoted by 3GPP, IMS uses SIP as its basic signaling protocol.

IMS uses SIP to register and authenticate the mobile user when joining a multimedia session, as well as to initiate the session by locating the destination of the session (either a multimedia server, or other mobile user, or other non-mobile user).

By selecting a standard protocol for multimedia services, the aim is to eliminate interoperability issues in the creation of multimedia sessions between mobile users, and between mobile users and users on the Internet.

Check Point's portfolio of cellular security solutions includes solutions for IMS security as well.

Long Term Evolution (LTE)

In response to the high demand for connectivity for new hand-held devices and mobile applications, the 3rd Generation Partnership Project, a collaboration between telecommunication associations and the European Telecommunications Standards Institute, has introduced the Long Term Evolution specification. LTE, also known as 4G, is a cellular internet protocol designed to increase the speed and download capacity of mobile (wireless) networks.

The LTE term also refers to SAE, or System Architecture Evolution. SAE is the core network architecture of the 3rd Generation Partnership Project's wireless communication standard.

Basic Components of GPRS/UMTS Networks

On the Network

PLMNClosed Public Land Mobile Network. (Public Land Mobile Network) - a mobile wireless network that uses land‑based radio transmitters or base stations.

PDNClosed Packet Data Network - a network that carries user data in packets (for example, Internet and X.25) (Public Data Network or Packet Data Network) - a network that provides packetized data services, such as the Internet.

GSNClosed GPRS Support Node. or xGSN (GPRS Support Node) - a generic term that refers to both SGSNs and GGSNs.

MSClosed Mobile Station - a portable device that connects subscribers to a wireless network, for example a cellular phone or a laptop with a cellular modem. (Mobile Station) - a wireless device that uses a radio interface to access network services.

GRX (GPRS Roaming eXchange) - an IP network that connects PLMNs, enabling MSs to connect to their home PLMNs through roaming partners.

APNClosed Access Point Name - the identifier of an external packet data network. (Access Point Name) - provides routing information for SGSNs

PDF (Policy Decision Function) - logical element that uses standard IP mechanisms to implement policy in the IP media layer. The PDF uses policy rules to make decisions in regard to network based IP policy, and communicates these decisions to the PEP on the GGSN.

PEP (Policy Enforcement Point) -logical entity that enforces policy decisions made by the PDF. It resides on the GGSN.

Interfaces

An interface is the point of connection between telecommunication entities. While there are many types of interfaces in a cellular network, this guide deals primarily with these:

Basic Components of LTE Networks

Interfaces

S5 and S8 are the main interfaces used for roaming. S5 is used in the Home Public Land Mobile Network (HPLMN) and S8 in the Visiting Public Land Mobile Network (VPLMNClosed Visited Public Land Mobile Network - the network where the MS is currently located.).

  • SGi interface - connects PGW to an external PDN.

  • S5 interface - connects SGW and PGW on same PLMN.

  • S8 interface - connects SGW on the VPLMN with the PGW at the HPLMN.

  • S11 interface - connects the MME to the SGW.

Signaling Protocols

GTP (GPRS Tunneling Protocol) - used to transport user data between GSNs. The data is encapsulated inside a packet, which consists of the data payload and a routing header. GTP versions have been updated to include new capabilities, however most GPRS/IPX networks maintain support for both.

GTP-C (GPRS Tunneling Protocol - Control) - used for control messages to create, update and delete GTP tunnels, and for path management.

GTP-U (GPRS Tunneling Protocol - User) - used for user messages to carry user data packets, and signaling messages for path management and error indication.

TEIDClosed Tunnel End Point Identification - The GTP version 1 uni-directional tunnel identifier. (Tunnel Endpoint Identifier) - used to unambiguously identify a tunnel endpoint.

G-PDUClosed A user data message, comprising a G-PDU and a GTP header. (GTP Protocol Data Unit) - used for data and control information.

PDPClosed Packet Data Protocol - a network protocol used by an external packet data network (usually IP). (Packet Data Protocol) - a network protocol used by an external packet data network (usually IP).

PDP addressClosed The MS's address in the external packet data network, also called End User IP address. - the address of an MS in the external packet data network, also called End User IP address.

PDP contextClosed Information sets held in MS and GSNs for a specific PDP address./session - a logical association between an MS and PDN. There are six types of PDP context commands:

  • Create

  • Update

  • Delete

  • Modify (only GTPv2)

  • Request

  • Response

For an extensive list of industry-specific terms, see the Glossary.

Comparing GTP Versions

The most important differences between GTP version 0 and version 1 arise from the fact that GTP version 1 supports several different services simultaneously, which in turn requires a clearer focus on Quality of Service (QoSClosed Check Point Software Blade on a Security Gateway that provides policy-based traffic bandwidth management to prioritize business-critical traffic and guarantee bandwidth and control latency.). While the difference between GTPv0/v1 to GTPv2 is due to the change in the network nodes and their roles the main new node presented is the MME which in turn introduced new concept of tunneling different the main difference is that GTP tunnel is now addressed as session, with bearers that act as data link context.

Port Changes

While the entire GTP version 0 communication is transmitted over a single UDP (3386), GTP version 1/2 packets are transmitted over two different UDP ports:

  • The Control plane, which includes the create, update, delete, modify and echo exchanges, now uses UDP port 2123.

  • The User plane, which includes the tunneled data packets, now uses UDP port 2152.

By separating signaling and mobile user traffic to two different ports, either one of these types of traffic can be encrypted without the other.

Multiple PDP Contexts for the Same PDP Address

In GTP version 0, an MS might have several simultaneous PDP contexts, but a single PDP address on a specific APN is uniquely associated with a single PDP context. For each combination of external packet network and MS-local address, there is one tunnel (PDP context).

In GTP versions 1 and 2, multiple PDP contexts/sessions are allowed per PDP address and APN. After a successful GPRS activation, where the MS establishes a PDP context/session and connects to the external network, the MS can initiate more PDP contexts/sessions with the same APN.

The new PDP contexts/sessions for the same PDP address differ in QoS requirements and charging characteristics, and are used to separate streams of different services or protocols.

This is useful for IMS, where the initial PDP Context/sessions (used for SIP registration) have low bandwidth requirements, but the following PDP Contexts/sessions (used for actual data streaming) require a higher bandwidth.