Data Infrastructure

Mobile data uses much of the same infrastructure as voice, with a structure similar to BTSs and BSCs. However, instead of telephone-style switches, it needs Internet-style routers, which send data in small packets rather than through a continuous circuit. Most voice operators are now upgrading their systems to handle packet data, with the most common upgrade being GPRS. As explained in Chapter 4, "PCS Standards" , this was designed for GSM and can easily be adapted for all TDMA networks. CDMA already has some packet facilities built in, but is still used by data networks.

Mobile operators are rushing to deploy GPRS because it will enable them to use their capacity more efficiently; data effectively rides free during gaps in voice conversation and when the network is not busy, though congestion will occur at peak time. It is also relatively cheap, with standards existing to keep the cost down. According to wireless vendor Nokia, a very basic GPRS upgrade can cost only $1 million, compared to the $8 billion that Vodafone Airtouch paid for a third-generation licence UK license. Such a basic upgrade would provide limited services, but the cost for a more comprehensive network is still measured only in tens of millions.

PCU

The most expensive part of a GPRS or D-AMPS+ network is the Packet Control Unit (PCU), needed to adapt the base station for packet data. Each brand of BTS is slightly different, and so operators need to purchase PCUs from the same company that supplied their original base stations. Newer networks are GPRS-ready, meaning that PCUs are already included or can be activated with a simple software upgrade.

The precise location of a PCU within a network depends on which company supplies it and the base stations. Some require a PCU at every base station, while others allow a single PCU to be placed at a BSC or even at an MSC. The PCU is the point where data departs from a voice network, so its position can have an important effect on issues such as latency and capacity. A PCU at every base station will bypass much of the existing infrastructure, but is much more expensive to deploy.

Many D-AMPS networks already have some packet data capacity, used by the existing CDPD services. Unfortunately, this is usually not sufficient for D-AMPS+, so an upgrade is needed.

SGSN

The Serving GPRS Support Node (SGSN) is the data equivalent of the MSC. Like the MSC, it is connected to various databases, which locate and authenticate traffic. Unlike the MSC, it does not need to be directly connected to each of the cells that it serves. In an ideal network, the SGSN would have a dedicated link to each PCU in its area, but traffic can also be backhauled over the voice network. This is slower and less efficient, but allows operators to get a GPRS system up and running quickly.

An operator could theoretically advertise full GPRS coverage with a single SGSN, though most traffic would be carried to it via the voice network. This is why operators' claims have to be taken with a heavy dose of skepticism. What matters is the density of coverage—how many SGSNs the operator has for a given area. In real networks, most operators will start with a few SGSNs and add more as the network becomes crowded.

Unlike PCUs, there are clearly defined standards for connecting SGSNs to cellular networks, so operators can mix and match their suppliers. This is important, because not all SGSNs deliver the same performance; there are plenty of options, such as queue management and quality of service. The type of SGSN deployed will depend on what kind of customers the operator is targeting.

For example, the German operator T-Mobil has a GSM network based on Nokia BTSs. When it upgraded to GPRS, it had to use Nokia PCUs, but opted for Ericsson SGSNs because they were a closer match to the needs of its customers.

The GPRS Backbone

The SGSN acts as router instead of a switch. While the MSC sets up a connection, which lasts for the duration of a call, it forwards each packet of data separately. A network of SGSNs and other GPRS devices passing data in this way makes up the GPRS Backbone.

All communications networks require protocols, sets of agreed standards used for signaling and data transfer. GPRS's is called GTP (GPRS Tunneling Protocol) and is based on the TCP/IP (Transport Control Protocol/Internet Protocol) used by the Internet.

GGSN

GPRS is designed to allow access to outside data networks, and so needs another device for this function. The Gateway GPRS Support Node (GGSN) can convert data packets to and from GTP data packets to and from TCP/IP, enabling an interface to the Internet. A network requires only one GGSN, though two or more can be used for redundancy.

The GGSN hides the complexity of GPRS from the Internet. Machines based on IP see it as a single router, like any other on the Internet, and don't know that its users are mobile. This means that popular Internet applications such as the Web should function over GPRS, capacity and latency permitting.

The GPRS specification also calls for GGSNs to support X.25, a packet protocol popular before the Internet became widespread. Many operators are ignoring this, though some might decide to implement it, as many corporate networks still use X.25 to connect remote sites. It is particularly dominant in banking, used by shops for credit card authorization and by banks themselves for ATMs (Automated Teller Machines). GPRS does not support the other type of ATM (Asynchronous Transfer Mode), a communications protocol used by most fixed voice and data networks.

Optional GPRS Infrastructure

The GPRS standard is still evolving, and many other infrastructure devices are appearing from the manufacturers or being requested by operators. Some of these are shown in Figure 8.4, and include

• The Point-to-Multipoint Service Center (PTMSC), a server which handles QoS (Quality of Service) issues. It prioritizes traffic for certain customers, usually those who have paid the most, and ensures that their data jumps to the front of the queue when the network is busy. Traffic can also be prioritized by type; for example, during a videoconference, most users consider the audio to be more important than the picture.

• The Border Gateway (BG), which deals with roaming. It is usually connected to a firewall, a device that blocks unwanted traffic.

• The GPRS Charging Gateway (GCG) is needed for specialized billing options. For example, operators might want to charge customers per byte of data sent, or change the charging scheme based on network congestion.

Ricochet WAPs

The MCDN system used by Metricom's Ricochet is based on a somewhat different architecture from traditional cellular networks. Because its cells are so small—each covers a radius of only about 300 meters—fixed networks are impractical for base station links. Instead, each of its small base stations acts as a relay (hence the name Ricochet), sending signals on to a larger BTS with a cell size of about 5 kilometers.

Confusingly, these larger BTSs are known as WAPs (Wired Access Points). Despite the unfortunate acronym, they are entirely unrelated to the more well-known WAP (Wireless Application Protocol). Indeed, one of Metricom's greatest boasts is that Ricochet offers the high bandwidths needed for true Web surfing, rather than WAP's scaled-down version.

Figure 8.5 shows how MCDN fits together. The microcell base stations are about the size of a shoebox and are usually placed on top of lampposts so they can use an existing mount and power supply. The WAP is usually much higher up, more closely resembling a traditional BTS. They have to be high up because the technology used to connect the BTS and WAP at high data rates requires a near line-of-sight; their transceivers don't need to be targeted exactly at each other, but they do need to be relatively free of obstructions.