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Wireless data is finally here to stay, and its usage – even by comparison with voice – is slowly creeping upwards. As with any new wireless technology, implementation is occurring in stages. First commercial implementations of data services have often been limited to the carrier’s network to which the user has subscribed, with roaming only coming later, and then only a few carriers at a time. Customers, however, have been spoiled by years of relatively seamless voice roaming, and have no emotions when roaming works, but certainly feel grumpy when it does not.
The general concepts of roaming are similar for wireless packet data and for voice services, but the devil is in the details. Services that both have in common are:
Voice roaming is based on the use of a MAP (Mobile Application Part) protocol allowing communication between the Serving MSC (Mobile Switching Centre) and the HLR (Home Location Register) and AC/AuC (Authentication Centre) for a particular subscriber. Whenever a mobile registers with a Serving MSC, the HLR is informed of the new serving system, the mobile subscription is validated and authenticated, and the serving system is provided with a service profile. Within a very short time the mobile is able to initiate calls and short messages from the Serving MSC, and with the help of the HLR, receive calls and short messages.
The technology to provide this was developed over the past 15 years. A decade ago, voice roaming services were probably more primitive than packet data roaming services are today. We are still in adolescent times, but services are improving quickly and it will not be long before packet data roaming operates as smoothly as voice.
The two major packet data technologies in use today are GPRS running on GSM systems and cdma2000 packet data services known as 1XRTT and HRPD (High Rate Packet Data, also referred to as 1XEV-DO). More advanced data services are in the works, including higher speed cdma2000 services, and similar services from UMTS/Wideband CDMA.
GPRS was designed around the GSM TDMA system. Eight (8) voice users take turns sharing a 200 kHz frequency assignment. GPRS assigns one or more of these time-slots (up to all 8, in theory, but only 1 to 3 currently) to be shared by a number of packet data users. Each time slot used provides approximately 14 kbps of aggregate bandwidth. Because bandwidth is shared, the amount available to any single mobile varies depending on the number of users on the GPRS channel, and the amount of data they are currently transferring.
The use of some GSM enhancements, such as EDGE, can increase the total bandwidth available from each timeslot. An alternative to GPRS is HSCSD (High Speed Circuit Switched Data), which can also combine timeslots (up to 4), but they are reserved for only a single data user, meaning that technically this service is not packet data. However, because of the dedication of time-slots to a single user, it may provide higher data performance for longer transmission sessions (e.g. transfers of large files).
GPRS is tightly integrated with GSM. A mobile that has both voice and data capabilities will report to the network whether it wants to be in GPRS or voice mode. If it is in GPRS mode, it can still receive short messages, and can be informed of an incoming voice call, and switch over to voice mode (assuming that the data application allows this).
Because of this integration, GPRS can take advantage of much of the mobility management and roaming technology already built into GSM. GSM MAP continues to track the mobile, provides the serving system with information about the level of GPRS service and allows the initiation of GPRS data sessions. GSM MAP messages may be carried over the SS7 network, or over the GPRS backbone, an IP network, through the use of GTP (GPRS Tunneling Protocol).
cdma2000 has defined a number of packet data protocols, each with a progressively higher data rate. One of the most recent is HRPD (High Rate Packet Data), also known as TIA/EIA IS-856 or 1xEV-DO. In a standard CDMA 1.25 MHz (1X) channel, the protocol can provide an aggregate forward (towards the mobile) data rate of from 38 kbps to 2.4 Mbps, depending on radio conditions. The reverse channel (from the mobiles) provides data rates from 9.6 kbps to 154 kbps.
HRPD relies more on off-the-shelf internet protocols than GPRS. Terminals access the network using PPP, which is familiar to anyone who has used a dial-up modem to access the internet from a computer. PPP assigns a temporary IP address to the mobile, that can be used during the mobile-initiated packet data session, but not very easily for mobile-terminated data services. Authentication, Authorization and Accounting are provided through the internet RADIUS protocol. The RADIUS protocol is standard, but the server needs to understand new CDMA packet data information elements.
Packet data networks in CDMA are defined by the TIA-835 specification, which defines the role of the PCF (Packet Control Function) and the PDSN (Packet Data Serving Node) as well as the optional Mobile IP Home Agent (HA) and Foreign Agent (FA). A subsidiary specification, TIA-880 defines some modifications to the ANSI-41 MAP protocol to better support packet data.
Wireless packet data is still relatively young, and the integration of the internet with wireless telecommunications network is still incomplete.
Security is a problem for any communications system, but particularly for those that rely on IP protocols. Denial of service attacks, viruses, Trojan horses, unwanted access to sensitive data and many other problems abound. The problem cannot be ignored by carriers. Both their control systems must be protected from attacks as well as their subscribers’ devices. GPRS, for example, requires that elements of the network backbone (SGSN – Serving GPRS Support Node and GGSN – Gateway GPRS Support Node) have IP addresses from the public address space, but inaccessible from the public internet. This is designed to protect these nodes from unauthorized communications, such as denial-of-service attacks. As the capabilities and popularity of packet data systems increases, security must be strengthened in parallel.
One of the biggest challenges is to provide terminating data services – those that are initiated by another party, activating a service on the mobile (SMS or MMS are examples). The internet has always done a good job of simulating terminating services (e.g. by regularly checking a server for incoming email), but sometimes true terminating services are required. A website is an excellent example, but perhaps unlikely on a mobile device. Location services are perhaps a better example. They are only useful for devices that move, and the mobile has no way to predict when another party might request its location. Consequently, there must be a way to route packets to the mobile without the mobile initiating the service. When terminating packet data sessions can be established with any data-enabled mobiles, services that rely on this will become much more common.
Terminating services also introduce more security challenges. Mobiles could be the target of denial-of-service attacks or faced by network villains attempting to take over their device, view data that might be stored on it (including credit card numbers for m-commerce) or simply annoy them with spam. Both networks and the mobile will probably have to implement firewalls and other more application-specific defences.
To be truly accessible to applications on the internet, the mobile device must have its own IP address, but unfortunately this creates a host of problems. The IETF standard method for providing this is through Mobile IP, but this protocol faces a number of hurdles, particularly when firewalls, NAT (Network Address Translation) and VPNs are used. The problems arise because the mobile will, while roaming, use an IP address that is not local to the service system.
Efficiency is also a problem, as all packets must be routed to the home system, and then ‘tunneled’ (prefixed with an IP header with a different IP address) to the mobile in the current serving system.
Terminating services would also create a resource exhaustion problem if all mobile devices were given IP addresses. The total IPv4 address space is only 4 billion, so it probably would be impossible to accommodate perhaps a billion mobile devices. IPv6 is a solution, but is not yet widely implemented. Worse yet, active mobile devices are already often also given a temporary IP address for PPP purposes, making the resource exhaustion problem even more critical.
Even mobile-initiated services pose some problems. GPRS allows packets either to be routed locally or via the Home system. Routing locally is more efficient, but only routing all mobile-initiated packets through the home system can ensure that all routing and security services are correctly applied. The use of VPNs from mobile devices, crucial to obtaining acceptance by corporate IT departments, is still relatively uncommon.
Packet data roaming creates a lot of inter-network relationships between carriers, and a heavy load of signaling and data traffic. This includes roaming control messages, DNS queries and billing-related data. To simplify routing of packets, a Roaming Exchange (GRX in GPRS terminology) may be used. This means that the “N” serving carriers do not have to maintain direct connections with “N” home carriers (resulting in N-Squared direct connections) but can each direct to one or a small number of exchange providers. CDMA packet data roaming currently relies mostly on direct connections between pairs of carriers, but it is likely that devices or services similar to a GRX will be provided as the number of roaming relationships grows exponentially.
Cellular packet data systems are growing up fast, but are not mature yet. The choice is largely between GPRS, which is well integrated with GSM and provides good roaming support, but has limited bandwidth, versus cdma2000 packet data services which provide higher data rates but less mature roaming capabilities.
The race of the future is going to be for the GSM community to get UMTS/Wideband CDMA data services up and running as a true high-speed, data-oriented 3G system, and for cdma2000 packet data services to maintain their lead in speed while achieving the same level of roaming capabilities.
For many years it was a standing joke in cellular that ‘next year’ would be the big year for wireless data. Many technologies were tried, but none were dramatically successful. But slowly that is changing. Packet data still has not taken over from voice as the killer app for wireless carriers. As devices become more capable, however, and more and more communication is carried via the internet or private IP networks, packet data becomes more a necessity than a luxury. Canadian wireless consumers are learning how packet data can be useful – more applications are available than ever before. Packet data has become a revenue source that carriers cannot ignore.
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