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Wi-Fi and Cellular represent two of the most spectacularly successful wireless communication systems since TV and broadcast radio. Both now are widely used by both workers and consumers. So far they are largely separate worlds. But a lot of people would like to see them develop a closer relationship.
Cellular began as a pure voice system in the early 1980s. For a long time, data was “coming next year,” as analog cellular data modems never achieved a big market. The transition to digital (2G) in the 1990s occurred without much data penetration. Data was still conceived of as ‘circuit oriented,’ and digital connections played even less well with modems than analog cellular.
SMS (‘texting’), while not a general-purpose data system, was a data application that did catch on with consumers who realized that they had in their hands a device that was useful for more than just talking. At first, SMS was just popular in Europe, where the ubiquitous GSM system guaranteed interoperability. When the CWTA worked with Canadian carriers to eliminate the barriers between multiple technologies (CDMA, GSM and TDMA), Canadian consumers also began to realize that, for some forms of communications, data was more appropriate than voice.
More general purpose data didn’t catch on in cellular until internet-based systems such as WAP became more available. Data really started to click with consumers when ... well, when it took just a click to download a ring tone, a game or wallpaper for their phone. Cellular data, whether GPRS/EDGE or 1X/DO, can also be used to provide a raw IP connection either from a PDA or from a PC-card. Over this connection, users can do anything that’s possible over a wired internet connection, including email and web surfing. Data speeds have rocketed from the modem-like speeds of analog (1-2 kbps) to speeds that are closing in on DSL and cable modems (in the megabit per second range at least when downloading).
Wi-Fi, more formally known as IEEE 802.11b, was always, by contrast, designed as a data service – voice was not a consideration. It was designed as a replacement for Ethernet, also an IEEE standard, for situations where wires are inconvenient. By providing 10 Mbps, Wi-Fi showed that wireless did not have to mean a loss of performance. Quite often the Wi-Fi radio link is not the limiting factor in a network. Even home users with high speed internet will have more radio bandwidth than on the wire leaving their home to their Cable or DSL provider.
Wi-Fi has become so popular that most personal computers now have it built in, or at least have the option of adding it cheaply. Wi-Fi access points are not much more expensive than wired Ethernet hubs, bringing Wi-Fi into the reach of most consumers who own multiple computers. The trade-off between cost and convenience has swung towards Wi-Fi rather than stringing Ethernet cables.
One of the most popular technologies of ‘next year’ is Voice Over IP (VoIP). If this becomes popular over Wi-Fi, then the circle will be complete. Cellular has adopted data capabilities, and Wi-Fi will have adopted voice.
Wi-Fi and Cellular obviously have considerable overlap, but should they work together or compete?
The answer in the market place is that the technologies provide complementary services more than they compete, and that interworking therefore makes economic sense. Wi-Fi can currently provide more bandwidth than cellular, but has only a tiny fraction of the coverage. Users can often get free or ‘all you can eat’ Wi-Fi access, making it cheaper, as well as faster, than cellular, but as soon as they leave the hotel, office or airport, they totally lose Wi-Fi coverage.
There are a number of different roaming options that consumers may benefit from. Carriers will benefit too, as people will see more value in wireless services, and will use them more:
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There are many aspects to Wi-Fi /Cellular interworking that make the process challenging. It will be some time before all are overcome and this service is ubiquitous. However, these have all been overcome for earlier technologies, so one can predict that it is just a matter of time before products become available, and carriers identify solutions, and verify that the marketplace demand is present.
Many of the challenges arise from fundamental philosophical differences between the designers of cellular and Wi-Fi systems. Cellular systems are rooted in telecom. Their backbone network, known as the Mobile Application Part (MAP), is in many important ways an extension of wireline telecommunications protocols, especially those from the SS7 family such as ISUP (ISDN User Part). Even though there are many differences between GSM MAP (used by GSM and UMTS/Wideband CDMA) and ANSI-41 MAP (used by CDMA, TDMA and Analog), there are important commonalities, such as the use of a Home Location Register to store subscriber information. Telecom protocols tend to be heavily integrated, with one message conveying a variety of information. MAP messages may contain security information, information for billing, network addressing, mobility management parameters, call processing, the current state of the mobile and the services to which a mobile has subscribed. Designed for relatively lo w bandwidth (but highly reliable) links, messages are designed to be very compact.
Wi-Fi comes from a computer communications heritage, and relies much more heavily on the internet family of protocols. These protocols tend to be much more modular. Mobile IP, for example focuses only on mobility management, while the AAA protocols RADIUS or DIAMETER focus on authentication and billing, and call processing might be provided by SIP and SDP. Designed for high bandwidth links, these protocols also tend to be much less optimized, especially at the higher layers.
Interworking between two such different technologies has to address a number of different requirement areas to produce a service that is desirable to consumers and makes money for both the Wi-Fi and cellular operators.
Mobile phones have traditionally been identified by the 10-digit MIN (Mobile Identification Number) or 15-digit IMSI (International Mobile Subscription Identity). Wireless internet devices often do not have a permanent IP address assigned to them, so cellular 3G internet protocols have introduced the concept of the Network Access Identifier (NAI), which looks like an email address. It is this identifier that will be used to provide a common identification in both worlds.
The NAI is sometimes not the most desirable identifier in the cellular network, so translation to the mobile’s phone number (aka MDN or MSISIDN), MIN or IMSI may be necessary to interface with legacy networks. This translation can be done inside the network, although it will require correct provisioning procedures to ensure that all required identifiers are present at the point of conversion.
Mobile devices not only have to identify themselves to the network, but the network also has to advertise itself to the mobile. Wi-Fi networks use the SSID (Service Set Identifier) to do this, while cellular systems use the SID (System ID) or a portion of an IMSI.
A good deal of software and memory in cellular phones is devoted to system selection. The carrier will often provide a complex algorithm that the phone must obey, to lock onto the system that will provide the customer and the carrier with the most effective service. There is no point in a phone locking onto a cellular system that does not have a roaming agreement with the mobile’s home carrier...even if that system happens to have the strongest signal (with an exception for emergency services where the strongest signal usually rules). Adding Wi-Fi into the picture makes life even more complex. The cellular carrier will likely have roaming arrangements with some hotspot providers, but not with others. Some roaming agreements will be preferred, while others will merely be tolerable. Mobiles will likely be programmed with a list of SSIDs that are valid, with each one given a priority. This will require careful management and constant updating to ensure that mobile s do not deny themselves service simply because they can’t recognize the systems they are roaming in.
The only simplification is that since Wi-Fi coverage is generally much spottier than cellular, and the throughput generally greater, a Wi-Fi hotspot will usually be preferred by the phone over cellular coverage, although there will be exceptions. In the absence of seamless handoff, for example, people initiating a long session may well want to stick with cellular, even though some of the session could be provided with higher bandwidth and possibly more cheaply by Wi-Fi.
Operators need to make sure that devices requesting access are valid, and that all usage is charged to some kind of an account. “Authentication” validates the identity provided by a mobile using cryptographic techniques. Once the identity has been verified, the subscription and service profile of the user can be examined. Only then can services be provided, at which time accounting records will be collected. The process of “Accounting” is to ensure that these records are converted into bills, deducted from a prepaid account or charged to a credit card.
The network device that performs these functions is known as the AAA (usually pronounced “Triple A”). Most public Wi-Fi systems are connected to one of these already, in order to validate users of the system. For Wi-Fi /Cellular interworking, it is critical that AAAs also be interworked. When a mobile is roaming in a Wi-Fi system, it will communicate with the local AAA. Based on the NAI, this will identify the Home AAA, which is the real source of the information. To minimize the complexity of the network, there may be a ‘Broker’ AAA in between. This reduces the number of systems with which each AAA has to be associated.
AAAs generally communicate today with the RADIUS protocol. DIAMETER is a future option.
Internet services (including VoIP) always require that the device has an IP address, if only for the duration of the service. The serving system’s AAA, among its other jobs, will most likely also be responsible for assigning a temporary IP address to the mobile. This is not the most desirable situation, because it complicates the transmission of data to the mobile, especially for mobile-terminated services (i.e. when a network server attempts to initiate a service rather than the mobile). However, the alternative service, Mobile IP, is not widely deployed because of other problems, including excessive consumption of IP addresses, routing and scalability. A temporary IP address works well for mobile-initiated, short-lived data sessions (such as web browsing or a download of emails).
Security in Wi-Fi hotspot is preferably provided by EAP (Extensible Authentication Protocol), either the version based on the internet TLS protocol, or one based on AKA, the 3G security algorithm required by UMTS and being developed for cdma2000. One of the problems with Wi-Fi security in the past has been provisioning of one key for each hotspot. EAP solves this problem but would seem to require that a second secret key has to be programmed into each mobile. This requirement can be eliminated if the Wi-Fi secret key is generated from cellular key material. This must be done using cryptographically strong algorithms to avoid a security lapse in one key resulting in the other becoming compromised.
Security algorithms are executed by an EAP Server that is usually connected to the home system’s AAA.
“Presence” is an internet term for identifying the activity status of a device and its internet location (IP address). This facilitates services like instant messaging. Presence information is derived from basic accounting data (RADIUS records), so when a presence-enabled device roams into a Wi-Fi hotspot, this information will probably be routed to the home carrier’s Presence Server via the chain of AAAs. As soon as this happens, the mobile user’s IM buddies will see the status change to active and instant messaging can resume.
No matter whether a Wi-Fi hotspot provides VoIP or not, emergency calls (e.g. 911) have to be supported. If necessary, the phone will simply drop the Wi-Fi connection, lock onto a cellular signal and make the call there. If VoIP is available in Wi-Fi mode, the phone will have to get its position from the Wi-Fi system so that its 911 call can be routed to the correct PSAP (Public Safety Answering Point).
One requirement that carriers won’t be talking about much publicly is the ability to allow law enforcement to eavesdrop on voice or data sessions that cross between cellular and Wi-Fi systems. Luckily for the carriers, one of the requirements of this service is “stealth,” which means that communications between the Wireless LAN and Cellular systems are forbidden. This means law enforcement agencies will have to contact both carriers separately to be sure that all communications by a “person of interest” are intercepted.
Voice is unlike most other data services, especially when the voice service is a phone conversation. When voice is digitized, it is better for packets to be dropped than delivered late or out of sequence. It is very difficult to converse when voice is significantly delayed (as over a satellite link) and even more difficult when the packet delays are random, resulting in distortion of the voice as well as delays. Telecommunication systems are generally designed for voice, so this is rarely a problem. Voice over IP on any system demands greater reliability from the underlying network than the original internet protocols could provide.
Increasing the reliability of communications demands Quality of Service. Essentially, the necessary characteristics of a traffic type needs to be defined, and then routers and other internet network devices programmed to handle that traffic type try to provide the quality that is required. QoS will not make much difference in a lightly loaded system because ‘best effort’ sharing of bandwidth will provide enough for all users. However, as the shared radio interface becomes more heavily loaded, QoS will become essential to give VoIP users the priority needed to maintain their conversations.
Wi-Fi was initially designed without support for Quality of Service. IEEE enhancements to 802.11 to support QoS may prove necessary before VoIP can be widely supported.
One of the ultimate features of Wi-Fi /Cellular interworking will be the ability to maintain communications while crossing the boundary between them. Most likely this will work from Wi-Fi to Cellular, but not in the other direction. This is because there will almost always be cellular coverage outside a Wi-Fi hotspot, but it is unlikely that there will be Wi-Fi coverage outside the boundaries of a cellular coverage area.
Handoff is very tricky for both the network and the mobile. Handoff can only be accomplished when the cellsite to receive the call is identified and resources assigned within it. Handoff will be easiest if the mobile device has dual radios, because then the cellular radio can be used for handoff purposes while the Wi-Fi radio is still providing service. With one radio it is more difficult to measure the signal quality in neighboring cellsites, although it may still be possible if the radio can quickly switch between the two different modes. It is likely that, at first, mobiles will be built with two radios and that later more highly optimized radios and handoff systems will emerge, resulting in smaller, cheaper and more efficient modules.
It is unlikely that handoff will be provided with initial Wi-Fi /Cellular interworking systems. Instead, sessions will have to be established in each system separately. Handoff will become more of a need when and if VoIP becomes a major application on Wi-Fi.
The ultimate in interoperability would be the ability to operate in both Wi-Fi and Cellular modes at the same time. The most likely scenario is a data session using Wi-Fi combined with a voice call using cellular. This would require the mobile to have two radios, and would also mean special handling in the network. Currently, a device can only be registered in one place at a time, so in some ways, the network would have to treat the mobile as two devices, perhaps knowing that it is supposed to route data services to the Wi-Fi twin when it is active, but voice services via the cellular network. Obviously this can get complicated when some (but probably not all) Wi-Fi hotspots can provide voice services.
Location services, often based upon GPS, are becoming more prevalent in cellular systems. One of the main reasons is to provide the location information required for processing emergency calls, and then routing help to the right place. Location in Wi-Fi might seem simpler because the ‘cells’ are so small, but it is actually a lot more challenging. If the location of the access point (Wi-Fi access point) is known, then for most applications, the job is done. That is a big “If”, however. Access points are not GPS-enabled, so the easiest approach is just to program the location into the access point. However, this complicates the job of installation and the access point’s location must be entered by a technician each time it is moved (with the consequent risk of error). Many cellular phones have GPS capabilities, but they require network support, and that is not available in Wi-Fi. Consequently, location services must be placed on the “To Do” ; list of wireless carriers. This capability cannot stay there all that long, however, because VoIP over Wi-Fi will require location for emergency calls, if nothing else.
Those who were expecting a quickie marriage between Wi-Fi and Cellular will be disappointed. The relationship will evolve slowly and carefully. Consumers will gradually see more capabilities evolve, but for the more sophisticated capabilities, they will need to have some patience. Patience may well be rewarded by a marriage made in heaven. One day in the not too distant future the average consumer may be able to blissfully forget the difference between Wi-Fi and Cellular.
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UMTS/GSM |
3GPP TR 22.935 | Location Services Feasibility Study |
| 3GPP TR 23.836 |
Quality of Service (QoS) and Policy |
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| 3GPP TS 22.234 |
Stage 1 (Requirements) |
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| 3GPP TS 29.161 |
Interworking Requirements |
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| 3GPP TS 29.234 |
Stage 3 (Protocol Details) |
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| 3GPP TS 32.252 |
Charging |
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| 3GPP TS 33.234 |
Security |
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cdma2000 |
3GPP2 S.R0087 |
Stage 1 (Requirements) |
| 3GPP2 X.S0028 |
WLAN/cdma2000 Interworking |
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AAA |
IETF RFC 2486 |
Network Access Identifier (NAI) |
| IETF RFC 2865 |
RADIUS |
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| IETF RFC 2866 |
RADIUS Accounting |
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| IETF RFC 3579 |
RADIUS Support for EAP |
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| IETF RFC 3580 |
Wi-Fi RADIUS Guidelines |
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| IETF RFC 3588 |
DIAMETER |
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Wireless LAN |
IEEE Std 802.11b-1999 |
‘Wi-Fi’ Wireless LAN (10 Mbps) |
David Crowe is a wireless standards, technology and numbering resource consultant based in Calgary. He can be reached at David.Crowe@cnp-wireless.com.
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