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In the unsettled times since September 11, 2001 the role of emergency workers and wireless communications systems have both achieved a higher profile. The unquestioning bravery of firefighters who went into the World Trade Center while others were leaving will not be forgotten for a long time. Nor will the furtive cellular phone conversations that inspired the passengers on one doomed plane to revolt at the cost of their own lives.
Communications is important when people are working or playing, but becomes critical in times of crisis. Emergency workers often have their own radio systems, but systems used by different agencies are often incompatible, and wireless phones often become a ubiquitous and compatible backup system. But during emergencies, wireless systems often become overloaded to the point that emergency workers have trouble getting critical calls through.
Emergency agencies in the United States, led by the National Communications Systems, have been promoting the concept of “Wireless Priority Service” (WPS) for several years, and now it appears like their dream may become a reality. The US Federal Communications Commission (www.fcc.gov) ruled in the year 2000 (WT Docket No. 96-86) that providing priority access to radio channels for emergency workers is allowable. Government agencies and industry representatives are now working feverishly to bring various levels of support for this service to fruition within the next two years. If this service is successfully deployed in the US, it is likely that Canada and other countries will implement similar systems.
Priority service provides emergency workers with preferred access to radio channels as they become available ... but does not knock people off channels who are currently in a call. A system based on pre-emption would quickly make the system unusable for all but the highest priority callers, because nobody else would be able to talk for more than a few seconds before their calls were interrupted.
Channels will be assigned to the priority caller who has been waiting the longest at the highest priority level. Five different priority levels are defined for the US service:
Level |
Assigned to... |
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1 |
Executive Leadership, Policy Makers, Critical Wireless Carrier Technicians |
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2 |
Disaster response, Military command and control |
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3 |
Public health, safety, and law enforcement command |
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4 |
Public services, utilities and public welfare |
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5 |
Disaster recovery |
It is quite likely that if a service like this is implemented in Canada, that priorities will be assigned according to our national requirements.
US wireless carriers have been unhappy with a number of government mandates that are either unfunded, or underfunded – Enhanced 911 (virtually unfunded), Lawfully Authorized Electronic Surveillance (underfunded) and Local Number Portability (with definite costs and only the possibility of additional revenue).
Priority Service is a refreshing change. It is a carrier option, although there may be significant pressure to implement it. More importantly, it will be funded by regular government payments to provide access to the service, plus additional fees for airtime that is used whenever the service is invoked. The US FCC has also promised that carriers will be immune from liability. This will be important if, for example, a carrier is sued by someone who believes that they were harmed – perhaps because they could not get an important call through during an emergency when priority service users were occupying all channels.
Emergency workers will not get priority without explicitly asking for it. They will have to dial a special feature code to initiate a priority call. These calls will be tracked, and they will have to account for any abuse of the service. Emergency workers will have their cellular phones listed in a database, along with the priority level that they are eligible to use.
One of the requirements for Priority Service is that any phone can be temporarily enrolled for service merely by dialing a special code. This is intended to allow emergency workers to borrow a phone from a civilian if they do not have a phone, or if their battery has died. This implies that any phone can be used for this service.
However, there are three important classes of phones that cannot be used with priority service. Analog phones are not supported because of the difficult of notification. TDMA (ANSI-136) phones were originally considered for inclusion, but were dropped because of the migration of major TDMA carriers AT&T Wireless and Cingular to GSM, and their consequent lack of interest in developing major new capabilities. Prepaid phones are also not supported, although the reasons for this restriction are not so clear. This means that, on some systems, an emergency worker stands only a small chance of being able to use a randomly selected mobile.
The service is designed to work on any post-paid (i.e. monthly billed) CDMA or GSM mobile. GSM systems will use a priority built into the Subscriber Identification Module (SIM, aka ‘Smart Card’). This service is known as Multi-Level Priority and Pre-emption (although the Pre-emption portion will not be used). GSM phones with low MLPP priority will not even attempt to access the system at times of overload. This priority will be verified by the HLR to ensure that SIM cards illegally or erroneously programmed do not allow priority access. Emergency workers will need to install their SIM into a borrowed phone (if they have one) in order to obtain priority access.
CDMA systems, in contrast, will have their priority controlled by the network. All calls dialed with the special WPS feature code will result in a query of the WPS database (WPS DB).
The similarities and differences between these techniques is shown in Figure 1.
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If priority service is invoked when no radio channels are available the mobile’s request will be acknowledged by a special audible alert. The mobile’s identity will then be added to the end of the queue for mobiles of the same priority. It will remain there until a radio channel becomes available, the mobile is used to initiate another call (priority or not) or until a maximum amount of time elapses. When a channel does become available, the mobile will be alerted again. Only if the user answers will the call be put through. If the user does not respond, the radio channel will be assigned to the next priority user. This is shown in Figure 2.
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Current requirements for priority service allow carriers to reserve some radio resources for public use, not priority use. This would guarantee that some calls by the general public could get through even though there are enough priority calls to use up all channels.
The US government has chosen a phased implementation for Priority Service for wireless (WPS). The first phase was a custom system to be implemented in New York, Washington and Salt Lake City (in time for the Winter Olympics). The next phase is known as Initial Operating Capabilities (IOC) and is to be in place by the end of 2002. It includes most of the capabilities described in this article. IOC requirements were first published in January, 2002.
The Final Operating Capabilities (FOC) add inter-carrier/inter-system operations and are to be implemented by the end of 2003 (a very aggressive schedule). This will allow wireless carriers to pass the priority of a mobile to the landline telephone network (PSTN) in an ISUP IAM message. This priority will be passed around the network with the call and will allow internal network resources (e.g. long distance trunks) to be assigned on a priority basis. This capability is defined in Telcordia GR-2931-CORE (High Probability of Call Completion – HPC).
If a priority call encounters an all-trunks-busy condition within the network, there is the possibility that it will be queued for the next available trunk, increasing the probability of call completion at times of high congestion even further.
If a priority call is destined for a mobile, the priority will be transmitted to the wireless carrier, which will allow the call to get priority access to available radio channels.
Inter-system support for queuing priority may also be supported in the FOC. If a mobile that is queued for a radio channel moves out of the coverage area of one cell and into another, it may not be queued at the end of its priority queue, but based on the amount of time that it was queued in other cells. If this is not done, mobiles in areas of poor coverage may continually be bumped out of the queue, making it even more difficult to get through.
Priority may also extend to inter-MSC handoffs, with radio channels being preferentially assigned to mobiles that were initiated with a priority request.
FOC requirements were still being refined when this article was written.
The implementation of Priority Service is not without risks. Perhaps the biggest is the risk of fraud. Temporary enrollment of phones could make it possible for people who discover the special enrollment codes (which will be sent over the radio interface, unencrypted, in many systems) to fraudulently obtain priority service, at no charge to them. This risk is currently being assumed by the US government. But many wireless experts remember the days in the mid-1990’s when analog cloning was a US$500 million annual loss for wireless carriers.
Another risk is that engineering of channel reservations will result in idle channels reserved for priority users (or public users) while the other category is over-utilized. Reservations may have to be ‘soft’, allowing all channels to be used by emergency workers if there are no calls by the public. This situation can easily occur when an area is evacuated due to a fire, earthquake or flood, and only emergency workers are left. The public reservation will cease to have value at this point.
Another risk with a priority system is that an emergency worker invoking priority for a not-so-important call, prevents a civilian with a potentially life-saving call from getting through. This is an intractable problem, and probably impossible to solve. One has to assume that emergency workers will not invoke priority unnecessarily, and infractions will have to be dealt with seriously.
Emergency workers have a job that is quite often extremely dangerous and can mean the difference between life and death for the people they are trying to evacuate from a burning building, rescue from a flood or extricate from a multi-car pileup on the 401. Communications are an essential part of this lifeline. Although wireless systems were not designed for emergency use, they have proved their adaptability on many occasions. They are an important backup/alternate system. But, because of their public status, and limited capacity, priority access by emergency workers can enhance their life-saving potential.
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