On Aug. 8, 2002, the Federal Communications Commission sounded the death knell for CDPD — Cellular Digital Packet Data services in the United States. On that date, the FCC set a five-year “sunset” period for the elimination of the requirement that cellular telephony service providers reserve a portion of their 800 MHz cellular spectrum for Advanced Mobile Phone Service (AMPS), better known as “analog” cellular.

For years, cellular telephony service providers have chafed at the requirement to carry analog cellular because those channels could produce more revenue when converted to digital.

CDPD was designed in the early days of analog cellular to fit data traffic “around” voice traffic on analog cellular systems.

In general, CDPD worked well enough, but did not ever become popular enough to earn back the R&D and deployment expenditures of equipment vendors and service providers.

The problem with CDPD was largely that service rates were too high — initially, priced by the kilobyte and only much later, at a flat rate acceptable to at least a few potential users.

What CDPD usage there was came in the last few years as carriers decided to earn at least some revenue from their investment by pricing CDPD services reasonably and at last finding a market in law enforcement.

Law enforcement used CDPD with in-car laptops to access criminology databases for such tasks as checking license plate numbers during routine traffic stops (freeing dispatch operators for higher-priority communications).

CDPD was attractive to law enforcement mostly because it did not consume precious public safety spectrum better used for voice communications and did not require construction of an infrastructure (not to mention an infrastructure based on TCP/IP data, which most public safety communications departments knew little about at the time CDPD was coming into use in public safety).

Now that analog cellular will be sunset, CDPD users face a transition. AT&T Wireless' CDPD users have already been notified that the company will discontinue CDPD service (accounts vary as to the date- some say “end of 2003,” others say June 2004.)

AT&T Wireless is “encouraging” CDPD users to rapidly transition to its new General Packet Radio Service (GPRS) network, part of its new GSM (Global System Mobile) network now being phased-in across the country.

Other CDPD service providers have not yet announced discontinuance of CDPD, but it is unlikely in the extreme that any cellular telephony service provider will continue analog cellular telephone service in urban areas a moment beyond the FCC sunset date.

The potential revenue resulting from conversion to digital, is simply too compelling.

There are a number of potential replacement solutions available for CDPD:

  • The data services of cellular telephony service providers, as well as Nextel, have improved considerably from several years ago, and no additional infrastructure expense is required. But, with such a service, there is the ongoing expense of usage-based service fees (rather than the flat fees offered for CDPD service).

  • Aerie Networks has resurrected the Metricom Ricochet service in Denver and San Diego in part by negotiating a service agreement with public safety users in return for “reclaiming” the poletop radios abandoned during Metricom's bankruptcy. One key issue in public safety use of Ricochet is that the poletop units, which provide the vast majority of the coverage area in a Ricochet network, have no battery backup; in a power outage much of a Ricochet network won't be accessible.

  • The FCC has promised additional public safety spectrum resulting from television broadcasters vacating television broadcasting channels 52-69… eventually… someday… maybe which could be potentially used for data. New infrastructure would have to be constructed, but going forward, there wouldn't be any service fees.

Several trends have converged to give rise to another alternative for data-to-the-car capability — use of license-exempt spectrum such as 2.4 GHz.

At first blush, license-exempt spectrum would not be seriously considered for public safety use, for reasons such as:

  • Not licensed, therefore the potential for interference is high
  • Very low power levels are dictated by the license-exempt rules
  • License-exempt equipment such as Wi-Fi simply isn't rugged enough for public safety duty

These factors argue that systems operating in license-exempt spectrum cannot be considered as “life-safety” quality.

Enter Alvarion

Alvarion is one of the largest vendors of systems specifically designed for the Broadband Wireless Internet Access (BWIA) industry. While the vast majority of Alvarion customers have deployed their equipment in stationary applications, over the last few years, Alvarion has received a number of requests for “mobility” wireless systems, including commercial aircraft-to-terminal communications, mega-cranes at international seaports, and even for providing Internet connectivity at highway speeds. To meet the “highway speeds” requirement, Alvarion developed a “fast handoff” algorithm for its BreezeNET product line, which results in no loss of data during inter-cell handoffs, even at highway speeds.

All these applications have three requirements in common:

  • High speed data communications - typically a minimum of 1 Mbps using TCP/IP
  • Use of license-exempt spectrum, typically 2.4 GHz for worldwide commonality
  • Robust and secure communications (even though using 2.4 GHz license-exempt spectrum)

To meet these requirements, and a rapidly-evolving market for homeland security/public safety requirements, Alvarion developed a new product member of its BreezeAccess II product line, the SU-M (Subscriber Unit — Mobile).

Some of the features of the SU-M (see Figure 1) include:

  • Integral shock mounting (see Figure 2)
  • Integral 12 volt DC power conditioning
  • Moisture/dust-resistant housing
  • Qualified for full temperature range required of car-mounted equipment
  • Full (Part 15) legal power output
  • Diversity antenna (can be operated with a single antenna if desired)

Alvarion is one of only a few companies to make use of Frequency Hopping Spread Spectrum (FHSS) for its license-exempt systems. Alvarion contends that FHSS systems offer far more robust performance in the presence of interference — largely because an Alvarion system's signal is a “moving target,” using a 1 MHz channel in a band 83.5 MHz wide, hopping in a pseudo-random pattern. If interference is encountered on a particular “hop,” the data is retransmitted on the next “hop” at least several MHz away, and thus unlikely to encounter the same interference.

In contrast, the popular 802.11b (Wi-Fi) standard, developed for use in Wireless Local Area Networks (WLANs) occupies a static 22 MHz portion of the 2.4 GHz band, and there are only three such channels that do not overlap.

A number of Alvarion systems have been deployed for public safety use.

One such system is the San Diego County Sheriff's Department Mobile Broadband system, with 650 vehicles accessing 40-50 base stations installed on county facilities such as station houses, courts and jails.

The SDCSD mobile broadband system was designed to allow patrol officers to perform law enforcement database queries and office functions from their patrol vehicles and remote facilities.

A secondary benefit is that the mobile broadband system offers sufficient connectivity speed for software and data updates to flow to the patrol vehicle's computer while on patrol, such as antivirus and database updates.

In general, the Mobile Broadband System has performed quite well.

“We wanted a high-speed wireless system that is rugged, dependable and extremely secure. And, of course, we wanted something that would require a very minimal infrastructure investment,” said Sergeant Lloyd Muenzer of the San Diego County Sheriff's Department. “Alvarion provided us with all that — a system which is reliable, robust, eases the strain on our office computers, and is easily implemented from both an IT and a fiscal stand-point.

Moreover, it eliminates the need for floppy disks, speeds up the transfer of information, and using push technology, allows officers to automatically download 5-10MB of data every time they log onto their mobile computing stations.”

To critics who say that license-exempt equipment cannot be relied upon for life-safety applications, vendors and users of license-exempt systems answer that license-exempt equipment merely supplements well-developed life-safety systems already in place, such as voice radio dispatch networks.

Alvarion Security issues and features

  • Alvarion's use of Frequency Hopping Spread Spectrum (FHSS) makes interception a more complex problem (the reason FHSS was developed in World War II) the hopping pattern is not easily discernable merely by monitoring. “War Driving,” which has been widely reported on, is limited to 802.11b/Wi-Fi systems and thus not generally applicable to Alvarion or FHSS (though it remains theoretically possible)
  • The Extended Service Set Identification (ESSID) must match between the user radio and base station radio before communication can begin; the user radio cannot “pick up” the ESSID from the base station but must be set in advance.
  • Alvarion radios cannot be set to “promiscuous” mode to “sniff” or passively intercept ESSIDs.
  • Each radio's unique Media Access Control (MAC) address can be registered in a database, and access can be restricted to only those MAC addresses that are registered in the database.
  • Alvarion has implemented robust Virtual Local Area Network (VLAN) technology. This allows multiple groups of users to use the same network. Separate VLANs for each group insure that each group's data is separated from other group's data.
  • Wireless Equivalent Privacy (WEP) encrypts data as transmitted (WEP has been compromised in widely publicized cracking attacks).
  • As is common with untrusted public networks such as the Internet, the highest layer of security implemented in an Alvarion network is Virtual Private Network (VPN), typically running on the user device (such as a laptop in a vehicle) and a VPN server positioned between the wireless network and the user's Intranet.