Public Wireless Packet Data Services

The demand for wireless packet data services has spawned the creation of public services, initially dominated by Ardis and RAM Mobile Data. These services are largely based on proprietary technology which can be licensed by other manufacturers. Nonstandard application program interfaces (APIs) lessen the attractiveness of porting applications to these services.

Public wireless packet data systems rely on third-party vendors, such as RadioMail, to provide gateways to the rest of the data networking world and perform store-and-forward functions. As a result, customer acceptance of these services has been somewhat slow, with approximately 30K and 40K subscribers on RAM and Ardis, respectively, at mid-1995.^9.16 These are considered to be public services because they are available on a subscription basis to whomever desires wireless capability.

Advanced Radio Data Integrated System (Ardis)

Ardis was created as a joint venture of Motorola and IBM. This 1990 partnership was intended to leverage the system Motorola had previously created for IBM field service technicians, who still comprise almost one-half of the Ardis subscriber base. Now owned solely by Motorola, Ardis retains its vertical market orientation with an emphasis on host connectivity; applications supported by Ardis include dispatch and field service .

Ardis is an 800-MHz band RF system built with a cellular-type architecture. Cellular architectures are used for one reason-the ability to reuse scarce channels for increased capacity-and Ardis exemplifies this goal. This single-frequency reuse system has up to 6 25-kHz RF channel pairs available in large North American cities (12.5 KHz channels in Europe), but only one so-called Nationwide Channel pair is available for accessing the system.

This limited channel set dictates a mode of operation which limits forward channel capacity. In areas where only a single channel is available, nearby base stations coordinate their transmissions to avoid interfering with one another; in a small cluster of cells only one base station can transmit at a time. This base station coordination is handled by area communications controllers (ACCs) upstream in the network.

However, a nice characteristic of the single-frequency reuse scheme is that overlapping cell coverages results in enhanced in-building reception of the reverse channel. Multiple base stations typically receive each mobile-originated packet; an ACC selects the best copy of the packet from these multiple receptions. The ACC then uses an intelligent "path sensing" algorithm to select the best route outbound to the mobile to minimize congestion. This routing algorithm also allows multiple simultaneous transmissions from non-interfering base stations by an Ardis system, gaining capacity of 1.5 to 3 times over a simulcast system.

A 19.2 Kbps radio link protocol known as RD-LAP is replacing the original 4800 bps MDC4800 protocol (which is still used for Nationwide Channel access). Both protocols are Motorola proprietary although RD-LAP can be licensed by other manufacturers. Four-level FSK modulation with a Gaussian filter is used on the RD-LAP-based system. A three-quarters rate Trellis-coded modulation with interleaving and CRC-32 supports an undetected block error probability of approximately 2 $^{{\rm -} {\rm 3}{\rm 2}{\rm .}{\rm } {\rm } }$

RD-LAP provides a slotted digital sense multiple access (DSMA) MAC protocol without collision detection by the half-duplex mobiles; higher layers are assumed to recover from errors. The maximum transmission burst is 2048 bytes. A stop-and-wait (SAW) link protocol is used by the half-duplex mobiles, which limits the throughput enjoyed by an application running over Ardis.

Taking all of this into consideration, the effective data rate for a 512-byte transmission is approximately 6.33 Kbps [WONG95]. The protocol efficiency is 57% and 61% for 125 and 250 octet messages, respectively.

Ardis is best suited for low bandwidth non-interactive applications; response times generally exceed 5 seconds. Ardis enjoys widespread deployment, providing coverage to some 80% of the population of the U.S. and 90% of U.S. business areas via more than 1400 base station sites, 35 area communications controllers (ACCs) and 3 message switches, as depicted in Figure 9.3.

Figure 9.3: ARDIS Architecture

The Ardis architecture defines protocols up to and including Layer 4 in the mobile. A so-called Native Command Language (NCL) in the bottom two layers provides a command/response type of transaction protocol between the Radio Packet Modem (RPM) and the subscriber's computer. The RPM uses RD-LAP in Layers 2 and 3 to access the network, which is X.25-based. The Ardis communications architecture is depicted in Figure 9.4.

Figure 9.4: ARDIS Communications Architecture

Mobility management in Ardis resembles CDPD. A Home Location Register (HLR) and Visitor Location Register (VLR) located in ACCs track the position of each RPM registered with the network. Whenever an RPM recognizes that it is in a new cell (by base station-transmitted information), it sends a registration packet to update the network location database. The serving ACC and VLR coordinate this with the home ACC and HLR, getting authorization prior to granting service.

Ardis provides a Motorola-proprietary security capability, involving authentication and symmetric key exchange. It defines communication protocols up to the Transport Layer of the OSI reference model. No access priorities are defined. Gateways are necessary to interface to the rest of the data networking world because of the nonstandard protocols and APIs in use.

Because the network consists of a single administrative domain, service and performance are consistent everywhere; there are essentially no interoperability or incompatibility problems. Ardis connects with the outside world via dedicated links or VANs (e.g., Advantis). As mentioned previously, the RadioMail service is typically used by subscribers for both store-and-forward and network gateway services.

Ardis features are compared with those of CDPD and RAM in Table 9.4.

RAM Mobile Data (Mobitex)

RAM Mobile Communications is a joint venture of Ericsson and Bell South in the U.S. This public wireless packet data network is based on the Mobitex network, the name by which it is known outside of North America. Mobitex was first deployed in Sweden in 1986 as a joint venture of Ericsson and the Swedish post office for voice and dispatch data services. RAM continues to have a horizontal market orientation with an emphasis on peer-to-peer messaging.

Typical applications of RAM include host access and dispatch. RAM has been widely deployed with over 1000 base stations in operation. RAM has worked with customers to co-finance base stations in remote areas where RAM might otherwise not build-out. RAM now provides nationwide coverage via cellular, paging, satellite, dial-up and private networks.

Architecturally, Mobitex has always been considered to be a wireless extension of X.25 packet switching networks, consistent with its European telephony roots. Mobitex is a cellular-based system with between 10 and 30 12.5 kHz channels available in markets in the 896-901 MHz band.^9.17 As a result, airlink capacity has never been a concern. Access to the system is provided by National System Channels, which control access to the particular channel set in operation in any given area; this provides the necessary support for mobility between markets.

Intercommunication is done at the lowest possible level of the tiered hierarchy of base stations, local switches called MOXs and regional/national switches called MHXs, as displayed in Figure 9.5. The MHXs are identical physically to the MOXs, but differ in their software soul. A network control center or NCC handles billing and management functions, but no user traffic. The modular nature of Mobitex architecture supports both small systems and extremely large systems with multiple levels of MHXs and redundant NCCs.

Figure 9.5: RAM Architecture

Sufficient intelligence is built into each node allowing intercommunication to be done at the lowest possible level: mobiles can interact directly via the base station (if they are registered to the same base). If a base station doesn't recognize the destination mobile, it will pass the message to the MOX. If the MOX knows the mobile, it will pass the necessary information on the destination mobile to the base station; if the MOX doesn't know of the mobile, it passes the message on to the MHX. Most fixed-location hosts are interconnected physically via land-lines at the local exchanges (MOXs), although the connection is also possible at the base station level. Occasionally the fixed host might also be connected to a main exchange (MHX).

External hosts can either connect directly to the MOXs or via public X.25 packet data networks. Other fixed host connection alternatives include dial-up gateways (preassigned to minimize PSTN charges), IP gateways (over either X.25 or PPP) and specialized gateways (including 3270 and 5250 emulation).

The 8 Kbps Mobitex MAC layer protocol is open but not a standard. It has two modes of operation-slotted Aloha for short messages and a modified CSMA for longer transmissions. The maximum transmission burst is 512 octets. A combination of a shortened (12,8) Hamming forward error correction with interleaving to a depth of 20 and CRC-16 parity provides an undetected block error rate of approximately 2$^{- 16}$. A selective ARQ (automatic repeat request) mechanism enables the retransmission of specific blocks.

Like Ardis, the half-duplex Mobitex mobiles have no capability for collision detection and so error recovery must be done at a higher layer. A go-back-N protocol is employed at the logical link control (LLC) sublayer by the half-duplex mobiles for error recovery.

The effective data rate for a 512-octet transmission is 4.6 Kbps [WONG95]. The protocol efficiency is 51% and 55% for 125 and 250-octet messages, respectively. Response times are typically over 4 seconds and highly variable. Channel access algorithm parameters such as maximum message length and priority levels can be dynamically adjusted by the network operator to optimize system performance in the face of changing traffic loads.

Protocols at the mobile are defined up through Layer 4, as displayed in Figure 9.6. MASC is the Mobitex Asynchronous Communications Protocol and MPAK is the Mobitex Packet assembly/disassembly protocol. MTP/1 is the Mobitex Transport Protocol and interface, which must be employed by applications developers to access the wireless data services. These are open, license-free protocols, but non-standard (except RS-232); there are several terminal manufacturers.

Figure 9.6: Mobitex Communications Architecture

The mobile handles its own radio resource management, like CDPD; received signal strength and data error rates are monitored by the mobile, which also scans the channels of adjacent base stations to determine optimum RF channel selection. Four user priority levels are defined; the highest is strictly for emergency messages. These priorities are used to control the flow of in-bound traffic to the system. The system advertises the lowest priority traffic it is willing to entertain. At coverage borders, mobiles can promote themselves one priority level; this is known as "distance access priority."

Mobility in Mobitex is also similar to CDPD. The mobile terminal notifies the network whenever it has changed base station coverage areas via a ROAM message. The MOX receiving this message in turn notifies other affected network nodes (the last serving MOX, etc.) of the mobile's change in location. Twenty-four-bit addressing supports international roaming.

Other features provided include sleep mode for mobile battery life extension, and store-and-forwarding of Email intended for mobiles with message storage notification to the sender. There has been criticism that RAM's primary shorcoming is poor in-building coverage, which RAM is addressing through continued deployment of additional base stations.

No security features are provided, as RAM believes that the complexity of airlink attacks is sufficient to discourage even sophisticated hackers.^9.18 Each subscriber device and fixed host in a Mobitex network is assigned a unique eight-digit Mobitex Access Number (MAN), which is stored in non-volatile memory. A unique Electronic Serial Number (ESN) is permanently stored in the radio modem and is validated by the network with the MAN whenever the mobile registers. A transferable subscription capability allows a user to provide an eight-character password when registering with a different mobile. All identifiers and passwords are transmitted in the clear.

RAM (Mobitex) features are compared with those of CDPD and Ardis in Table 9.4.

Table 9.4: CDPD, RAM and ARDIS Feature Comparison

RadioMail

RadioMail is not a wireless service provider at all, but rather a service providing the backend mailbox solution for wireless services such as RAM and Ardis. In late 1995, fewer than three thousand subscribers were using this service, despite its having received great fanfare in the press. One might conclude from this that relatively few RAM and Ardis applications were horizontal in nature at that time.