The A interface doesn't get much respect. Everyone appreciates the features that the radio interface provides. In fact, many consumers believe this is the only interface in a wireless system. But how and why could an interface between a base station and a switch possibly be exciting?

The concept of an open interface to connect base stations to MSCs first was established by GSM. The desire to open this interface was not academic — the base station and MSC are some of the most expensive wireless system components. A proprietary interface forces carriers to buy both from the same vendor. An open interface allows carriers to choose. Even if they don't exercise diversity, their suppliers know that it wouldn't be impossible to replace them. Every time a network expansion is planned, the carriers can use the competition for price negotiations.

The A Team

GSM defines a highly developed series of specifications for the A interface. Not only does the series define the interface between a GSM base station subsystem (BSS) and the MSC in TS 48.001 through 48.008, but also between the BSS and the GPRS SGSN (router for GPRS packets) in TS 48.018. A newer interface between the BSS and the BTS (Base Transceiver Subsytem; the radio equipment) is defined in TS 48.051 through 48.058.

AMPS systems in North America didn't put a premium on the A interface. In the late 1980s, industry officials attempted to standardize the A interface for analog, but eventually abandoned the effort. Carriers who later started TDMA service didn't consider this important either.

CDMA carriers, particularly Sprint, viewed it differently. An open A interface became a priority for them (and still is). Asian CDMA carriers also are active participants in standards development, particularly KDDI in Japan and SK Telecom and KT Freetel in Korea.

The first CDMA A interface standard (IS-634, published in 1995 by TIA TR-45.4) was closely modeled on GSM, which was based on a circuit-switched voice with SS7 signaling. It tried to be technology-agnostic but, with only CDMA carriers showing an interest, became more focused on CDMA.

The interface assumed that base stations would be connected to MSCs over T1s, as is common even with proprietary interfaces. Each of the timeslots within the T1s (DS0s) would carry either voice or SS7 with IS-634 as the application layer carrying information regarding important functions such as call setup and teardown, handoff and registration.

Changes in radio interfaces directly affect the A interface. In fact, IS-634 has been revised in parallel with several of the revisions to the CDMA standards (IS-95, TIA/EIA-95 and IS-2000). The identity of the standard changed to IS-2001 in June 2000, and revisions have so far paralleled the IS-2000 CDMA standard that it supports.

Exerting Pressure

As changes introduced more options, it became clear that the goal of interoperability was fading away. Sprint and the CDG then developed their own specification, known as the interoperability standard (IOS). They then applied some not-so-subtle pressure on vendors to focus the standard.

Data has become more of an issue for the A interface as it has developed. The goal has been to route data to the Internet or other data networks at the base station, rather than through the voice/circuit-oriented MSC.

1XEV-DO is supported by a November 2001 standard published as TIA-878. GSM, similarly, has had to adapt its A interface to the Internet-oriented GPRS protocol.

Many people believe that all communications, including voice, from the base station eventually will become packet-oriented. All interfaces then will be IP-based, running a variety of other IETF protocols at higher layers. Voice-over-IP matches wireless quite well because voice is already digitized and compressed and, with variable rate voice coders, is well-suited to transmission over a packet interface.

Tandem-Free Ops

Transmitting coded voice presents an attractive alternative even before the all-IP concept is a practical reality. Radio interfaces generally run voice coders at 8kb/s to 13kb/s (vs. 32kb/s to 64 kb/s in landline networks), and speeds may go even lower in the future. The fewer bits used by voice coders, the more subscribers who can be supported by the same amount of spectrum.

However, there's a fly in the ointment — ensuring that there's a compatible voice coder at the destination. Most radio interfaces use different voice coders from one another, and each supports multiple voice coders of its own. There's no guarantee that two handsets will use the same voice coder, even if they are using the same radio interface. It's also true that the same voice coder could be in use by handsets using two different radio interfaces.

Using 32kb/s to 64kb/s voice not only uses more bandwidth, but paradoxically reduces voice quality because it implies four encoding or decoding stages. In fact, if speech is recorded by a voice mail system, even more coding steps are applied. Each coding or decoding loses information and reduces the transmission quality.

Tandem-free operation (as it is known) requires only encoding by the transmitter and decoding by the receiver, and results in better voice quality. The ideal solution is to have end-to-end signaling to negotiate a voice coder. However, that is impractical when calls are transmitted through the PSTN.

A surprising solution has been uncovered (in TIA/EIA-895, published in September 2001) to support tandem-free operation that only requires the participation of base stations. Through a technique known as bit robbing, the digital DS0 facilities can transmit both coded and uncoded voice. The bits that are robbed are the least significant bits of the 56kb/s channel and make little difference to the voice quality. By including the voice, both coded and encoded, it's possible for the terminating base station to decide which stream to use. It's also possible to switch between coded and decoded voice, which is sometimes necessary when a handset participates in features such as 3-way calling that may disrupt the flow of the robbed bits.

An A interface is going to be in everybody's wireless future. Perhaps it's only a coincidence that the wireless protocols with the most promise for the future (cdma2000 and the GSM family, including UMTS) are the only ones that have opened up this important interface. Or perhaps, as with roaming, what was once seen as this year's luxury will become next year's necessity.

Crowe (crowed@cnp-wireless.com) is a wireless standards consultant and editor of Cellular Networking Perspectives.