Getting ready

If you are a government agency or government contractor subject to federal regulations on disability access to telecommunications services and IT, be aware that current voice-over-IP standards do not include support for TTY interfaces. A TTY is a special type of teletypewriter designed specifically for people with hearing impairments.

The specific government regulations that apply here are Section 255 of the Telecommunications Act of 1996 and Section 508 of the Rehabilitation Act of 1973, as amended in the Workforce Investment Act of 1998. Any claims by vendors that certain devices used in pure VoIP networks are exempt from these rules need to be investigated carefully.

While traditional circuit-switched telephony sends and receives voice communication in the same order it was spoken, packet telephony breaks up the conversation and sends bits and pieces across different pathways. The packets don't arrive in the same order in which they were generated, creating jitter that must be smoothed out. If this process takes more than 150 milliseconds, the delay becomes unacceptable.

To reduce this delay, VoIP phones do packet loss concealment: If a packet is missing for too long, the phones look at the adjacent packets for context and try to fake the missing one. There is no standard for this process, so each vendor is using a proprietary method.

Each character typed on a TTY teletypewriter spans about eight 20-millisecond VoIP packets. Consequently, a packet loss or delay of 0.5% - a good score when conversations are going across today's WAN links - means that 4% of the teletypewriter characters will be received incorrectly or not received at all. This problem is further complicated by the fact that nearly half the people who use TTY devices are hearing impaired but can speak clearly, and prefer to receive on a TTY and speak in response.

"So, with 20-millisecond packets and 0.5% packet loss, the theoretical best case is now corrupting 4% of TTY characters," says Paul Michaelis, a member of Avaya's technical staff and inventor of the TTY user interface for the Intuity AUDIX voice messaging system. "It's actually worse, because if the corruption is taking place in the start or stop bits, it messes everything up. So we can expect approximately 6.5% of TTY characters to not come through."

The problem is not solved by using a TTY-friendly encoding scheme for digitizing audio, such as G711. That doesn't address the gaps caused by the packet loss.

Since a pure IP environment cannot currently support TTY access, Michaelis said, Avaya is advocating a hybrid solution that accommodates standard analog and digital connections.

Cisco has been delivering TTY interface support in its pure IP telephony networks for several years, and addresses the packet-loss problem by using its quality-of-service technology to mark voice packets for special treatment. "Turning on QoS can ensure that high-priority voice packets are not dropped, even in the face of severe congestion on the network where there may be large losses occurring for low-priority packets," asserts William King, technical marketing manager for Cisco's enterprise voice and video business unit.

However, none of the TTY solutions provided by VoIP vendors are based on standards that make them interoperable. Accessibility workgroups are working on the problem, but meanwhile any IP-based solutions being sold by vendors are proprietary.

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