5 The problem

26.2263GPPCellular text telephone modemGeneral descriptionRelease 17TS

Due to the fact that digital cellular phone systems are optimized for speech signals and the radio transmission may be subject to relatively high error rates, the transmission of text telephone modem signals via the speech path in cellular networks is in some cases unsatisfactory: i.e. received messages show an unacceptably high character error rate (CER). One reason for this is that the digital cellular phones use speech coding in order to compress the signal. Since this coding is optimized for speech-like signals, modem signals are more or less distorted. A further problem is in the transmission errors and the applied error concealment in digital cellular phone systems in case of poor channel conditions. The repetition of speech frames results in character errors or even in error propagation.

The traditional text telephone modulation technique, designed years ago for PSTN lines, was not developed for these sophisticated, speech-optimized transmission channels.

Data transmission methods exist in the wireless services, but for various reasons, a text telephone transmission method for the speech path is desired. Two reasons are: 1: text telephony is acknowledged as a way to contact the emergency services, and emergency services in wireless networks are so far only defined for speech calls. 2: Alternating speech and text in a call is desired, and one simple way to accomplish that without special service support (like multimedia) is by alternating the use of the speech channel.

5.1 US regulatory issues

The FCC under the US Government has required an urgent solution for all emergency (911) calls for one specific text telephone protocol of the ITU-T V.18 standard, called "Baudot Code". This TS addresses these FCC requirements and this section points out specific US regulatory issues.

The minimum performance requirements for the CTM transmission are included in [1]. That specification shall be applied for verifying performance.

The FCC requires that the solution for cellular systems is fully compatible to traditional text telephone standards, at least it shall support Baudot modulation at 45.45 baud in the landline end.