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IS-130/IS-135 Data/Fax

In theory, establishing a data/fax call on an IS-136 TDMA traffic channel should have been no different from a similar landline connection. However, voice coding techniques on the traffic channel made this method of communication unreliable. For that reason, the IS-130 and IS-135 standards were developed to provide a reliable transport mechanism, air interface protocol and command set for the ansynchronous transfer of data and fax information.

With IS-130 and IS-135, a cellphone could function as an external modem and enable a computer to send and receive faxes, files and access the internet.

Diagram of laptop connected to internet via cellphone acting as external modem
Laptop surfing internet via a cellphone configured as an external modem to the laptop.

Although, mobile browing technology was not well developed at the time, when not tethered to a PC, the cellphone could still send and receive faxes and files.

Raw data throughput was 9.6 Kbps which through compression could be increased to 38.4 Kbps. At the time, dial up landlines were still the main means of residential internet service so, the throughput while now considered slow was at the time acceptable. Channel bonding while pricey due to additional traffic channel requirements could be used to boost throughput even higher.


The protocol of IS-130 provided a reliable data channel over which IS-135 commands and information could be exchanged.

On the uplink side, octets of data from the higher layers are compressed into smaller codewords. Codewords are blocked together and if requested, encrypted. Control information is then appended to the codewords with the result being a protocol data unit (PDU). These PDU's are then concatenated and a CRC added. Further error protection is added by convolutionally encoding the block. The RLP1 frames are then sent to the physical layer for transmission out the radio link.


The IS-135 data specification defines the setup, supervision and clearing of fax and data calls.

An AT command set very similar to the Hayes command set was defined for the basic asychronous data and fax calls. Special cellular specific AT comands were defined for commands such as signal strength and battery level. Vendor specific AT commands were also defined outside of IS-135.

Interim Technology

IS-130/IS-135 was ultimately an interim technology providing data/fax capabilities to second generation cellphones. While many IS-136 TDMA phones supported the technology, support by base stations was limited and coverage spotty. Ultimately, IS-130/IS-135 proved to be a technology that pointed the way towards third and fourth generation cellular packet data technologies.

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