Monday, June 7, 2010

We Still Have Unlimited SMS

AT&T recently announced changes to their data plans, eliminating the unlimited data plans and replacing them with tiered plans offering up to 2GB of data per month. Yet AT&T still offers unlimited messaging, including SMS and MMS. Thus, there is a mechanism to game the system.

IP via SMS

SMS relies on the GSM 03.38 character set, which is 7 bits but using an escape code for additional characters. UTF-16 is used for non-Latin alphabets, and an 8 bit clean data alphabet is also supported (but not required).

For widest support we should rely on GSM 03.38, the only character encoding which handsets are required to implement. TCP/IP packets can be encoded into at most 160 characters of 7 bits each, for a total of 1120 bits or 140 bytes. An <ESC> can be followed by only a few valid characters, so an <ESC> followed by any arbitrary character might be rejected at some point through the delivery path. An HDLC character-stuffing technique can be used to eliminate <ESC> from the data stream, details to be worked out later.

SMS does define a concatenated option to support longer messages, but it occupies 7 characters in the payload. As a compressed IP header would be smaller than 7 characters, it is recommended that SMS concatenation not be used. A 140 byte packet MTU will pack more efficiently than a larger MTU plus concatenation overheads.

With such a small MTU, header compression is a must. Robust Header Compression defines profiles for TCP+IP, UDP+IP, and RTP+UDP+IP compression, which can reduce the typical header stack down to about 3 bytes. For deployment use we'll need to define a new ROHC profile for IP Fragments, an RFC will be drafted later.

IP via MMS QR code

MMS can be used to send small pictures and movies. Initially this seemed very promising: wrap the IP packet inside a JPG header and send it. Unfortunately to save cost, most mobile operators re-sample and re-encode any images sent via MMS to reduce their size. Straight data inside an image payload would be destroyed.

Instead the IP packet should be encoded as an image of a QR code, which can be robustly decoded even if the image is resampled. QR Code also includes error correction, helpful in this application.

QR Code can handle over 2 KBytes of data in a single barcode, easily enough to handle the maximum 1.5K ethernet frame size. ROHC header compression is not required.

Closing Notes

Yes, this is a joke. Material posted on Mondays is not intended to be taken seriously. After all, nothing productive happens on Monday.