Retronetworking

h1. ITU-T Recommendation V.110

V.110 is an ITU standard for bit-rate adaptation of low-speed async serial signals to 64 kbit/s ISDN B-channels.  It was primarily used in the context of interworking between GSM Circuit Switched Data (CSD) and ISDN.

h2. Synchronous

Rate adaptation of synchronous serial signals of 600, 1200, 2400, 4800, 7200, 9600, 12000, 14400, 19200, 24000, 28800 or 38400 bit/s

h3. Frame structure

The protocol uses an 80bit (10 byte) frame.  The Frame contains a frame sync pattern (similar to e.g. E1 TS0 or TRAU frames) consisting of the first byte and the first bit of every following byte.

Each frame consists of 

* 17 bits of above-mentioned frame sync pattern

** this is used to esetablish frame alignment

* 48 D-bits (D1..D48)

** this is the actual serial bitstream of user data. No distinction is made between data, parity, start, stop bits

* 7 E-bits (E1..E7)

** E1/E2/E3 used to indicate the _user signaling rate (synchronous)_

** E4/E5/E6 used for support of _network-independent clocking_ of synchronous serial signals

** E7 os normally '1' but occasionally 0 for compatibility with X.300 

* 8 S/X-bits (S1,S3,S4,S6,S8,S9,X,X)

** SA (S1, S3, S6, S8)

*** used for Signal 107/DSR (ISDN->TA) and 108/DTR (TA->ISDN)

** SB (S4, S9)

*** used for Signal 109/CD (ISDN->TA) and 105/RTS (TA->ISDN)

** X

*** used for Signals 106/CTS (ISDN->TA) and 133 (TA->ISDN)

h3. Rate Adaptation Functions

h4. RA1

The rate-adaptation function RA1 converts from the V-series rates (e.g. 2400bps) to an intermediate rate of 8/16/32/64 kbit/s.

h4. RA2

RA2 converts from the RA1 output rate of 8/16/32/64 kbit/s to 64 kbit/s and is actually specified in I.460.  It's quite simple:

* in 8 kbit/s, only bit 1 (first bit) of every octet is used

* in 16 kbit/s, only bit 1+2 (first two bits) of every octet are used

* in 32 kbit/s, only bits 1,2,3,4 (first four bits) of every octet are used

All unused bits are set to binary "1".

h2. Asynchronous

Rate adaptation of asynchronous serial signals of 50, 75, 110, 150, 200, 300, 600, 1200, 2400, 3600, 4800, 7200, 9600, 12000, 14400, 19200, 24000, 28800 or 38400 bit/s

h3. Rate Adaptation Functions

h4. RA0

The RA0 function is only used with asynchronous V-series interfaces. Incoming asynchronous data is padded by the addition of stop elements to fit the nearest channel rate defined by 2 × 600 bit/s. The resulting synchronous bit stream is then processed with RA1 and RA2 like in the synchronous serial case.