retronetcall / B-channel protocols

master
Harald Welte 2 months ago
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commit ae35b7395c
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@ -0,0 +1,266 @@
ISDN B-Channel protocols: V.110, V.120, X.75, H.221
===================================================
:author: Harald Welte <laforge@gnumonks.org>
:copyright: 2022 by Harald Welte (License: CC-BY-SA)
:backend: slidy
:max-width: 45em
== Overview
* quick recap on D-channel
* overview of B-channel classes/formats
* V.110
* HDLC/syncPPP
* V.120
* X.75
* H.221
== ISDN D-channel protocols
* many have seen ISDN D-channel protocol traces
** Q.921 (LAPD) as Layer 2
** Q.931 as Layer 3 for call control
* several FOSS implementations around
* quite a bit of literature
* protocol decoders in wireshark
== ISDN B-channel protocols: Why?
* we had modems to transmit serial data over analog
* now that we have a digital ISDN B-channel, why are protocols needed?
* _serial data_ is traditionally not just a stream of bytes, but
** modem control signals (CD, DSR/DTR, RTS/CTS, ...)
** variable number of data bits, stop bits, parity
** ... and what about the break condition?
== ISDN B-channel protocols
* transport of
* interoperability with legacy data (modem, *V-series*) applications
** V.110
** V.120
** X.75
* video telephony
** H.221
* IP or other network traffic
** bit-synchronous HDLC
** octet-synchronous HDLC
== ISDN Terminal Adapters: Interfaces
* R interface (V-series / "RS-232")
* S interface (S0 in Germany)
* U interface (Uk0 in Germany)
== V.110
* rate adaptation between classic serial modem rates and ISDN
* support synchronous and asynchronous user communications
image::v120_figure1_connection_services.png[]
== V.110 for synchronous users
* bit-rates 600, 1200, 2400, 4800, 7200, 9600, 12000, 14400, 19200, 24000, 28800, 38400
* two rate adaptation functions
** RA1
** RA2
image::v110_figure1.png[width=1200,align="center"]
== V.110 RA1 generic frame structure
image::v110_table2_frame_structure.png[]
* 17bit sync pattern
* 48 D-bits (user data)
* 7 E-bits (encode frame format/rate)
* 8 S/X-bits (RTS/CTS/DTR/CD/... status lines)
== V.110 RA1 specific frame structure
image::v110_tables6abcd_frames.png[width=1200,align="center"]
* same as generic frame structure
* rate specific D-bit repetition
* rate specific E1/E2/E3 bit
== V.110 RA0 for asynchronous users
* bit-rates 50, 75, 110, 150, 200, 300, 600, 1200, 2400, 3600, 4800, 7200, 9600, 12000, 14400, 19200, 24000, 28800, 38400
* async user rate is adapted to nearest sync user rate
** new RA0 rate-adaptation function
** all it does is _stop-bit manipulation_, i.e. inserting,removing extraneous stop bits between characters
* RA1 + RA2 like in synchronous case
image::v110_figure4.png[width=1200,align="center"]
== V.110 RA0 for asynchronous users
image::v110_table8.png[width=1200,align="center"]
image::v110_table8contd.png[width=1200,align="center"]
== V.110 RA2 from 8/16/32k to 64k
* V.110 just points to I.460
* in the end, RA2 just means
** use only LSB of each byte for 8k
** use two LSB of each byte for 16k
** use four LSB of each byte for 32k
It reminds a bit 16k sub-slots as in GSM Abis.
However, V.110 doesn't allow multiplexing of multiple 8/16/32k channels in one 64k channel.
== V.110 synchronization entry/exit
image::v110_figure3.png[width=1200,align="center"]
== V.110 misc features
* synchronous rates of 48 + 56 kbps
** separate rate adaptation / frame format for those
* In-band parameter exchange
* network independent clocking
** in case you have synchronous communications, but running off a different clock than ISDN
== V.110 interworking with modems
* what if an ISDN subscriber with a V.110 TA wants to talk to another subscriber with an analog modem, e.g. V.32?
** in theory, the ISDN network could provide an IFW (inter-working function)
** in practice, that didn't really happen, so it was not supproted in real-world networks
== V.110 usage
* major usage in the context of GSM
** GSM offers CSD (circuit switched data), a V.110 derivative
** CSD uses modified V.110 frames
** GSM-specific interworking function between GSM and ISDN converts to plain V.110
== HDLC with syncPPP (RFC1618 / RFC1549)
* Flag octet (0x7E)
* Address octet (0xFF; all-stations addres)
* Control octet (0x03; UI frame)
* FCS (CRC16)
* optional *compression* suppressing Address + Ctrl on transmit
== X.75
* HDLC based protocol
* 0x7E flag octets
* address octet
* 8/16/32bit control field
* variable-length information field
* 16bit FCS (CRC16)
* your usual HDLC/LAPD/LAPB/... type commands: I/RR/RNR/REJ/SABM/DISC/FRMR/UA/DM
== V.120
* TA for providing V-series applications over ISDN
** same purpose as V.110
* unlike V.110: Not based on rate adaptation functions and its own framing format, but HDLC/LAPB based
* supports operation over _unrestricted_ (64kBps) and _restricted_ (56kBps) [and other] channels
** made it popular in the US, where restricted 56k user channels were common
* optionally supports multiplexing of multiple streams as _logical links_ into one B-channel
* optionally supports V.42bis compression
== V.120
* protocol stack
** Q.922 as data link layer (LAPB framing)
** Q.931 as L3 (SETUP/CONNECT/RELEASE/RELEASE_COMPLETE) for establishing logical links
image::v120_figure3_frame_formats.png[align="center"]
== V.120 flow control / modes
* protocol sensitive asynchronous mode
** I-frames/ABM: flow control by data link layer (RNR frames, withholding V(R) updates)
** unacknowledged: flow control by RR bit in control state informatoin octet
* protocol sensitive synchronous mode
** no flow-control possible
* bit transparent mode
** over/underruns may happen in case of clock problems
== X.75
* predates ISDN: Specified as part of Packets Switched Public Data Networks
** X.25 is the UNI (user-network interface) of PSPDNs
** X.75 is the NNI (network-network interface) of PSPDNs
** so if German Datex-P interconnected with French Transpac or US Telenet, that was supposedly using X.75 in the 1970s
image::x75_ulema_figure1.png[width=1000,align="center"]
* X.75 SLP (single link procedure) was extended in 1988 for use on ISDN B-channels
* T.90 contains the specific rules how X.75 is used in ISDN
== X.75 Frame Formats
* looks just like LAPB, LAPD, LAPDm, LAPSat, or any of the other many HDLC derivates
* but: note the absence of UI-frames! No unacknowledged data transfer
image::x75_table1.png[]
== X.75 Commands and Responses
image::x75_table7.png[]
== X.75 Adresses
image::t90_figure2.png[width=800,float="right"]
* X.75 only uses two addresses (it's point-to-point...)
** Address _A_ (0x03)
*** commands from callee to caller
*** responses from caller to callee
** Address _B_ (0x01)
*** commands from caller to calee
*** responses from callee to caller
== H.221
* bit-stream TDMA system; much more like V.110 or E1 multiframe structure than HDLC based bearers
* not just frames (like V.110), but _multiframes_ (like E1 or GSM)
** frame: 80 octets
** multiframe: 16 frames (1280 octets)
== H.221 frame structure
image::h221_figure1.png[width=900,float="right"]
* FAS: _Frame Alignment Signal_
** alignment pattern (0011011) to lock on multiframe structure
** control (E/C-bits) and alarm information (A-bits)
* BAS: _Bit-rate allocation signal_
** codeword to describe capability/capacity
* ECS: _Encryption control signal_
** optional 800 bit/s channel
== H.221 Frame Alignment Signal
image::h221_figure3.png[width=1400,align="center"]
== H.221 Frame Alignment Signal Multiframe
image::h221_figure4a.png[width=1400,align="center"]
* N1-N4, N5: Mulitframe number
* C: CRC4
* TEA: Terminal Equipment Alarm
== EOF
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