Initial draft of CSD slides

2023/osmodevcall-gsm_csd/gsm_csd.tex

@@ -0,0 +1,397 @@
+
+\newcommand{\degree}{\ensuremath{^\circ}}
+%\documentclass[handout]{beamer}
+\documentclass[aspectratio=169,11pt]{beamer}
+
+% This file is a solution template for:
+
+% - Talk at a conference/colloquium.
+% - Talk length is about 20min.
+% - Style is ornate.
+
+
+
+% Copyright 2004 by Till Tantau <tantau@users.sourceforge.net>.
+%
+% In principle, this file can be redistributed and/or modified under
+% the terms of the GNU Public License, version 2.
+%
+% However, this file is supposed to be a template to be modified
+% for your own needs. For this reason, if you use this file as a
+% template and not specifically distribute it as part of a another
+% package/program, I grant the extra permission to freely copy and
+% modify this file as you see fit and even to delete this copyright
+% notice. 
+
+
+\mode<presentation>
+{
+  \usetheme{CambridgeUS}
+  \usecolortheme{whale}
+
+%\setbeamercolor{titlelike}{parent=palette primary,fg=black}
+\setbeamercolor{frametitle}{use=block title,fg=black,bg=block title.bg!10!bg}
+% from beamercolorthemeorchid.sty to make it look more like warsaw
+\setbeamercolor{block title}{use=structure,fg=white,bg=structure.fg!75!black}
+\setbeamercolor{block title alerted}{use=alerted text,fg=white,bg=alerted text.fg!75!black}
+\setbeamercolor{block title example}{use=example text,fg=white,bg=example text.fg!75!black}
+
+\setbeamercolor{block body}{parent=normal text,use=block title,bg=block title.bg!10!bg}
+\setbeamercolor{block body alerted}{parent=normal text,use=block title alerted,bg=block title alerted.bg!10!bg}
+\setbeamercolor{block body example}{parent=normal text,use=block title example,bg=block title example.bg!10!bg}
+\setbeamertemplate{navigation symbols}{}
+
+
+
+  % or ...
+
+  %\setbeamercovered{transparent}
+  % or whatever (possibly just delete it)
+}
+
+\mode<handout>{
+	\usepackage{misc/handoutWithNotes}
+	\pgfpagesuselayout{2 on 1 with notes landscape}[a4paper,border shrink=5mm]
+	\usecolortheme{seahorse}
+}
+
+% ensure the page number is printed in front of the author name in the footer 
+%\newcommand*\oldmacro{}
+%\let\oldmacro\insertshortauthor% save previous definition
+%\renewcommand*\insertshortauthor{%
+%  \leftskip=.3cm% before the author could be a plus1fill ...
+%  \insertframenumber\,/\,\inserttotalframenumber\hfill\oldmacro}
+
+\usepackage[english]{babel}
+\usepackage[utf8]{inputenc}
+\usepackage{times}
+\usepackage[T1]{fontenc}
+
+\usepackage{subfigure}
+\usepackage{hyperref}
+\usepackage{textcomp,listings}
+%\usepackage{german}
+\lstset{basicstyle=\scriptsize\ttfamily, upquote, tabsize=8}
+
+
+\title{Circuit Switched Data (CSD) in GSM}
+%\subtitle{(Circuit Switched Public Data Network)}
+\author{Harald~Welte}
+\institute{Osmocom project}
+\date[March 2023]{OsmoDevCall}
+
+
+% - Use the \inst command only if there are several affiliations.
+% - Keep it simple, no one is interested in your street address.
+
+% - Either use conference name or its abbreviation.
+% - Not really informative to the audience, more for people (including
+%   yourself) who are reading the slides online
+
+%\subject{Having fun with DECT}
+% This is only inserted into the PDF information catalog. Can be left
+% out. 
+
+
+
+% If you have a file called "university-logo-filename.xxx", where xxx
+% is a graphic format that can be processed by latex or pdflatex,
+% resp., then you can add a logo as follows:
+
+% \pgfdeclareimage[height=0.5cm]{university-logo}{university-logo-filename}
+% \logo{\pgfuseimage{university-logo}}
+
+
+
+% Delete this, if you do not want the table of contents to pop up at
+% the beginning of each subsection:
+%\AtBeginSubsection[]
+%{
+%  \begin{frame}<beamer>{Outline}
+%    \tableofcontents[currentsection,currentsubsection]
+%  \end{frame}
+%}
+
+
+% If you wish to uncover everything in a step-wise fashion, uncomment
+% the following command: 
+
+%\beamerdefaultoverlayspecification{<+->}
+
+
+\begin{document}
+
+\begin{frame}
+  \titlepage
+\end{frame}
+
+
+% Structuring a talk is a difficult task and the following structure
+% may not be suitable. Here are some rules that apply for this
+% solution: 
+
+% - Exactly two or three sections (other than the summary).
+% - At *most* three subsections per section.
+% - Talk about 30s to 2min per frame. So there should be between about
+%   15 and 30 frames, all told.
+
+% - A conference audience is likely to know very little of what you
+%   are going to talk about. So *simplify*!
+% - In a 20min talk, getting the main ideas across is hard
+%   enough. Leave out details, even if it means being less precise than
+%   you think necessary.
+% - If you omit details that are vital to the proof/implementation,
+%   just say so once. Everybody will be happy with that.
+
+%\include{part-introduction}
+
+
+%\part{Java SIM}
+
+%\section{Cicruit Switched Packet Data Networks}
+
+\begin{frame}{CSD? What is that?}
+\begin{itemize}
+	\item a circuit-switched call, like a classic voice call
+	\item bearer data (user plane) is not voice, but data
+	\item resembles the kind of service known previously from ISDN data calls or POTS modem calls
+	\item the only data service provided by classic 2G/GSM besides SMS (before 2.5G/GPRS was introduced)
+	\item CSD is also used to transport {\em Group 3 Facsimile} (Telefax) over GSM
+\end{itemize}
+\end{frame}
+
+\begin{frame}{CSD Users: Modems}
+\parbox{.50\textwidth}{
+\begin{itemize}
+	\item GSM Modems (external/RS-232 attached or in PCMCIA form factor) for laptop users in the 1990s
+	\begin{itemize}
+		\item Dial-up access to BBSs, Databases, etc. (pre-internet days)
+		\item Dial-up access to other networks like X.25/Datex-P (gateways)
+		\item Dial-up access to Interenet or private IP networks (SLIP/PPP)
+	\end{itemize}
+\end{itemize}
+}\hfill\parbox{.50\textwidth}{
+	\includegraphics[width=60mm]{siemens_mc35i.jpg}
+}
+\end{frame}
+
+\begin{frame}{CSD Users}
+\begin{itemize}
+	\item End-to-end encrypted telephones (like older/original GSMK Cryptophone)
+	\item At least in theory, there's also a spec for Teletex over GSM
+	\begin{itemize}
+		\item unclear if it was ever deployed
+		\item Teletex used 2400 bps synchronous X.21 CSPDN in Germany; see related RetroNetCall talks
+	\end{itemize}
+	\item Early Machine-to-Machine (M2M) communication; most well-known user is data connection between
+		train engines and controllers in GSM-R
+\end{itemize}
+\end{frame}
+
+\begin{frame}{CSD Users: Fax}
+\parbox{.55\textwidth}{
+\begin{itemize}
+	\item GSM Telefax devices (yes, they exist[ed])!
+	\begin{itemize}
+		\item Fax from one GSM subscriber to another GSM subscriber
+		\item Fax from GSM subscriber to wired (ISDN/PSTN) fax machine
+	\end{itemize}
+\end{itemize}
+}\hfill\parbox{.45\textwidth}{
+	\includegraphics[width=75mm]{greta_fax.jpg}
+}
+\end{frame}
+
+
+
+\begin{frame}{CSD Services}
+\begin{itemize}
+	\item both synchronous and asynchronous services
+	\item both transparent and non-transparent services
+	\item service bit rates of 300/1200/2400/4800/9600/14400 bps
+	\item later extended to HSCSD (high-speed CSD), bundling 2-4 TCH for up to 57600 bps
+\end{itemize}
+\end{frame}
+
+\begin{frame}{Background: Synchronous Interface}
+\begin{itemize}
+	\item {\it synchronous} means that receiver and transmitter share a common clock
+	\item bits are then transmitted one synchronous with that clock, usually 1 bit per clock cycle
+	\item synchronous interfaces do not need start/stop bits, so they have higher user data throughput at same bps
+	\item simple synchronous interfaces need separate wires for clock, in addition to data
+	\item example for other synchronous serial interfaces: SPI, I2C
+	\item sometimes, uC will have peripherals called USART for an UART that's extended also for synchronous communications
+\end{itemize}
+\end{frame}
+
+\begin{frame}{Background: Asynchronous Interface}
+\begin{itemize}
+	\item data bits are not synchronous to any clock
+	\item baud rate just tells the (rough) rate of bits, but not the phase
+	\item transmitter can start to transmit a character at any given point in time
+	\item asynchronous interfaces need start/stop bits, so they have lower user data throughput at same bps
+	\item most common example for asynchronous interface: RS-232 serial port
+\end{itemize}
+\end{frame}
+
+\begin{frame}{Transparent (T) vs. Non-Transparent (NT)}
+\begin{itemize}
+	\item Transparent
+	\begin{itemize}
+		\item bit stream passed transparently end-to-end
+		\item air interface FEC, but no reliable (re)transmission in case of loss
+		\item defined latency, but possible loss
+		\item available for sync and async services
+	\end{itemize}
+	\item Non-Transparent
+	\begin{itemize}
+		\item works only for async services
+		\item characters sent via RLP (Radio Link Protocol)
+		\item control lines (RTS/CTS/DSR/DTR/DCD) serialized via L2RCOP (Layer 2 Character Oriented Protocol)
+		\item RLP is a HLDC-style protocol with ABM
+		\item frames are re-transmitted as needed
+		\item indeterminate latency, but guaranteed delivery (or disconnect)
+	\end{itemize}
+\end{itemize}
+\end{frame}
+
+
+\begin{frame}{CSD async-T vs. sync-T vs. async-NT}
+\begin{figure}[h]
+	\centering
+	\includegraphics[width=75mm]{csd_model_1b_2b_3b.png}
+\end{figure}
+\end{frame}
+
+\begin{frame}{CSD signaling in Layer 3 Call Control}
+\begin{itemize}
+	\item Normal Q.931-derived GSM Layer 3 Call Control with SETUP / CONNECT / etc.
+	\item Only difference: Bearer Capabilities IE
+\end{itemize}
+\end{frame}
+
+\begin{frame}{CSD on the Um (MS-BTS) interface}
+\begin{itemize}
+	\item A variety of new convolutional coding/interleaving schemes
+	\item TCH/F9.6, TCH/F4.8, TCH/H4.8, TCH/H2.4, TCH/F14.4
+	FIXME: TABLE
+\end{itemize}
+\end{frame}
+
+\begin{frame}{CSD on the Abis (BTS-BSC) interface}
+\begin{itemize}
+	\item Classic GSM uses TRAU frames over E1 timeslots/sub-slots
+	\item Special TRAU frame formats specifically for CSD
+	FIXME
+\end{itemize}
+\end{frame}
+
+\begin{frame}{CSD on the A (BSC-MSC) interface}
+\begin{itemize}
+	\item Voice calls in the GSM core network are just ISDN calls (64k PCM timeslots)
+	\item CSD calls in the GSM core network are just ISDN data calls (64k V.110 timeslots)
+	FIXME: something about V.110 from the RetroNetCall
+	\item for more details about V.110, see the 12/2022 RetroNetCall about ISDN B-Channel protocols at
+		\url[https://media.ccc.de/v/retronetcall-20221207-laforge-isdn-b-channel-protocols]
+\end{itemize}
+\end{frame}
+
+
+
+
+
+
+\begin{frame}{Interworking with ISDN data calls}
+\begin{itemize}
+	\item The GSM MSC is just a fancy ISDN switch anyway
+	\item Call Control is using SS7/ISUP like other switches in ISDN; no difference between voice and daat
+		other than the indicatd service (voice vs. UDI/RDI)
+	\item Transparent CSD is just a slightly modified V.110, so CSD-modified V.110 frames are mapped 1:1
+		to standard ISDN V.110 frames (happens in the TRAU; typically BSC-colocated)
+	\item Non-transparent CSD requires Inter-Working Function (IFW; typically MSC-colocated); IWF
+		terminates RLP and L2RCOP layers and acts as TA towards ISDN
+\end{itemize}
+\end{frame}
+
+\begin{frame}{Interworking with POTS modem calls}
+\begin{itemize}
+	\item Specs say that operators could deploy an IWF (typically MSC-colocated) which terminates the
+		transparent or non-transparent CSD call and implements an ITU V-Series Modem towards the POTS
+	\item This has been tested in some trials; not sure if any operator had deployed this
+		in production. Reason: Expensive modem banks with one modem per concurrent call required.
+	
+\end{itemize}
+\end{frame}
+
+\begin{frame}{Interworking with POTS Telefax}
+\begin{itemize}
+	\item 
+\end{itemize}
+\end{frame}
+
+\begin{frame}{CSD Fax Adaptation across the network}
+\begin{figure}[h]
+	\centering
+	\includegraphics[width=99mm]{csd_model_5a.png}
+\end{figure}
+\end{frame}
+
+
+\begin{frame}{How to speak CSD in 2023?}
+\parbox{.50\textwidth}{
+\begin{itemize}
+	\item public operators (a least in Germany) have sadly phased out CSD
+	\begin{itemize}
+		\item Vodafone in 12/2020
+		\item Deutsche Telekom in 12/2022
+	\end{itemize}
+	\item alternative: {\em test equipment} like the Racal 6103
+	\item alternative: {\em private} networks
+	\item Osmocom support for CSD never really existed
+	\begin{itemize}
+		\item Ancient (2012) OsmoNITB branch {\tt tobias/csd} for use with nanoBTS
+		\item Current WIP for proper CSD support
+	\end{itemize}
+\end{itemize}
+}\hfill\parbox{.50\textwidth}{
+	\includegraphics[width=70mm]{racal6103.jpg}
+}
+\end{frame}
+
+\begin{frame}{Omsocom CSD plans}
+\begin{itemize}
+	\item support for 2400/4800/9600 bps CSD across CNI ({\tt osmo-bts/bsc/msc/mgw})
+	\begin{itemize}
+		\item BTS support focus on {\tt osmo-bts-trx} only
+	\end{itemize}
+	\item CSD to CSD calls between subscribers (sync/async, T/NT)
+	\item support for V.110 / RTP CLEARMODE interworking with SIP ({\tt osmo-sip-connector})
+	\item support for V.110 interworking with ISDN (tbd)
+\end{itemize}
+\end{frame}
+
+\begin{frame}{Omsocom CSD status}
+\begin{itemize}
+	\item convolutional [de]coder, [de]interleaving implemented
+	\begin{itemize}
+		\item verified against air-interface traces ({\tt burst\_ind} captures) between Racal 6103E and Siemens GSM Modem
+	\end{itemize}
+	\item V.110 frame synchronization, encoding/decoding
+	\begin{itemize}
+		\item verified against V.110 ISDN calls in OCTOI network
+	\end{itemize}
+	\item GSM-modified V.110 frame synchronization, encoding/decoding
+	\begin{itemize}
+		\item verified against air-interface traces ({\tt burst\_ind} captures) between Racal 6103E and Siemens GSM Modem
+	\end{itemize}
+	\item RFC4040 complaint RTP CLEARMODE in osmo-mgw
+	\item osmo-bsc handling on Abis and A interface; MGCP towards osmo-mgw
+\end{itemize}
+\end{frame}
+
+
+\begin{frame}{The End}
+EOF
+\end{frame}
+
+\end{document}