Asynchronous voice band modem for IP data protocols

1
Asynchronous voice band modem for IP data
protocols

• overview
• features
• glossary
• exercises

2
Inside the modem Hardware

• Line Interface
• Lightning protector
• Ring voltage detector
• Off-hook relay (also used for pulse dialing)
• Isolation transformer
• Analog/Digital Conversion
• Digital Signal Processor
• Control Processor
• UART or UART emulation

3
Inside the modem Firmware

• Line Control
• Signal Processing - V.34
• HDLC - V.42
• Compression - v.42bis
• Command Processor
• UART emulation or control

4
PC Serial Ports (UART)

• NS8250
• dual character buffer
• interrupt for every byte
• NS16550A
• 16 byte FIFO buffer for transmit
• 16 byte FIFO for receive
• interrupts on specified FIFO level or timer
• fewer interrupts, less critical timing

5
Serial Port Speed Buffering

• DTE port speed is fixed (115200 bps)
• DCE link speed is fixed (33600 bps)
• Data compression ratio is variable
• Flow control is required
• All modem applications must use hardware flow
  control (RTS/CTS)

6
Modem Control Signals

• FROM DTE
• Transmit data
• DTR
• RTS (Ready to receive)
• TT (Terminal Timing)

• FROM DCE
• Receive data
• DSR - Modem is ON
• DCD - connected
• CTS (Clear to Send)
• RI (Ring Indication)
• ST (Send Timing)
• RT (ReceiveTiming)

7
Using Async Modems for Data Protocols

• Special requirements
• Dial and login scripts
• Error handling
• Multi-modulation interoperability
• Echo on/off in commands
• Speaker on/off
• Flow control

8
Dialing for clients versus dialing for LANs

• Single User
• one IP address - can be temporary
• user is known by user name
• mailbox on servicehost
• LAN
• needs a route to a subnet
• needs stable addresses
• ideally, wants a two-way dialing link

9
Async. modem glossary
CCITT InternationalTelegraph and Telephone
Consultative Commitee (now ITU-T) ITU-T Internatio
nal Telecommuncations Union, Telecommunication
Standardization Sector (former CCITT) V.nn ITU-T
(CCITT) recommended standards
modulation standards Bell 103, V.21 110, 150,
300 bps Bell 212A, V.22 1200 bps V.22bis 2400
bps V.32 2400, 4800, 9600 bps V.32bis 7200,
9600, 12000, 14400 bps V.Fast preliminary
V.34 V.34 28800, ..., 14400 bps V.34bis 33600,
..., 14400 bps V.90 56.700 bps modulation
fallback V.90 --gt V.34 --gt V.32bis --gt V.32 --gt
V.22bis --gt V.22
10
Async. modem glossary
speed buffering interface speed connection
speed flow control RTS/CTS hardware control by
special wires XON/XOFF software control through
special characters (Ctrl-Q/Ctrl-S) ASL Au
tomatic Speed Leveling during retraining err
or control MNP 1, 2, 3, 4 Microcom Networking
Protocol (de facto standard) V.42 Link
Access Protocol for Modems (LAPM) error-cont
rol fallback V.42 --gt MNP4 --gt MNP3 --gt MNP2 --gt
MNP1 --gt none
11
Async. modem glossary
data compression MNP3, 4 protocol optimization
(108-120) MNP5, V42bis true compression, (up to
400, depends on data) compression
fallback V.42bis --gt MNP5 --gt none DTE Data
Terminal Equipment (computer) DCE Data
Communication Equipment (modem) DTE-DCE
signalling DTR Data Terminal Ready, set by
DTE DSR Data Set Ready, set by modem RTS,
CTS Reday To Send, Clear To Send, used to
hanshake on sped buffering CD Carrier
Detect, set by modem when communication to
remote modem is established
12
Async. modem exercise

• modem description
• US Robotics Courier V.34
• data format, binary, serial, synchronous/asynchro
  nous
• Enhanced AT command set
• Error correction MNP 2-4, V.42
• Data compression MNP 5, V.42bis
• Interface speed up to 115200, speed buffering
• Features NVRAM configuration, remote
  configuration, call-back
• front panel lights
• HS high speed (gt2400 bps)
• AA auto answer on
• CD carrier detected
• OH off hook
• SD/RD sending/receiving data
• TR terminal ready
• MR modem ready
• RS request to send
• CS clear to send

13
Async. modem exercise

• modem installation
• power adapter cord
• RS232 cable to COM port
• telephone cord with RJ11
• software tool which may be used
• ZOC under Windows 95
• Minicom, C-Kermit in Linux
• designed to
• dial out to remote site
• emulate terminal to remote site
• enable user to transfer files between remote and
  local machine
• COM port customization using ZOC
• in ZOC--gtOptions--gtSettings--gtDevice set
• baud rate 115200,
• data bits 8, stop bits 1, parity none
  (8N1)
• hardware flow control (RTS/CTS handshaking)

14
Async. modem exercise

• COM port customization using Linux Minicom
• minicom -s
• --gtserial port setup--gtserial device /dev/ttyS0
• --gtbaud rate--gtH--gtO 115200, 8N1
• --gtsave setup as dfl
• COM port customization using Linux C-Kermit
• kermit
• ? help with C-Kermit commands
• set line /dev/ttyS0
• set speed 115200
• set flow rts hardware flow control
• c switch to data mode

15
Async. modem exercise

• modem customization
• dynamically using modem initialisation function
  at ZOC or minicom startup
• specify modem initialisation commands
• in ZOC--gtOptions--gtSettings--gtModem (tilde)
  in command string means 1/3 second delay
• in minicom--gtmodem dialing
• initialisation command e.g. ATF
• dialling command - usually ATDT - tone dialling
• 4 different commands may be prepared
• hangup command - usually ATH
• statically using modem NVRAM
• interactive data sent from the machine to the
  modem in modem command mode are interpertted as
  modem comands
• modem is in comand mode after power on or may be
  switched to command mode by string of
  followed by approx. 1 second delay
• switch
• ZOC to data modeby Device--gtInitialise
• minocom to data mode by exiting configuration
• write modem commands
• save the modem configuration in modem NVRAM by
  ATW command

16
Async. modem exercise

• statically using modem NVRAM
• interactive data sent from the machine to the
  modem in modem command mode are interpertted as
  modem comands
• modem is in comand mode after power on or may be
  switched to command mode by string of
  followed by approx. 1 second delay
• switch
• ZOC to data modeby Device--gtInitialise
• minocom to data mode by exiting configuration
• write modem commands
• configuration set by modem commands may be saved
  in modem NVRAM by ATW command

17
Async. modem exercise

• AT commands basics
• ATZ reset
• ATV1, ATIn print out the configuration
• ATW save the configuration in NVRAM
• ATE0/1 echo the commands
• ATXn
• ATX3/4 ignore/detect dialtone, detect busy tone,
  signal CONNECT
• ATH0/1 go on/off-hook
• ATP/T set pulse/tone dialing
• ATD dial the specified number and try to
  connect
• try call to
• terminal servers - they should hang off and
  connect
• modem at the next PC - RING should appear on its
  screen if it is in modem command mode
• responses to dial command (when ATV1 is set)
• NO DIALTONE, BUSY, NO CARRIER, CONNECT
• A cancel dialing

18
Async. modem exercise

• ATA answer incoming call (when RING is signaled)
• responses to aswer command (when ATV1 is set)
• NO CARRIER, CONNECT
• automated answering to incoming calls is set in
  S-register 0
• ATS0 answer after of incoming rings
• ATS0? display content of S-register 0
• ATDn
• ATD0 ignore DTR (status of local machine)
• ATD2 hang up when DTR drops
• ATD3 reset when DTR drops
• data transfer mode
• established after successful connection
• escaped by sequence
• resumed by ATO command