Sem 6 – Chap 2

All You Need to Know about Modems

Modem Function

• PCM– standard for encoding analog to digital

• Each sample is then measured, or quantified, so that it can be encoded as a binary value (typically eight bits).

• These approximate values can be used to reconstruct the waveform digitally

North America Signaling Standards

• RS-232 (officially EIA/TIA-232-C)

• V.35

• HSSI (High Speed Serial Interface) – Asynchronous serial modems are connected to

end stations using EIA/TIA-232– if you are connecting a router to a digital local

loop using a CSU/DSU over a leased line, you are likely to use V.35 or HSSI.

EIA 232 Standards

• specifies a cable that uses a 25-pin connector (DB-25). However, only eight pins of the cable are used

• Many RS-232 cables use a DB-9 or RJ-11/RJ-45 connector instead of DB-25

• Eight pins categorized by groups– Data transfer group 2,3,7– Hardware flow control group 4,5– Modem control group 6, 8, 20

• Pin 6 (DSR) and pin 20 (DTR) control how the modem operates; Pin 8 (CD) is controlled by the DCE

Connecting External Modems

• To router– AUX (Auxiliary)

– Console

– Serial interface (on some models)

– Asynchronous interface (on some models)

• To PC– Internal Modem

– External Modem - via a rollover cable to a 9-pin serial port on a PC

Line Types

• CON (Console line) - Typically used to login to the router for configuration purposes; this line is also referred to as CTY.

• AUX (Auxiliary line) - EIA/TIA-232 DTE port used as a backup asynchronous port (TTY)

• TTY (Asynchronous line) - Same as asynchronous interface; available on access server models (Cisco 2509, 2510, 2511, 2512, AS5100, etc); used typically for remote dial-in sessions that use such protocols as SLIP and PPP. A serial interface configured as asynchronous is a TTY connection.

• VTY (Virtual terminal line) - Used for incoming Telnet, local-area transport (LAT), X.25 packet assembler/ disassembler (PAD), and protocol-translation connections into synchronous ports (e.g., Ethernet and serial interfaces) on the

router.

Telnet/Reverse Telnet

• telnet 131.108.30.40 2001– This command indicates a Telnet connection to

line 1 (2000 + 1). If you want to reverse Telnet to a modem on line 14, you would use TCP port 2014

• Show line– Will give line numbers

Configure Reverse Telnet

• RTA#configure terminal

• RTA(config)#line 10

• RTA(config-line)#transport input all

• RTA(config-line)#modem inout

Group Range Commands

• RTA(config)#interface group-async 1

• RTA(config-if)#group-range 1 7– specify which individual interfaces are

members of the group

Interface Configuration

• RTA(config)#line 2• RTA(config-line)#login• RTA(config-line)#password letmein• RTA(config-line)#speed 115200• RTA(config-line)#flowcontrol hardware• RTA(config-line)#stopbits 1• RTA(config-line)#transport input all• RTA(config-line)#modem inout

Using the Console Port

• Advantages

Using the Console Port - Disadvantages

• The console port does not support EIA/TIA-232 modem control (Data Set Ready/Data Carrier Detect (DSR/DCD), data terminal ready (DTR) the modem connection will not drop automatically; the user will need to manually disconnect the session.

• The EXEC session will not automatically reset; This can present a security hole, in that a subsequent call into that modem will be able to access the console without entering a password; The hole can be made smaller by setting a tight exec-timeout on the line. However, if security is important, it is recommended to use a modem that can provide a password prompt.

• The console port does not support hardware Clear to Send/Ready to Send (CTS/RTS) flow control. It is recommended to use no flow control. If data overruns are encountered, however, software (XON/XOFF flow control may be enabled.

• The console ports on most systems only support speeds of up to 9600 bps. • The console port lacks reverse telnet capability. • A console port cannot be used for dial-on-demand routing; it has no corresponding

async interface.

Configuring Async Ports on Access Servers

• Network protocol support such as IP, IPX, or AppleTalk

• Encapsulation support such as PPP

• IP client addressing options (default or dynamic)

• IPX networking addressing options

• PPP authentication

Dialers

• dialer-list 1 protocol ip permit 192.168.1.1– Defines interesting traffic

• dialer-group 1– Assigns dialer-list to an Async interface

Modem Configuration

• Manual (typically via reverse Telnet)

• Automatic discovery (called autodiscovery)

• Automatic configuration using a database called autoconfigure

Configure Modem To

• Answer a call • Perform hardware flow control • Lock DTE speed to ensure that the modem will

always communicate with the access server at the specified speed.– when you use an async interface, you lock the speed to its

theoretical maximum of 115.2 kbps. The router speed command sets both transmit and receive speeds

• Hang up when you quit a session • Have the CD signal truthfully reflect the carrier

state

Essential Modem Commands

• Hardware flow control - Uses CTS and RTS signaling.

• Lock DTE speed - Sets the serial port of the modem to a fixed data transfer rate..

• Error correction - Sets error control. • Compression - Uses the best compression algorithm

that can be negotiated between the two communicating modems.

• Show configuration - Shows current modem settings. • Getting help and saving the configuration

Commands

• physical-layer async – Must be issued in interface configuration mode– Used for out-of-band management

• Show line– shows the type of modem configured on a line.

• Clear line 5– returns a line to its idle state

• Show modemcap– verify modemcap support for this modem.

• Debug confmodem– displays the modem configuration process