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 (DB25). 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
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