User guide

Internetworking Primer A-39

Modems

Both the access server and the modem must be conﬁgured for correct operation on each of

the pairs of wires. Understanding the conﬁguration concerns for these signals simpliﬁes

modem setup and troubleshooting.

Data Transfer (TX/RX)

The data transfer wire pair is used for the transmission of user data (characters and packets)

between the access server and the modem. The conditions that must be met when setting

data transfer speeds are as follows:

• The access server asynchronous port and the modem must agree on the speed of data

transfer used on this wire pair. (The modem must not change the speed on its

EIA/TIA-232 port when a session is negotiated with a remote modem.)

• The speed used should take into account modulation speed (V.32 bis, V.34),

compression ratio achieved (V.42 bis), and the length of the EIA/TIA-232 cables.

Clear To Send (CTS) Input Output DCE signals to DTE that it can

continue to accept data into its buffers.

Data Terminal Ready

(DTR)

Output Input DTE indicates to DCE that it can

accept a call. Lowering and raising

instructs DCE to drop the active call

and return to the stored conﬁguration.

Data Carrier Detect

(DCD)

(Connected to DSR

input on access

server.)

Input Output DCE indicates to DTE that a call is

now present and established with a

remote modem. Dropping DCD

terminates the session.

Signal Name

Asynchronous

Port (DTE)

Modem

(DCE) Function