This document provides an explanation of transmit (Tx) and receive (Rx)
levels on modems.
For more information on document conventions, refer to the
Cisco Technical Tips
There are no specific prerequisites for this document.
This document is not restricted to specific software and hardware
The Tx level is the power in decibels per milliwatt (dBm) at which a
modem transmits its signal. The Rx level is the power in dBm of the received
signal. The server modems normally transmit at -13 dBm by default. Ideally, the
Rx level should be in the range of -18 to -25 dBm. If the Rx level is under -25
dBm, the Signal-to-Noise Ratio (SNR) is likely to decrease, meaning that the
speed also decreases. If the Rx level is too high, you may see signal
distortion or the receiver's Digital Signal Processor (DSP) being overdriven,
and erratic connections are possible.
In some modulation standards, such as V.34, a receiver can tell its
peer that the signal level is too high and the transmitter then reduces the
level at which it transmits. (If this behavior is widespread, try configuring
the transmitter to transmit at a lower level.) Modems that use other modulation
standards (such as K56 Flex) may not be able to do this, resulting in
Therefore, an effective Rx level is a function of the peer's initial Tx
level, the negotiated dBm reduction (if any), and the attenuation in the voice
circuit. The voice circuit attenuation is, in turn, a function of link
attenuation and of analog or digital pads, which are telephone company
circuitry designed to insert attenuation into the voice circuits.
If you need to reduce or increase your Tx level, this is attainable
with the following modems and modulation standards:
If you need to reduce or increase your Rx level, you need to do this
either at the peer transmitter (although this is not feasible if there are
thousands of peers) or within the telephone company (more likely), by
increasing or decreasing the padding.
On a live connection, you can see or infer the Rx and Tx levels as
Microcom modems???Initiate a
telnet session and issue the AT@E1
MICA modems???Issue the show modem
NextPort modems???Issue the show port
Some MICA modem examples are as follows:
router#show modem operational-status 1/0
Parameter #8 Connected Standard: V.34+
Parameter #20 TX,RX Xmit Level Reduction: 0, 0 dBm
Parameter #22 Receive Level: -22 dBm
In this case the Rx level is -22, which is fine. The peer has not
requested that the modem attenuate its Tx, so you can infer that it is
transmitting at the default output level of -13 dBm. You can also infer that
the signal level is not too high for the peer's receiver, because the peer has
not requested a reduction in signal strength (though it could still possibly be
too high???you cannot be certain without directly interrogating the peer).
Another example is as follows:
router#show modem operational-status 2/14
Parameter #8 Connected Standard: V.34
Parameter #20 TX,RX Xmit Level Reduction: 0, 3 dBm
Parameter #22 Receive Level: -19 dBm
In this case there is a good Rx level of -19, but the peer has asked
this modem to reduce its Tx level by 3 dBm. Therefore, it starts to transmit at
-16 dBm instead. This modem's signal is arriving with excessive strength at the
peer. If this occurrence is widespread, you might want to cut back on your
configured Tx level globally through S39. In this case, the problem appears to
be an issue with this particular peer, so there is no need to do so.
You can also check the output of the show modem
operational-status command for other potential issues and fixes
(registered customers only)
Telephone companies can insert a digital or analog pad, which is
circuitry designed to add attenuation on a per-channel basis. Padding ensures
that end-to-end circuits that take various paths through the Public Switched
Telephone Network (PSTN) end up with comparable signal levels. For instance, if
a modem transmits at -13 dBm, the receivers see a signal at the right
For purely analog carriers (V.34 and earlier standards), pads are
useful if they result in the desired levels being received. If the Rx levels
being observed are too high on a widespread basis, then pad insertion can make
analog carriers perform better.
However, the effect of pads on a digital (Pulse Code Modulation (PCM))
carrier (K56 Flex and V.90) can be problematic. An analog pad (line pad), which
merely attenuates the signal, is not a problem for a PCM carrier. However, a
pad in the Network Access Server's (NAS's) T1 line to trunk, or within the
telephone company's trunk-to-trunk connection, can have implications for PCM
Digital pads remap the PCM data, which can disrupt communication. The
general rule is that zero-dB digital pads are optimal for PCM connects.
However, zero-level padding is less than optimal in other cases; for example,
K56 Flex modems are less tolerant of Rx levels that are too high.
Different kinds of PCM modems can adapt to different flavors of digital
pads. Rockwell K56 Flex modems (as well as Microcom and MICA modems) can handle
zero-, three-, or six-dB pads. Lucent modems have a finer granularity of pad
handling, and can cope with one-, four-, five-, and seven-dB pads as well. V.90
modems can handle zero to seven dB of padding in one-dB increments. If you see
good V.34 connections, but poor or no K56 Flex connections, and if you know
that there is no extra A-to-D conversion in the circuit path, then you may have
a digital padding issue. In that case you need to contact your telephone
company to resolve the problem. In such a case it may be helpful to conduct
circuit traces of the suboptimal connections.