User Manual

Modems Modem and SCS Interaction
9-9
If the modem responds with the Connect String, the call will succeed. If the modem responds with the No
Carrier, Error, No Dial Tone, or Busy strings, or if no response is received in 60 seconds, the call will fail
and the modem will be reset (60 seconds is the default wait period; this can be configured using the Define
Ports Modem Carrierwait command).
Note: Define Ports Modem Carrierwait is discussed on page 12-94.
9.4.3 Incoming Calls
The SCS will detect an incoming call when a port receives the Ring string. The port will then be in a
“ringing” state; outgoing calls cannot be made from this port during this period. The SCS will send the
Command string followed by the Answer string forcing the modem to answer the call.
When a modem asserts the DCD signal, the incoming call will be permitted. If more than 60 seconds pass
between ring signals or before the assertion of DCD, the SCS will assume that the caller hung up or that the
connection attempt failed. Sixty seconds is the default wait period; this can be configured using the Define
Ports Modem Carrierwait command. The port will then be available for outgoing calls.
9.4.4 When a Port is Logged Out
Each time a port is logged out (for example, when a user hangs up), the SCS will send the Attention string
to the modem. The OK string is expected in return. When this string is received, the SCS will send the
Command Prefix string and the Reset string.
When the modem receives the Reset string, it will read its configuration from NVR. Any temporary
configuration, such as changes made by an outbound modem user, will be cleared at this point. If a user
made changes during an outbound call and saved them to the modem’s NVR, the modem will be returned
to that changed state.
9.4.5 Compression
The compression setting in a modem profile enables or disables data compression in the modem. Data
compression enables a modem to transfer a larger amount of data in the same amount of time. When
compression is used, uncompressed data arrives on the modem’s serial port and the modem compresses the
data before sending it over the phone line.
The advantage of compression is increased throughput. For example, a modem might compress data to 1/2
its original size, doubling the modem’s throughput; twice the data could be sent in the same amount of time
required to send uncompressed data.
The disadvantage of compression is increased latency. Latency is the delay before data transfer occurs,
caused by the additional time the modem requires to compress the data before it is sent. In situations where
the delay is undesirable (for example, during interactive use over a long distance line), compression should
not be used.
The "compressability" of data depends on what is being compressed. Some data can be compressed to less
than half its original size, while other data cannot be compressed at all. As the type of data to be sent
changes, the modem’s throughput will change.