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Concepts and Principles of Operation
15
diagram, traffic decreases temporarily before increasing again. Because bandwidth requirements can
change suddenly like this, the second B-channel waits for a period of time before closing down. In the
above diagram, this value has been set to 5 seconds. You can set this time to suit your own
requirements.
At point 4, data drops below the lower traffic load percentage value. Because traffic volume must
remain below this threshold for a certain length of time, the second B-channel does not close until
point 5 (5 seconds later) has been reached.
Data Compression
Data compression allows the router to send and receive data more efficiently by compressing the data at
sending and de-compress the data at receiving. When compression is enabled, the Hypertec Router
negotiates using a STAC LZS compression scheme using the Compression Control Protocol (CCP).
Hypertec ISDN 10T Router supports both Ascend’s pre-standard compression scheme, and the standard
compression scheme as specified by ITEF. Depending on the actual data pattern, the STAC compression
algorithm may accomplish the compression ratio of 4:1 against an ASCII file. No data compression can be
gained against a file which has already been compressed. When working with the Multilink PPP protocol,
the compression algorithm is inappropriate as the frequent packet out-of-sequence condition can cause
either end to request a compression history reset.
Calling Line Identification (CLID)
Calling Line Identification (CLI) or Caller ID is a mechanism for identifying incoming calls. You may be
able to order CLI from your telephone company as part of your ISDN Service. The Hypertec Router
allows one CLID for each Profile. In many circumstances, the incoming call indication signal does not
include the Calling Line Identification (directory number) of the device which has just called you, or the
Calling Line Identification (CLID) may not be passed to the Hypertec Router . You may want to disable
CLID authentication, instead, use the Challenge-Handshake Authentication Protocol (CHAP) for
additional security if these circumstances apply.
Callback
Callback is available on ISDN circuits. It allows a router to request a return call from a remote router,
meaning that when an incoming call is received on a circuit, the router drops that call and calls back
the originating router. This feature allows you to take advantage of any differences in call charges and
obtain centralized billing. Callback also provides additional security.
There are two scenarios of Callback operation:
1. Caller ID (CLID)
A call arrives at the Hypertec Router, which has been configured to accept Callback requests with
the CLID enabled. The HypertecRouter checks if the CLID matches its CLID configuration. If a
match is found, the Hypertec Router rejects the incoming SETUP message and calls back the
originator using the number presented by Caller ID. This method means that there is either no
charge or a minimal call setup charge (service provider dependent) to the originator. If the
originator is not configured to request Callback but the receiving circuit is configured to call back
the originator, then when a call arrives it will still be called back. If the originator is configured to
request Callback but the receiving end is not configured to call back, the receiving end will accept
the call and the originator will pay the cost of the call.
2. PPP
If CLID is disabled in configuration in the Hypertec Router, a call is accepted by the Listener
circuit then Callback is negotiated using PPP authentication. This method will mean a small cost is
incurred at the originating end since the call has to be accepted before Callback is negotiated. If the
PPP negotiation fails then the call is dropped as usual.
Packet Filtering, IP Access List
To provide an additional level of security once the connection is established, the Hypertec Router
allows users to configure access lists against the IP traffic. The access list specifies what kind of data