Operation Manual

NT/MPRI – Glossary 116
D Channel
The D channel transmits management information, such as the type of
ISDN service in use or the ISDN number of the communications part-
ner. The bandwidth is 16 kbit/s for the basic rate interface (BRI) and 64
kbit/s for the primary rate interface (PRI). In ISDN, charge information
(AOCD, AOCE) and the caller’s number can be transmitted over the D
channel. In Germany, these features must be specifically requested.
DSS1
A standardized European D channel protocol. All new ISDN lines in Ger-
many use DSS1.
Filters (special)
In practice, certain applications constantly exchange network packets,
which in the case of WAN connections over ISDN could lead to connec-
tions being set up unnecessarily often. For this reason, the NT/MPRI
contains special packet filters to intercept network overhead traffic. In
this way, SNMP packets in UDP and TCP can be filtered, for example, as
well as NetBIOS broadcasts in IP and IPX, so that they are not transmit-
ted over ISDN. The actual filtering criteria are the source and destina-
tion ports. These filter mechanisms are not negotiated with the remote
site, but set in the NT/MPRI.
Firewall
The NT/MPRI’s firewall filters protect against unauthorized access to
the network and select the resources and services to be made avail-
able for outside access. Various mechanisms are used to implement
firewalls. The NT/MPRI’s firewall is implemented as a packet filter. The
NT/MPRI checks every incoming and outgoing data packet against the
security criteria. These include the packet’s source and destination ad-
dresses (network address and mask) as well as the service (such as
FTP). The security criteria are saved in global IP filters and destination
filter profiles. The filter rules specify what measures are to be taken
with a packet: send it through, simply discard it, or stop it and send
back an error message. See also the section “IP-Masquerading / Net-
work Address Translation (NAT)” on page 118.
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