30-2 Rapier Switch Software Reference
Software Release 2.6.1
C613-03072-00 REV A
Introduction
This chapter describes the Time Division Multiplexing (TDM) support
provided by the switch, and how to configure the switch to use E1/T1 TDM
and BRI TDM.
Time division multiplexing is a mechanism for dividing the bandwidth of a
link into separate channels or time slots. The switch supports TDM in two
forms—E1/T1 TDM and BRI TDM. BRI and PRI interfaces are available as
optional Port Interface Cards (PICs).
E1 TDM provides a 2.048Mbps communications link divided into 32 slots of
64kbps each. T1 TDM provides a 1.544Mbps communication link divided into
24 slots of 64kbps each and an 8kbps channel for synchronisation and
maintenance. E1 and T1 TDM were first used by telephone companies for the
transport of digitised voice, but since there is no difference between digitised
voice and other kinds of data E1 and T1 TDM are now also used for wide area
network links.
Of the 32 time slots in a E1 TDM link, time slot 0 is usually reserved for
framing. For Primary Rate ISDN running over E1 where calls need to be set up
and cleared dynamically, time slot 16 is reserved for signalling. This leaves up
to 30 time slots, each of 64kbps, for information transfer over ISDN calls and
up to 31 time slots for static TDM links. A mixture of static links and dynamic
ISDN calls is also possible. An E1 TDM link may also be used in an
unstructured mode where all of the 2.048Mbps of bandwidth is available for
data transfer. In this mode only a single link of the full bandwidth is possible
and slot 0 is not used to demarcate the slot structure.
For T1 Primary Rate ISDN, time slot 24 is reserved for signalling and the other
23 time slots are available for ISDN calls. When the T1 interface is not used for
ISDN, slot 24 may be used for a TDM link. Unstructured mode is not
supported for T1 TDM.
E1/T1 TDM support is provided on the switch by the Primary Rate interface
(PRI) card. The E1/T1 TDM facility can be used as the data link layer transport
mechanism for Primary Rate ISDN and as the data channel over which one or
more static PPP links can be configured. It is this later functionality which is
described in this chapter. See Chapter 10, Integrated Services Digital Network
(ISDN) for more information about using a Primary Rate interface for Primary
Rate ISDN connections.
BRI TDM support is provided by a switch Basic Rate Interface which can be
used for Basic Rate ISDN and as a data channel for one or more static PPP
links. The Basic Rate Interface has 2 time slots of 64kbps. See Chapter 10,
Integrated Services Digital Network (ISDN) for detailed information about
configuring ISDN Basic Rate interfaces.
A powerful feature of the switch’s support for TDM is the ability to use an
interface for ISDN and static PPP links simultaneously. The slots available on
the interface are statically apportioned, by command, between static TDM and
dynamic ISDN use. For example on an E1 Primary Rate interface slots 1 to 20
could be reserved for ISDN calls and slots 21 to 31 for static TDM links. Note
that not every telecommunication service provider is able to support
simultaneous static and dynamic use of Basic Rate and Primary Rate ISDN
services.