Figure 5.2 E&M signalling, type V
Signalling is carried out with direct current via the E & M control wires for call setup and tear down, pulse dialling and remote blocking. DTMF signals can alternatively be used for dialling.
The E&M signalling can operate in several modes:
- continuous signal;
- wink start signal;
- delay dial;
- immediate dial.
-/ 5.1.1.3 E1/CAS trunk
CAS (Channel Associated Signalling) exists in many varieties that operate over analogue or digital interfaces. A common digital interface with CAS signalling is called E1 (European version).The physical layer works in accordance with the ITU recommendation G.703/G.704 for PCM30/32.
The endpoints continually send Backward and Forward marks in 16 TSLs (Timeslot of PCM30/32, bits ABCD) as a supervision signal to indicate various states of the connection.
Additionally, the MFC-R2 (Multi Frequency Compelled) signalling is used (in TSL 1-15 and TSL
P.137
[IP Telephony Cookbook] / Setting Up Advanced Services
17-31) to support for several features:
- malicious call tracing (used to transfer calling party numbers);
- override authorisation;
- free calls;
- called party hold.
-/ 5.1.1.4 ISDN Access Interfaces
ISDN (Integrated Service Digital Network) is a currently preferred method of PBX to VoGW
interconnection. It is described in more detail and some illustrative examples of exchange of its Q.931 signalling messages are given. ISDN is a system of digital connections allowing the establishment a call with end-to-end digital connectivity of nx64kbps. Original recommendations for ISDN were in CCITT Recommendation I.120 (1984), which described some initial guidelines for implementing ISDN.The first commercial implementation of ISDN was in PBX
Hicom300 (Siemens AG). Several different signalling protocols have emerged. It was the 1TR6
protocol in Germany, NI (National ISDN) in the USA and the French National ISDN VN3
protocol in France.The absence of an international standard led each European country to make its own version of ISDN, which meant incompatibility and increased costs.Twenty-six communication organisations signed the ‘Memorandum of Understanding on the Implementation of a European ISDN’ in 1992.The signing countries were obliged to offer a common technological substructure for ISDN network development, connecting all of Europe. As a result, Q.931 signalling has been internationally standardised.
Two types of access methods exist for ISDN:
- BRI (Basic Rate Interface);
- PRI (Primary Rate Interface).
BRI delivers two 64 kb/s B channels and one 16 kb/s D channel.The reference configuration of ISDN defined in the ITU specification I.411 is illustrated in Figure 5.3
S
T
U
ISDN
TE1
NT2
NT1
local
exchange
Transmission
R
S
line
TE2
TA