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What is Signalling ?
Sending control signals that start and stop a transmission or other
operation. The signals are the commands that request an operation to be
performed. For example, in telephony, a control signal is sent to establish a
call, and later one is sent to tear down (disconnect) the call.
What are different type of signalling ?
Following are different ways in which Signalling cane be categorized.
In-band & Out-of-band
In-band signaling: A signaling method in which network control and call
setup signals are sent over the same path as the user's transmission. With
in-band call setup signals occupying the communications pathway, transmission
of the user message must wait until the call setup process is completed.
Out-of-band signaling: A signaling method in which network control and
call setup signals are sent over a separate digital channel, called a
signaling link. This method allows for the transport of more data at higher
speeds since setup signals and transmission of user messages can be sent
simultaneously.
Channel Associated Signaling(CAS) & Common Channel Signaling(CCS)
Channel Associated Signaling(CAS) is same as In-Band signaling. Example of
a CAS signaling is R2MF.
R2 is a 1960s- and 1970s-era channel-associated-signalling signalling
protocol used outside of the former Bell System to convey information along a
telephone trunk between two telephone switches in order to establish a single
telephone call along that trunk.
R2 is the name given to two broad protocol groups: R2 line signalling and
R2 register signalling. More details about R2 can be found at link below :
http://www.answers.com/topic/r2-signalling
CAS is not a preferred signaling mode due to disadvantages like Loop Disconnect, dedicated signaling resource for every circuit, Slow-Long post dialing delay, limited information transfer etc.
Common Channel Signalling (CCS) is the transmission of signaling information (control information) on a separate channel to the data, and, more specifically, where that signalling channel controls multiple data channels.
Its Out-of-Band Signaling which additional specification of Single signalling channel handling multiple voice channels (as Single channel each for Voice & Signalling will also be Out-of-Band Signalling)
Advantages of CCS over CAS
CCS offers the following advantages over CAS:
•Faster call setup.
•No interference between signalling tones by network and frequency of human speech pattern.
•Greater trunking efficiency due to the quicker set up and clear down, thereby reducing traffic on the network.
•No security issues related to the use of in-band signalling with CAS.
•CCS allows the transfer of additional information along with the signalling traffic providing features such as caller ID.
The most common CCS signalling methods in use today are ISDN and SS7, while common CAS signalling method is R2MF whose usage in telecom domain has gone down to almost Nil.
Some differences between R2MF & SS7 based IVR Service node deployment are :
1.No SS7 hardware(card or signaling gateway) is required for R2MF based IVR Services. Simple Voice cards would be enough to integrate your IVR system.
2.R2MF can support advanced services like Conferencing, Call forwarding etc.
3.You can’t provide multi-model services on R2MF. e.g. If you have deployed your IVR system on shortcode 12345, then only calls terminating on number 12345 can be routed to this IVR. R2MF based IVR won’t be able to handle 12345xxx kind of service through which you could have given direct access to any service menu or level.
Difference between ISDN PRI and SS7
PRI can be termed as subset of SS7.
The difference between PRI and SS7 is the kind of signaling that occurs. SS7 is the signaling network for telcos. Like ISDN, inter-telco signaling is Out-of-Band.
However, SS7 links carry only signaling. And signaling is not solely related to call passing. LNP (Line Number Portability) transactions occur on this kind of link. You may think of ISDN as an IGP and SS7 as an EGP. ISDN is for processing calls between the telco and the customer. SS7 is for processing signaling,generally between telcos.
Differences can be summarized as below :
1. Both are similar in many ways, but SS7 is more powerful and core to the network. PRI is a subset of SS7.
2. Main difference between PRI & SS7 which telecom professionals or IVR Integrators/Developers would notice is that You can’t route multiple shortcodes (i.e. IVR Service Code e.g. 123) to Single Trunkgroup.i.e. Trunkgroups/Circuits can’t be overlapped between different shortcodes. While this is a facility offered in SS7. You can have shared trunkgroup/circuits between different shortcodes or service codes. So SS7 allows optimal utilization of resources.
3. ISDN PRI link must directly be connected to switch. It has no capability to route the call intelligently. Using SS7 every node in the network can be accessed. Nodes need not be connected directly, SS7 has got built in intelligence to redirect packets to the correct destination.
4. SS7 allows to access the telecom companies databases, like the subscriber data, and other configuration information. Now a days most of the call routing is based on database information about the customer and the destination number.
ISDN is Call Signalling/Control protocol only and can’t be used for querying databases..
An example for this would be, the credit limit approaching message that we hear when our credit limit is about to expire. Whenever a call is placed, subsriber information is looked up from the database the appropriate message is played or routed to the destination.
5. SS7 is used to interconnect the switches. Its not a consumer end protocol. Modern day switches convert SS7 to PRI before giving it to a vendor or a consumer. Interconnection protocol between HLR, VLR , MSC and various Mobile Networks core nodes is SS7 wheareas almost all Call Centres & Connect Centres run on ISDN.