There are lots of things in this world that don’t make a lot of sense unless you know how they have evolved to their current state. One of those things is the migration of circuit switched telephony to the IP world with the Bearer Independent Core Network concept specified in ITU Q.1901 and introduced in GSM and UMTS starting with 3GPP Release 4.
Here’s my take at it:
With BICN, the circuit matrix of the MSC (Mobile Switching Center) that creates a physical voice circuit between two subscribers is replaced by a media gateway. The media gateway maps the concept of a circuit connection to an IP stream between two parties. The stream is then transmitted together with many other streams over a shared packet switched link, which is for example based on Ethernet.
The Signaling System Number 7 (SS-7) used in the circuit switched world is still used in this architecture but has been changed to some degree. The protocols of this family are used for the following tasks:
- For the establishment of voice calls
- For the interaction between different network nodes (e.g. between the switching center and the subscriber database node)
- For communication between the switching center, the radio network and the mobile device
The main difference with SS-7 over IP is that the lower layer MTP protocols have been replaced by IP, SCTP and M3UA, so the messages can be transported over IP instead of a circuit switched timeslot. The figure on the left shows the MTP based SS-7 protocol stack in comparison to the IP based SS-7 protocol stack.
Above the MTP layers, the ISUP protocol that is used for establishing voice circuits has been replaced by BICC (Bearer Independent Call Control). BICC is very similar to ISUP. Message names are the same and only some parameters have been changed as the messages are now used to control media streams instead of circuit connections.
Protocols for the interaction between different network nodes (e.g. between the voice switch and the subscriber database) such as SCCP, TCAP and MAP have not been altered at all. DTAP (Direct Transfer Application Part), the protocol used for interaction between a mobile device and the switching center for purposes such as authentication, location updates, etc., has also remained unaltered. In other words, applications that use these protocols are not aware if the messages are transported over signaling timeslots of a circuit switched network or over an IP link.
To enable IP based SS-7 nodes to communicate with MTP based nodes in the network, Signaling Gateways are used to translate MTP into IP / SCTP / M3UA. This way, a traditional circuit switched MSC is able to communicate with a subscriber database node that is connected to the network over an IP connection.
And finally, from a mobile point of view, the air interface between the base station and the mobile device also remains unchanged. This means that GSM and UMTS mobiles have no visibility what kind of access or core network technology is used.
Today, both traditional circuit switching and BICN can be found in live networks so knowing only one of them won’t do, at least for the moment. So I’ve decided to coin two terms:
- “Traditional circuit switching”, i.e. the origins of circuit switching with voice calls transported over physical circuits and SS-7 messages being transported over circuit timeslots.
- “Virtual circuit switching over IP”, i.e. a voice channel is transported over an IP stream and the SS-7 protocol is used in a modified form in the IP world.
Traditional circuit switching vs. virtual circuit switching over IP. Do the terms make sense to you?