How to Explain the Thoughts Behind BICN

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The main difference with SS-7 over IP is that the lower layer MTP protocols have been replaced by IP, SCTP and M3UA, so the messages can be transported over IP instead of a circuit switched timeslot.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Above the MTP layers, the ISUP protocol that is used for establishing voice circuits has been replaced by BICC (Bearer Independent Call Control).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?

Comments

[David]

Hello Martin,
Tome it feels a bit strange that you discuss only the transport of signalling and then conclude with definitions including the transport of voice, maybe some details on voice could be interesting. Also, "traditional" or "origins" of circuit switching come from 19th century while SS7 was specified in 1980. Then, I am not sure SCTP/IP creates a single path, so "virtual circuit switching" is a bit misleading, while it is fully accurate for ATM (above which M3UA was operating through SSCOP & AAL5).
One question: what were the motivations for the change? Cheap IP link availablity? Something else?

[Rohit]

Martin

Thanks for explaining this. So you mean today even if we say voice is CS, it still can be virtual CS ? Since you say that shared link is used to carry voice in case of virtual CS, how is the QoS contraints for voice guarenteed. Meaning by things like jitter ? Voice frames are created every 20ms so how exactly would the conditions be met ?

You can also point me to material that explains this in detail .

Rohit

[Martin]

Hi David,

thanks for your comments. The voice part of BICN is described at the beginning of the post. As this change is pretty much straight forward (replacement of the connection matrix), there's not as much to say about it as about the signaling changes.

Agreed, when going even further back, there's tradition before SS-7 :-)

Yes, you are right, at the beginning it would be good to mention why the architecture has changed. In my opinion, because running two different types of networks alongside each other becomes more expensive every day.

Thanks!
Martin

[Martin]

Hi Rohit,

not sure if how to ensure QoS is specified for this application. It could be by oversubscription or by some other means. Not sure where to start looking for that.

Kind regards,
Martin

[Steve Carr]

Martin -

I think your description is incomplete and the term “Virtual circuit switching over IP” is not so relevent for the following reasons.

With BICN, only the core uses IP, and the radio is still circuit switched. Thats why the DTAP is unchanged.

In fact, (I think) for a call between two handsets controlled by the same MSC, the call would stay circuit switched, or at least be switched between two media gateways controlled by the same MSC without BICC. BICC would only get used for a call needing to transit between two different MSCs.

Only when the radio network is changed to IP - for LTE, or for VoIP over HSPA, would you need to change DTAP. In fact, for pure IP, you would use the SIP protocol of IMS instead of DTAP. Then you would have end-to-end VoIP.

There is also work in 3GPP to define CS over HSPA, where virtual circuits are carried over IP over the air interface. In this case, DTAP is still used, but I assume it would have to be modified to indicate virtual circuits. Another approach for IP over the air interface is VoLGA.
Both these would fit the label “Virtual circuit switching over IP”.

Steve

[Martin]

Hi Steve,

very insightful comment, thanks a lot! You are right, I should include the following additions:

-Rather than just explaining BICN the whole thing should be extended to say "The Thoughts behind BICN and SS-7 Over IP", to also include the evolution of SS-7 signaling e.g. to the HLR from MTP to IP.

-I think over time, the Iu(cs) and A-Interface will migrate over IP. 3GPP standards are in place now. I assume Abis and Iub will also be transported over IP once network operators start the transfer from E-1/T-1 links to broadband when the demand for Internet connectivity rises beyond their current capacity. At this point, everything except the air interface would be IP based, no more circuit switching anywhere.

-I think even for the case today were a call doesn't leave an MSC-S / MGW site, things have already changed since the physical switching matrix has gone and the media gateway packetizes everything as soon as it arrives at an external interface.

Thanks again, appreciate it very much!

Martin