Protocol Interworking

Protocol interworking means providing functionality that enables two different systems using different signalling protocols to work together with the aid of an interworking function (IWF).

Protocol interworking is needed for example in peering between operators using different protocols in their networks. Both H.323 and SIP implementations are present in the networks of long distance and international carriers, who utilize VoIP to carry PSTN and PLMN traffic. In order to exchange traffic between carriers using different signalling protocols, conversion is required.

The need for protocol interworking is not limited to carrier to carrier peering. SIP is frequently used in systems providing communication services for consumers and enterprise. On the other hand, H.323 based video conferencing is very common and widely deployed in the enterprise environment. Protocol interworking is needed to connect the two. Figure 25 presents architecture for implementing IWF.

H.323

Figure 25 Architecture of a SBC with SIP-H.323 IWF [Com04]

In addition to providing interworking between two completely different protocols such as SIP and H.323, protocol interworking may be required even between two systems using the same signalling protocol as the basis of their operation.

This is the case in connecting the SIP services found in the Internet with the SIP based IMS services specified by 3GPP and ETSI TISPAN. This incompatibility within the same protocol is the result of different SIP profiles used by the two [Cam05]. The IMS specification defines profiles of IETF RFCs for 3GPP usage [3GP05].

SBCs are used to provide interoperation between different signalling protocols such as SIP and H.323 [Com04]. The SIP-H.323 interworking has existed since the first SBCs.

5.2.8.1 Protocol Interworking Approaches in SBCs

The interworking implementations in SBC are similar to the SIP B2BUA approach found in protocol repair function, but in this case the other endpoint may be other than a SIP user agent, such as H.323 gateway. The signalling is terminated by the IWF and then regenerated using another signalling protocol or protocol profile. Converting between signalling protocols to achieve interworking is a complex process and interworking functions require quite a lot of processing resources.

5.2.8.2 SIP-H.323 Interworking

The requirements for SIP-H.323 IWF have been defined by the IETF in RFC 4123 [Sch05]. This RFC was published in July 2005, but the SBC literature studied does not specify conformance to this specification or any of the draft versions [Agr01] through [Sch04].

The RFC 4123 states that a SIP-H.323 IWF contains functions from the following list among others:

1. Mapping of the call setup and teardown sequences

2. Registering H.323 and SIP endpoints with SIP registrars and H.323 gatekeepers 3. Resolving H.323 and SIP addresses

4. Maintaining the H.323 and SIP state machines 5. Negotiating terminal capabilities

6. Opening and closing media channels

7. Mapping media-coding algorithms for H.323 and SIP networks 8. Reserving and releasing call-related resources

9. Processing of mid-call signalling messages 10. Handling of services and features

The functions seem fair and practical and it is very possible, that many SBC IWF implementations perform them.

The requirement specification states that IWF should not process media and assumes media exchange to take place directly between endpoints. If a particular service requires, the IWF is allowed to handle media. The IWF then simply forwards media packets without modification from one network to the other.

SBCs frequently participate in media processing to perform other functions such as NAT traversal or topology hiding. It is allowed although not encouraged by the specification.

Figure 26 shows common IWF configurations or deployment scenarios listed by the specification. They are similar to the SBC deployment scenarios. The first four ones describe scenarios where endpoints using different signalling protocols communicate.

The last two ones are examples of a situation, where a network used as a transit network uses a different protocol than the access networks.

H.323 EP

H.323 GK SIP proxiesSIP proxies SIP UASIP UA H.323 EP

Calls using both H.323 GK and SIP proxy

SIP trunking between H.323 networks

H.323 trunking between SIP networks

Figure 26 Common SIP-H.323 interworking scenarios [Sch05]

While the basic functionality described in SBC literature matches that of the IWF requirements no compliance to RFC 4123 is mentioned. The IWF requirement specification covers the following main areas (sub categories not presented here):

• Pre-Call requirements

• General interworking requirements

• Transport

• Mapping between SIP and H.323

• Security considerations

The material available from vendors does not describe details of the implementations. As a result, it is not possible to analyze how the requirements in any of the areas handled are met by SBC implementations.

5.2.8.3 Generic SIP and IMS Interworking

As of writing this thesis, there is emerging support for TISPAN based IMS services by some SBCs [New05b], [Acm05a]. The standardization of extended IMS architecture by ETSI TISPAN is work in progress.

TISPAN NGN Functional Architecture Release 1 [ETS05] specifies an Interworking Function (IWF). The IWF performs the interworking between protocols used within TISPAN NGN service control subsystems and other IP-based protocols e.g. between the SIP profile used in the IMS and other SIP profiles or IP-based protocols such as the H.323 protocol.

The IWF has two interfaces between which it performs it function. For protocols used in TISPAN, it interfaces to another TISPAN defined functional element called The Interconnection Border Control Function (IBCF) shown in Figure 27. For non TISPAN protocols, such as SIP version not compatible with SIP in IMS the IWF interfaces to the functional elements in other IP networks [ETS05]. IMS is not in the central focus of this thesis, but SBCs may be used to implement IWF and IBCF. These functions are marked by the dashed line in Figure 27.

The TISPAN specifications do not yet define any internal functions or requirements of the IWF, thus no comparison between it and the functions found in SBCs can be made.