vb.net barcode generator SIP: Alternative Softswitch Architecture in Software

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SIP: Alternative Softswitch Architecture
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Figure 5-7 H.323 using Fast Start
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as the terminals must have configuration information for all locations they will contact using the Fast Start procedures. Furthermore, not all terminals support the Fast Start procedures, so this further limits the usefulness of this procedure. Fast Start was developed assuming a fairly closed system, where the participants are limited and have intimate knowledge of each other, such as in an office environment where users are calling each other.12 An additional aspect of H.323 s complexity is its duplication of some of the functionality present in other parts of the protocol. In particular, H.323 makes use of RTP and RTCP. RTCP has been engineered to provide various feedback and conference control functions in a manner that scales from two-party conferences to thousand-party broadcast sessions. H.245, however, provides its own mechanisms for both feedback and simple conference control (such as obtaining the list of conference participants). These H.245 mechanisms are redundant and have been engineered for small to mediumsized conferences only.13
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Scalability
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H.323 and SIP differ in terms of scalability. The three main concerns are managing a large number of domains, server-processing capabilities, con-
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Ibid. Schulzrenne and Rosenberg.
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SIP: Alternative Softswitch Architecture
SIP: Alternative Softswitch Architecture
ferencing, and feedback. H.323 was originally conceived for use on a single LAN and not a large number of domains. Issues such as wide area addressing and user location were not a concern. The newest version defines the concept of a zone and defines procedures for user locations across zones for email names. However, for large numbers of domains, and complex location operations, H.323 has scalability problems. It provides no easy way to perform loop detection in complex multidomain searches (it can be done statefully by storing messages, which is not scalable). SIP, however, uses a loop detection algorithm similar to the one used in BGP, which can be performed in a stateless manner. Another factor for scalability is server processing. As regards server processing in an H.323 system, both telephony gateways and gatekeepers will be required to handle calls from a multitude of users. Similarly, SIP servers and gateways will need to handle many calls. For large, backbone IP telephony providers, the number of calls being handled by a large server can be significant. In SIP, a transaction through several servers and gateways can be either stateful or stateless. In the stateless model, a server receives a call request, performs some operation, forwards the request, and completely forgets about it. SIP messages contain sufficient state to enable the response to be forwarded correctly.14 Because the protocol is simpler, SIP requires less code to implement than H.323. This is reflected in lower fixed (code space) and dynamic memory requirements for both the protocol stack itself and the host application. Because of the smaller number of messages that the processor needs to handle to set up a call, SIP is faster than H.323. This means that SIP can process more calls per second than H.323 and that SIP requires a less powerful CPU to handle the same number of calls. The result is higher calls handled for a given system s usage. This results in larger message sizes for SIP than for H.323. However, it is better to send a few large frames of data than a lot of small frames except for slow links.15 SIP can be carried on either Transmission Control Protocol (TCP) or UDP. In the case of UDP, no connection state is required. This means that large, backbone servers can be based on UDP and operate in a stateless fashion, reducing significantly the memory requirements and improving scalability. H.323, on the other hand, requires gatekeepers (when they are in the call loop) to be stateful. They must keep the call state for the entire duration of a call. Furthermore, the connections are TCP based, which
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