IP Telephony Gateway

If a call is to be made from a PC or specialized IP phone to a regular PSTN telephone or vice versa, both the PSTN and IP network are involved in making the call. The PSTN may also use an IP network for so-called trunk replacement, or IP trunking, where the long-distance portion of the PSTN voice traffic between two PSTN telephones is carried over an IP network.

When it comes to delivering real-time voice, the PSTN and an IP network are different in a number of ways, as summarized in Table 1. For establishing a call, for example, SS No. 7 has been traditionally used within the PSTN, while H.323 is the most prevalent protocol suite [with Session Initiation Protocol (SIP) as a viable alternative] in the Internet to date. In general, the connection of two dissimilar networks is achieved through some sort of a device—called a gateway—that compensates for the differences in the networks. There is no exception when it comes to interconnecting the PSTN and IP networks for supporting IP telephony. In this case, the interconnecting device is called an IP telephony gateway and will link users of IP telephony with a billion or so PSTN users.

Table 1: Key Differences between Telephony over the PSTN and the Internet

Distinguishing Aspect	PSTN	Internet
Bandwidth allocation for voice transport	A dedicated circuit (e.g., 64 kbps) set up for each two-party communications session.	Best-effort delivery of IP packets.
Numbering and addressing scheme	14-bit point code for network nodes and E.164 for endpoints.	4-byte IP address, domain name system (DNS), e-mail address, uniform resource locator (URL), etc.
Voice representation	Typically analog in the loop and G.711 (either A-or m-law) in the backbone.	G.711, G.723.1, G.729, etc.
Signaling protocols	Signalling System No. 7 Q.931, etc.,	H.323, SIP, etc.
Availability	99.999% (5 min downtime per year).	99% (88 h downtime per year).

Figure 1 illustrates the integration of the PSTN and the Internet (or any IP network) through gateways in support of IP telephony. It distinguishes four types of IP telephony gateways based on the PSTN interfaces and certain specific functions that they support.

Figure 1: PSTN-Internet integration through gateways.

1. 
   
   Trunking gateway. Connects a central office (CO) switch to an IP router. Such a gateway typically has an SS No. 7 signaling interface and manages a large number of 64-kbps digital circuits and Real-Time Transport Protocol (RTP) streams. It is used in the trunk replacement application where the long-distance portion of a call between two telephones is made over the IP network instead of the PSTN. (In IP telephony parlance, such calls are known as phone-to-phone calls.)
2. 
   
   Access gateway. Connects telephones or PBXs to an IP router through an access interface [such as ISDN primary rate interface (PRI)]. It supports calls between two telephones with the IP network as an intermediary transport or between a telephone and a PC. (Again, in IP telephony parlance, calls between a telephone and a PC are also known as PC-to-phone calls or phone-to-PC calls.)
3. 
   
   Network access server. Connects a central office switch to an IP router. (Though previously discussed, it is included for completeness, because this type of gateway can be controlled in exactly the same manner as others.) Such a gateway may have an ISDN interface similar to that of the access gateway.
4. 
   
   Residential gateway. Connects analog phones to an IP router. Such a gateway typically supports a small number (two to four) of analog lines and is located on the customer premises. It brings the Internet interconnection point directly to the curb and maximizes the use of the IP network for calls between two telephones as well as between a telephone and a PC.