Chapter 5. Speaking the Gateways Languages

Chapter 5. Speaking the Gateways' Languages

What You Will Learn

After reading this chapter, you should be able to

	Explain the need for gateway control languages.
	Identify characteristics of H.323.
	List the features of MGCP.
	Describe the operation of SIP.

The whole world hasn't migrated to Voice over IP (VoIP) telephony yet. Therefore, your VoIP networks need a portal to the rest of the world (that is, the public switched telephony network or public switched telephone network [PSTN]). Chapter 1, "Touring the History Museum of Telephony," introduced a VoIP gateway. Recall that a gateway acts as a threshold between a VoIP network and another type of network (for example, a private branch exchange [PBX] or the PSTN). For example, a gateway might have an Ethernet interface connecting to a VoIP network and a series of Foreign Exchange Office (FXO) or T1 interfaces connecting to the PSTN.

Gateways and CCM servers need a common language for communication; that is, a protocol. Think of gateway protocols as languages of love between VoIP devices. This chapter discusses the generic functions of these protocols and considers three specific gateway protocols:

• H.323

• MGCP

• SIP

Before delving into the specifics of individual gateway control protocols, first consider the general characteristics of gateway protocols.