VOIP, INTRODUCTORY TECHNICAL DESCRIPTION
of the physical transport layer. They are controlled by a call control function housed in a
media gateway controller. A media gateway with its associated gateway controller is at
the heart of the network transformation to packetized voice. Several of the media gateway
functions are listed here:
Carries out A/D conversion of the analog voice channel (called compression in
Converts a DS0 or E0 to a binary signal compatible with IP or ATM.
Supports several types of access networks including media such as copper (including
various DSL regimes), fiber, radio (wireless), and CATV cable. It is also able to
support various formats found in PDH and SDH hierarchies.
Competitive availability (99.999%).
Capable of handling several voice and data interface protocols.
It must provide interface between media gateway control device and the media
gateway. This involves one of four protocols: SIP , H.323 , MGCP, and
Can handle switching and media processing based on standard network PCM, ATM,
and traditional IP.
Transport of voice. There are four transmission categories involved:
1. Standard PCM (E0/E1 or DS0/DS1)
2. ATM over AAL1/AAL2
3. IP-based RTP/RTCP
4. Frame relay
The most powerful gateway supports the public network or PSTN requiring a high
reliability device to meet the PSTN availability requirements. It will be required to process
many thousands of digital circuits. As shown in Figure 15.4, it has a network management
capability most often based on SNMP (see Chapter 21).
A somewhat less formidable gateway is employed to provide VoIP for small and
medium-sized business. Some texts call this type of gateway an integrated access device
(IAD) if it can handle data and video products as well. An IAD will probably be remotely
The least powerful and most economic gateways are residential. They can be deployed
in at least five settings:
Set-top box (CATV), which provides telephony as well
Broadband last mile connectivity (to the digital network)
Figure 15.5 shows gateway interface functions via a block diagram. On the left are
time slots of a PCM bit stream (T1, in this case). The various signal functions are shown
to develop a stream of data packets carrying voice or data. The output on the right consists
of IP packets.
The first functional block of the gateway analyzes the content on a time-slot basis.
The time slot may contain an 8-bit data sequence where we must be hands-off regarding