The Session Initiation Protocol (SIP) is an application-layer
control protocol that can establish, modify, and terminate multimedia
sessions or calls. Like MGCP, SIP is text-based. SIP
came out of the Internet Engineering Task Force (IETF) in
1998 as an RFC. It has rapidly gained widespread support,
including Microsoft’s announcement that SIP will be supported
in the next generation Windows XP product.
SIP uses a “request-response” model like that used in
Hypertext Transfer Protocol (HTTP). There is one major difference
between MGCP and SIP—a call agent is not necessary
to mediate between clients. An SIP interface is shown in
Figure 2-19.
The usefulness of SIP for multimedia is almost limitless.
Sessions can be unicast or multicast and include multimedia conferences, distance learning, VoIP, or similar applications.
Some examples of multicast protocols include email, news
groups, Web pages, and the like. SIP also supports the ISDN
and Intelligent Network telephony subscriber services for personal
mobility, which is important for the Wireless Internet.
SIP is reliable, scalable, and can be used with other protocols.
Development is fast because it is very similar to HTTP,
thus making the addition of feature-rich applications very quick
to implement. Initially H.323 and MGCP may be the protocols
of choice for tomorrows’ media gateways and soft switches but
SIP and Megaco will be strong contenders as they mature.
No matter which protocol or protocols become the favorite,
soft switches will be flexible enough to adapt. This adaptability
makes network service providers very happy. Unlike old legacy
switches, this new breed of switches will be quick to accept
added features or changed services without waiting months for
a manufacturer to modify the switch design. 104
IT Certification CCIE,CCNP,CCIP,CCNA,CCSP,Cisco Network Optimization and Security Tips
MGCP/MEGACO
The Media Gateway Control Protocol (MGCP) specifies communication
between call control elements and telephony gateways.
It is a text-based protocol. Media gateways are telephony
gateways that convert circuit-switched voice signals to data
packets for multiservice packet networks. The Internet
Engineering Task Force (IETF) created MGCP to address
some of the perceived shortcomings of H.323. See Figure 2-18.
The main purpose of MGCP is to place control of call signaling
and processing intelligence in call agents or media gateway
controllers. (Call agents and media gateway controllers are
synonymous with and similar to the gatekeeper functions in
H.323 and are also called soft switches.) A new version of
MGCP, released in August 2000, is called Megaco or H.248.
Although Megaco was created for the same purpose, Voiceover-
IP, it differs from MGCP because it supports a broader
range of networks and devices such as ATM, Remote Access
Servers, Multi-Protocol Label Switching routers (MPLS),
Digital Subscriber Line Access Multiplexers (DSLAMs), and
more.
Because Megaco is very new, interoperability testing is
ongoing. It appears to answer many of the deficiencies of
H.323 and will become very important as we move toward
Voice-over-IP networks. It is somewhat unclear at this time if
Megaco will replace MGCP or just supplement it. Megaco is
more suited for media applications than MGCP, but MGCP
may be a better choice for nonmedia-centric applications, such
as MPLS-based session control.
between call control elements and telephony gateways.
It is a text-based protocol. Media gateways are telephony
gateways that convert circuit-switched voice signals to data
packets for multiservice packet networks. The Internet
Engineering Task Force (IETF) created MGCP to address
some of the perceived shortcomings of H.323. See Figure 2-18.
The main purpose of MGCP is to place control of call signaling
and processing intelligence in call agents or media gateway
controllers. (Call agents and media gateway controllers are
synonymous with and similar to the gatekeeper functions in
H.323 and are also called soft switches.) A new version of
MGCP, released in August 2000, is called Megaco or H.248.
Although Megaco was created for the same purpose, Voiceover-
IP, it differs from MGCP because it supports a broader
range of networks and devices such as ATM, Remote Access
Servers, Multi-Protocol Label Switching routers (MPLS),
Digital Subscriber Line Access Multiplexers (DSLAMs), and
more.
Because Megaco is very new, interoperability testing is
ongoing. It appears to answer many of the deficiencies of
H.323 and will become very important as we move toward
Voice-over-IP networks. It is somewhat unclear at this time if
Megaco will replace MGCP or just supplement it. Megaco is
more suited for media applications than MGCP, but MGCP
may be a better choice for nonmedia-centric applications, such
as MPLS-based session control.
H.323
H.323 defines packet standards for terminal equipment and
services for multimedia communications over local and wide
area networks communicating with systems connected to
telephony networks such as ISDN. The initial version of this
standard came from the International Telecommunications
Union (ITU) in June 1996.
It defines communication over IP-based local area networks
(LANs). A later version (v2), adopted in January 1998,
extended it over wide area use and general-purpose IP networks.
Several subprotocols are included under H.323 relating
to call setup and signaling.
Four components for a multimedia communication system
as shown in Figure 2-17 include terminals, gateways, gatekeepers,
and multipoint control units (MCU). Gateways and
gatekeepers are used in negotiation for PSTN connections,
whereas MCUs enable multiparty audio and videoconferences.
One drawback of H.323 is that it is somewhat complex and
inflexible. However, it is ISDN-based and relatively easy to
build applications across it. For many applications, H.323 is
satisfactory, but falls short for more advanced implementations
and solutions.
All things considered, the most likely scenario is that multiple
protocols will be used with H.323, such as SIP for
exchange between soft switches and gateways and MGCP for
call setup, because H.323 is too complex and time consuming
to set up a call.
services for multimedia communications over local and wide
area networks communicating with systems connected to
telephony networks such as ISDN. The initial version of this
standard came from the International Telecommunications
Union (ITU) in June 1996.
It defines communication over IP-based local area networks
(LANs). A later version (v2), adopted in January 1998,
extended it over wide area use and general-purpose IP networks.
Several subprotocols are included under H.323 relating
to call setup and signaling.
Four components for a multimedia communication system
as shown in Figure 2-17 include terminals, gateways, gatekeepers,
and multipoint control units (MCU). Gateways and
gatekeepers are used in negotiation for PSTN connections,
whereas MCUs enable multiparty audio and videoconferences.
One drawback of H.323 is that it is somewhat complex and
inflexible. However, it is ISDN-based and relatively easy to
build applications across it. For many applications, H.323 is
satisfactory, but falls short for more advanced implementations
and solutions.
All things considered, the most likely scenario is that multiple
protocols will be used with H.323, such as SIP for
exchange between soft switches and gateways and MGCP for
call setup, because H.323 is too complex and time consuming
to set up a call.
HOME RF
Another industry group, the Home Radio Frequency Working
Group (HRFWG)—made up of members of industry leading
companies such as Compaq, Ericsson, HP, IBM, Intel,
Microsoft, Motorola, and others—created the Home RF
Standard Specification. Home RF combines elements of
802.11 and Digital Enhanced Cordless Telecommunications
(DECT) but supports only up to 2 Mbps. It is aimed at homes
and small businesses.
The price of Home RF is generally less expensive than
802.11 but performance is considerably less. The devices operate
in the 2.4 Ghz ISM band just as 802.11 devices do. In actuality
Home RF competes more with Bluetooth than 802.11. It
was designed for embedded applications in appliances and
computing equipment such as printers. Only time will tell if
this standard prospers.
Group (HRFWG)—made up of members of industry leading
companies such as Compaq, Ericsson, HP, IBM, Intel,
Microsoft, Motorola, and others—created the Home RF
Standard Specification. Home RF combines elements of
802.11 and Digital Enhanced Cordless Telecommunications
(DECT) but supports only up to 2 Mbps. It is aimed at homes
and small businesses.
The price of Home RF is generally less expensive than
802.11 but performance is considerably less. The devices operate
in the 2.4 Ghz ISM band just as 802.11 devices do. In actuality
Home RF competes more with Bluetooth than 802.11. It
was designed for embedded applications in appliances and
computing equipment such as printers. Only time will tell if
this standard prospers.
HIPERLAN2 FEATURES
Other than the high data rate and
QoS features, HiperLAN2 includes including the following:
• Automatic frequency allocation
• Security support
• Mobility support
• Network and application independent
• Power save mode
Automatic frequency allocation is especially important
because this allows for easy installation without the need for
complicated frequency planning such as that required for cellular.
The access points use a built-in support for automatic
transmission frequency allocation.
HiperLAN2 networks also supports authentication and
encryption. A handoff mechanism is managed by the mobile
terminal based on received signals from each access point.
Connections are maintained just in cellular (hopefully maybe
even better). The HiperLAN2 network may also integrated with
a variety of fixed networks.
A power save mechanism is based on mobile terminal-initiated
negotiation of sleep periods. A request is made to the
access point for a low power state and a specific sleep period.
At the end of the sleep period, the mobile terminal searches for
a wake up indicator from the access point, and in the absence
of that, sleeps the next period, etc.
QoS features, HiperLAN2 includes including the following:
• Automatic frequency allocation
• Security support
• Mobility support
• Network and application independent
• Power save mode
Automatic frequency allocation is especially important
because this allows for easy installation without the need for
complicated frequency planning such as that required for cellular.
The access points use a built-in support for automatic
transmission frequency allocation.
HiperLAN2 networks also supports authentication and
encryption. A handoff mechanism is managed by the mobile
terminal based on received signals from each access point.
Connections are maintained just in cellular (hopefully maybe
even better). The HiperLAN2 network may also integrated with
a variety of fixed networks.
A power save mechanism is based on mobile terminal-initiated
negotiation of sleep periods. A request is made to the
access point for a low power state and a specific sleep period.
At the end of the sleep period, the mobile terminal searches for
a wake up indicator from the access point, and in the absence
of that, sleeps the next period, etc.
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