> interactive Media Service: Media Servers

Interactive Media Service: Media Servers
On the other hand, there is another kind of media server that actually contains DSP
resources that it uses to process speech or video (and perhaps one or more additional
form of media).These may be involved with generating and receiving DTMF tones,
executing the logic of an IVR system, converting text-to-speech or handling
streaming or document content in response to speech or DTMF input. Or it may
orchestrate multiway call traffic, conference calls, handle translation between codecs,
or even fax processing. Media servers of this class may provide VoiceXML interpretation
for interactive, dynamic voice applications.



Call or Resource Control: Media Servers
This class of media server is responsible for managing communications resources at a
higher level, such as handling call control while managing media gateways that have
DSP and other gateway resources for the actual media manipulation. Most Media
Servers support VoIP protocols but are likely also to support others as well, such as
digital voice or video trunks, or even analog voice through media gateways.
Examples of this kind of media server include call control servers from PBX vendors
that control separate gateways, voice processing servers that manage and redirect DSP
resources located elsewhere, and call distribution systems that manage off-board call
handling resources such as switches and IVR systems.
The H.323 Gatekeeper
This gatekeeper is the manager of one or more gateways, and is responsible for providing
address translation (alias to IP address) and access control to VoIP terminals
and gateways. A gatekeeper acts as the central authority for other gateways, allowing
an administrator to quickly and authoritatively roll out changes across a voice network.
Gatekeepers limit the number of calls at a given time on a network by implementing
control over a proxy. A gatekeeper works something like this: A user wants
to make a call to another user at a different physical location, and his phone registers
with a local gateway.The gateway then passes on his call information to the gatekeeper,
which acts as a central hub to other gateways and users.The gatekeeper then
passes call setup information to the gatekeeper at the other office, which in turn
hands it to the appropriate destination gateway, and finally to the desktop of the
called party. Many call control media servers include an H.323 gatekeeper.



Registration Servers
In a traditional PSTN or PBX switching system, where each user is at a fixed location,
usually tied in place by copper wires, routing calls is (relatively speaking)
simple. So-called find-me/follow-me services on PSTN or PBX switches can add
PSTN mobility. Forwarding or extension-to-cellular features can increase this sense
of mobility, but all these solutions require active user programming or rely on fixed
forwarding algorithms and are rooted in the PSTN.
But with VoIP, a user can be geographically located virtually anywhere on the
planet (as long as minimum QoS conditions are present). A registration server acts as
a point of connection for mobile users. Johnny can log in to the registration server
from his hotel room in Amsterdam with an unknown IP address and the registration
server will let the gateways know where to route his traffic.That way, Johnny can
keep the same phone number no matter where he is physically located. A similar
example can be seen with instant messaging networks. A user can log in using his
screen name from home and be reachable to the same users as if he had logged in
from work. In the H.323 world, registration is a function of a gatekeeper; however,
this can be a separate function in the SIP realm.




Redirect Servers
A SIP redirect server acts as the traffic light at the VoIP intersection.Very much like
a web page with a redirect tag built in, a redirect server will inform a client if the
destination the caller is trying to reach had changed. Armed with the updated information
from the redirect server, the client will then rerequest the call using the new
destination information.This takes some of the load off proxy servers and improves
call routing robustness. In this way, a call can quickly be diverted from a proxy, rather
than require the proxy to complete the connection itself.



Media Gateways
A gateway is a device that translates between protocols in general by providing logic
and translation between otherwise incompatible interfaces. A voice or media gateway
in particular tends to translate between PSTN (trunking) protocols and interfaces
and local line protocols and interfaces (though that’s not universally true). In addition,
the potential protocols and interfaces that a voice gateway now might support
include Ethernet and VoIP protocols as well.The voice gateway could have H.323
phones on one side and an ISDN trunk on the other (both digital) or a VoIP phone
on one side and an analog loop to the carrier, or even VoIP on both sides (say,
H.323 to the station and SIP trunking to the carrier).The point is that there are literally
hundreds of different equipment classes that all fall under the voice gateway
moniker and thousands of classes that fall under gateway to begin with.
One class of VoIP media gateway connects traditional analog or digital phone
equipment or networks to VoIP equipment or networks. A simple home-user implementation
of a VoIP gateway like this is an ATA, or Analog Telephone Adaptor. At a
minimum a VoIP media gateway will have both a phone interface (analog or digital)
and an Ethernet interface. For an ATA, a regular analog phone is connected to the
adaptor, which then translates the signal to digital and passes it back over the
Ethernet. Of course, media gateways can get much more complex than this. PBX
vendors have split out the line-card cabinet portion of their product and recast it as a
media gateway, with the gateway under the control of a media server. IP routing
companies have added analog and digital voice/video interfaces to routers and recast
them as media gateways. And in many respects these products do contain overlapping
functionality even though they may not be equivalent.




Firewalls and Application-Layer Gateways
Within a firewall, special code for handling specific protocols (like ftp, which uses
separate control and data paths just like VoIP) provides the logic required for the IP
address filtering and translation that must take place for the protocol to pass safely
through the firewall. One name for this is the Application Layer Gateway (ALG).
Each protocol that passes embedded IP addresses or that operates with separate data
(or media) and control streams will require ALG code to successfully pass through a
deep-packet-inspection and filtering device. Due to the constantly changing nature
of VoIP protocols, ALGs provided by firewall vendors are constantly playing a game
of catch-up. And tests of real-time performance under load for ALG solutions may
reveal that QoS standards cannot be met with a given ALG solution.This can cause
VoIP systems to fail under load across the perimeter and has forced consideration of
VoIP application proxies as an alternative.