Understanding Gateways
A articulation aperture functions as a translator amid altered types of networks. Gateways
allow terminals of one type, such as H.323, to acquaint with terminals of another
type, such as a PBX, by converting protocols. Gateways affix a aggregation arrangement to
the PSTN, a PBX, or alone analog devices, such as a buzz or fax.
Following are the two types of Cisco admission gateways:
■ Analog gateways: There are two categories of Cisco analog admission systems:
■ Analog abject gateways affix an IP telephony arrangement to POTS. They provide
FXS ports to affix analog telephones, IVR systems, fax machines, PBX
systems, and voice-mail systems.
■ Analog block gateways affix an IP telephony arrangement to the PSTN central
office (CO) or a PBX. They accommodate FXO ports for PSTN or PBX admission and Ear
and Mouth (E&M) ports for analog block affiliation to a bequest PBX. To minimize
any acknowledgment and abstract administration issues, use agenda gateways whenever
possible. Analog direct-inward-dialing (DID) is additionally accessible for PSTN connectivity.
■ Agenda gateways: Cisco admission agenda block gateways affix an IP telephony
network to the PSTN or to a PBX via agenda trunks, such as PRI or BRI common
channel signaling (CCS) and T1 or E1 approach associated signaling (CAS). Digital
T1 PRI trunks ability additionally affix to assertive bequest voice-mail systems.
IP telephony gateways should accommodated these amount affection requirements:
■ Aperture agreement support: Cisco articulation gateways abutment assorted signaling protocols,
depending on the accouterments platform. Cisco gateways abutment H.323, MGCP,
SIP, and SCCP. H.323 and SIP gateways do not charge a alarm ascendancy agent. Therefore,
they can be deployed on networks in which alarm agents, such as Cisco UCM, are not
present. MGCP and SCCP are automated protocols that assignment alone on a arrangement in
which a alarm agent, such as Cisco UCM, is present. Cisco IP Phones use SCCP, which
is a lighter-weight protocol. SCCP uses a client-server model, admitting H.323 is a
peer-to-peer model. MGCP additionally follows a client-server model.
22 Authorized Self-Study Guide: Cisco Articulation over IP (CVOICE)
Note Agreement alternative depends on site-specific requirements and the installed abject of
equipment. For example, abounding alien annex locations accept Cisco 2600XM Series or
3700 Series multiservice routers installed. These routers abutment H.323 and MGCP 0.1,
beginning with Cisco IOS Release 12.2(11)T and Cisco UCM Release 3.1 or later. You
might adopt MGCP to H.323 because of simpler configuration. This advantage additionally works
well with earlier IOS versions because of abutment for alarm survivability during a Cisco UCM
failover from a primary to a accessory Cisco UCM server. On the added hand, you might
prefer H.323 over MGCP because of the added alternative of interfaces supported.
The Simplified Message Desk Interface (SMDI) is a accepted for amalgam voice-mail systems
with PBXs or Centrex systems. When abutting to a voice-mail arrangement via SMDI
and application either analog FXS or agenda T1 PRI connections, you will use either the SCCP or
MGCP agreement because H.323 accessories do not analyze the specific band actuality acclimated from a
group of ports. The use of H.323 gateways for this purpose agency the Cisco Messaging
Interface cannot accurately associate the SMDI advice with the absolute anchorage or channel
being acclimated for an admission call.
■ Advanced gateway functionality: The gateways should support the ability to transmit,
without corruption, touch-tone digits (that is, Dual Tone Multifrequency
[DTMF] tones) and also support a collection of other user services, as follows:
■ DTMF relay capabilities: Each digit dialed with tone dialing is assigned a unique
pair of frequencies. Voice compression of these tones with a low bit-rate codec
can cause DTMF signal loss or distortion. Therefore, DTMF tones can be separated
from the voice bearer stream and sent as signaling indications through the
gateway protocol (H.323, SCCP, or MGCP) signaling channel instead.
■ Supplementary services support: These services provide user functions such as
hold, transfer, and conferencing, and are considered to be fundamental requirements
of any voice installation.
■ Work with redundant Cisco UCM: The gateways must support the capability to
“rehome” to a secondary Cisco UCM in the event of a primary Cisco UCM failure.
■ Call survivability in Cisco UCM: The voice gateway preserves the RTP bearer
stream (the voice conversation) between two IP endpoints when a Cisco UCM server
to which an endpoint is registered is no longer accessible.
■ Q Signaling (QSIG) support: QSIG is becoming the standard for PBX interoperability
in Europe and North America. With QSIG, the Cisco voice packet network
appears to PBXs as a distributed transit PBX that can establish calls to any PBX or
other telephony endpoint served by a Cisco gateway, including non-QSIG endpoints.
■ Fax and modem support: Fax over IP enables interoperability of traditional analog
fax machines with IP telephony networks. The fax image is converted from an analog
signal and is transmitted as digital data over a packet network.
Gateways are deployed usually as edge devices on a network. Because gateways might
interface with both the PSTN and a company WAN, they must have appropriate hardware
and utilize an appropriate protocol for that network. Figure 1-9 represents a scenario
where three types of gateways are deployed for VoIP and PSTN interconnections.
The scenario shown in Figure 1-9 displays the unified communications network of a company
that was recently formed as a result of a merger of three individual companies. In
the past, each company had its own strategy in terms of how it connected to the PSTN:
■ The San Jose location used a Cisco UCM environment with a MGCP-controlled unified
communications gateway to connect to the PSTN.
■ The Chicago location used a Cisco UCM Express environment with an H.323-based
unified communications gateway to connect to the PSTN.
1-9 Aperture Deployment Example
■ The Denver area acclimated a Cisco SIP proxy server and SIP IP phones as able-bodied as a
SIP-based unified communications aperture to affix to the PSTN. Because the
Denver area is alone a baby office, it does not use the WAN for IP telephony traffic
to the added locations. Therefore, Denver’s bounded VoIP arrangement is affiliated only
to the PSTN.