The Gatekeeper Function
The gatekeeper is a Cisco router that runs the H.323 MCM feature set, and provides
the H.323 centralized call admissions control for the enterprise, call setup,
and related management issues. Among these functions is the decision regarding
whether the destination path can support the required bandwidth requirement of
the device placing the call.To illustrate this concept, let’s go through a call, referring
to Figure 11.1 as a common reference point.
A user on Site A wants to call a user on Site C.When the Site A user picks up
the phone and gets a dial tone, this person types in the digits of the destination
phone.This request is sent to the CallManager on the head office backbone, which
determines that the destination device is on Site C, and then contacts the gatekeeper.
The gatekeeper looks at the request in regards to the amount of bandwidth
requested, the type of services requested, and then makes the determination as to
whether the total amount of bandwidth is available to the site.
www.syngress.com
Figure 11.1 A Typical Centralized VoIP Design
Backbone
Router (R1)
3524 Switch
CallManager Unity
Site A
Router (R4)
3524 Switch
T-1
512K
Frame
Relay
Cloud
Site B
Router (R5)
3524 Switch
512K
Site C
Router (R6)
3524 Switch
512K
Network
Exchange
MGCP (R3)
Gateway
PRI
Telco
FXO
Telco
FXO
Telco
FXO
Telco
H.323 (R2)
Gatekeeper
Head Office
Designing and Implementing Multisite Solutions • Chapter 11 395
The gatekeeper knows these things because it keeps track of the amount of
calls currently placed to Site C, and the amount of bandwidth dedicated to that
site.With the WAN link currently set at 512 Kbps, the average g.711 call uses 64
Kbps of bandwidth, which means that 8 simultaneous calls are possible to Site C
from any other site, provided that none of the 512 Kbps is used for data streams.
If other compression techniques are used, the voice streams can be compressed to
as low as 5.3 Kbps with the high-complexity digital signal processor (DSP)
CODECs in the voice-capable gateways.
However, Cisco design rules state that no more than 75 percent of the circuit
capacity should be used for voice traffic. Furthermore, overhead in the IP packets
can raise the total per-call bandwidth requirement for a G.711 call to 80 Kbps per
call. Using these parameters on the same 512 Kbps connection now yields the primary
reason many VoIP designs fail to meet expectations: 75 percent of 512 Kbps
is 384 Kbps. Divided by 80 Kbps per G.711 call, we now have a maximum of four
possible calls at the same time.This is quite a difference from the previous paragraph,
and illustrates how and why these designs sometimes go wrong.
NOTE
The gatekeeper does not handle the actual voice stream between the
two endpoints, but rather assures that the proper bandwidth is available
between the two endpoints.