3Com Nbx 100 And Ss3 Nbx Administration Guide

Connection Considerations431
Most people notice latency when the end-to-end delay is above 200 ms. 
(The round-trip delay is typically no greater than 400 ms.) Conversations 
sound most natural when latency is below this range. Network latency 
can be measured by “pinging” the network connection, but the network 
connection is only part of the delay. The entire end-to-end delay also 
includes the H.323 gateway, firewall or router, and the LAN itself. System 
administrators can control some local device...

432APPENDIX A: CONNEXTIONS H.323 GATEWAY
Adaptive Jitter Buffering
All IP network devices use buffers to retime the packets that they receive 
from a network. Retiming allows these devices to compensate for the 
variable delays that occur as the packets pass through an IP network. 
H.323 calls take different paths through a network so the ConneXtions 
gateway uses an adaptive “jitter buffer” to minimize delay variability. 
Initially, the jitter buffer delays the entire packet stream by 50 ms, an 
amount...

Connection Considerations433
NBX systems use the latest developments to address voice packet priority 
concerns at the Layer 2 Ethernet level and at the Layer 3 IP network level.
Layer 2NBX systems address Layer 2 priority concerns through the 
802.1(p and q) standards. These standards have two parts. The first part 
addresses the way Ethernet frames get onto the local “wire.” The NBX 
system uses a special “back-off” algorithm that gives voice frames a 
higher priority when both voice frames and data...

434APPENDIX A: CONNEXTIONS H.323 GATEWAY
Special IssuesThis section describes issues related to H.323 telephony in general and to 
ConneXtions gateways in particular. These include:
■Firewall Security
■Gateway Load
■Remote Access
■PBX Connections
■Class of Service
■IP Type of Service and Differentiated Services
■Alternate Gatekeepers
Firewall SecurityFirewalls determine which packets can cross the boundary between a 
protected network (intranet) and the public internet. The network 
administrator...

Special Issues435
3Com recommends that a high-performance PC be dedicated to the 
ConneXtions software.
The question of whether an operating system is adequately “secure” is a 
subject of debate. The concern is that Windows has many IP ports of its 
own. One way to deal with these ports is to set up a firewall that limits 
the range of externally accessible ports. However, some organizations 
connect the ConneXtions gateway directly to the Internet through a 
second NIC that bypasses the firewall...

436APPENDIX A: CONNEXTIONS H.323 GATEWAY
The ConneXtions gateway uses these default port assignments:
■For UDP traffic, ConneXtions uses ports 8000-8099 by default. Calls 
require four UDP ports each.
■ConneXtions uses ports 1025-5000 for TCP traffic. You can configure 
TCP ports during installation.
During ConneXtions installation, you can configure the TCP ports that are 
used for incoming calls. For outgoing calls, no control is possible. Port 
1720 must be preserved.
You must configure a firewall to...

Special Issues437
You can use Microsoft’s VPN Dial Up Networking (version 1.3) to establish 
a virtual private network connection between a roaming laptop and the 
NBX system LAN. One end of the VPN connection is in the laptop while 
the other end must be located in a VPN server between the router and 
firewall.
The VPN server provides caller authentication and a secure (encrypted) 
channel across the internet. After a caller has been authenticated, the 
connection is passed to the firewall, which sees...

438APPENDIX A: CONNEXTIONS H.323 GATEWAY
Tie-line connections between NBX and PBX systems require technical 
people from both ends of the connection to collaborate in these major 
areas, discussed next:
■H.323 Interoperability
■IP Addressing
■Voice Ports
■Extension Dial Plans
■Extension Delay
H.323 Interoperability
H.323 protocol stacks provide the foundation for H.323 compatibility. 
Each consists of a collection of engineered software products that 
implements the H.323 standard. Although PBX...

Special Issues439
Outgoing IP addresses can be entered:
■As pre-programmed speed dial numbers that forward callers to the 
Auto Attendant at a remote NBX system.
■By modifying the dial plan.
You can configure the speed dial numbers to include an appended 
extension if a person in one NBX system needs to make frequent calls to 
someone in another NBX system. Alternatively, you can configure the dial 
plan to route these calls seamlessly to the caller.
NBX system calls to outside numbers must use IP...

440APPENDIX A: CONNEXTIONS H.323 GATEWAY
Extension Delay
Call setup times for digital connections, compared to analog connections, 
are instantaneous so there is no need to include a delay between the 
IP address and an appended extension.
Incoming H.323 calls to an NBX system usually go directly to the 
Automated Attendant. Although the Auto Attendant can respond with 
voice instructions, the call does not have to wait until the end of the voice 
instruction to respond. The Auto Attendant accepts...