Citel EXTender PBXgateway Manuale
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Copyright 2007 Citel plc All Rights Reserved
CITEL’S POSITIONING FOR xDSL AND CABLE MODEM
NETWORKS
Citel manufactures the EXTender™ family of distributed voice solutions that work over serial and
packet networks. Many of these solutions are designed to work using Internet Protocol (IP), but require
a “managed” network connection that guarantees bandwidth, reduces latency on the network and
assures timely delivery of packets across the wide area network (WAN). Absent a “managed” network
connection, voice quality and service may be compromised. The Internet uses IP as a protocol, but
does not have built-in management features to eliminate the latency issues that often interfere with
business quality voice.
Citel recommends its solutions to run over managed IP connections for best results. A “managed”
connection means a network where end-to-end jitter, latency and packet loss can be monitored and
acceptable parameters can be set. Citel supports all of its products with a full range of services,
including installation, support, time and materials repair and advance replacement.
Citel Products
EXTender™ 4000 for IP
EXTender™ 6000 for Branch Offices
EXTender™ 7000 for Branch Offices
EXTender PBXgateway™
EXTender PBXgateway II™
For the purposes of this document, we will confine our discussion to the EXTender 4000 for IP, a
single-user client for home offices and teleworkers. As this is the product most often associated with
home office usage, many customers ask if an existing xDSL or Cable modem connection to the Internet
could serve as the network. The general discussion, however, applies to all the other IP-compatible
Citel solutions.
Citel Technologies. markets and supports solutions that utilize Internet Protocol as a transport
mechanism. CItel recommends running its solutions over managed IP network connections, where
jitter, latency and packet loss, can be monitored and controlled.
This is due to the Internet’s inherent lack of management and inability to guarantee Quality of Service
(QoS) across the network.
This situation is not unique to Citel, but applies to all voice solutions that work over
Internet Protocol (IP) networks. The problem arises from the confusion between the
Internet, which is a network, and IP, which is a transmission standard.
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Copyright 2007 Citel plc All Rights Reserved
SUMMARY OF KEY POINTS
•Citel recommends the deployment of its EXTender 4000 for IP over managed networks.
This applies most directly to the EXTender 4000 for IP, a single user product installed in
a home office or teleworker environment.
•Citel solutions, such as the EXTender 4000 for IP, utilize Internet Protocol (IP) and
require certain network characteristics that guarantee voice packets transmission
across the network in a timely fashion.
•Citel supports all of its products with a full range of services, including installation,
support, time and materials repair and advance replacement.
•Many xDSL and Cable modem connections utilize connections to the Internet
•The internet utilizes IP, but contains no management features that guarantee QOS,
Quality of Service across the network.
•Due to the nature of the Internet, Citel cannot guarantee the operation of this application
over an unmanaged network. Citel will work with the customer to ensure that factors
that could impede business quality voice – such as latency, jitter and packet loss - are
minimized.
•Many people confuse the protocol (IP) with the network (Internet); this causes the false
belief that a delay-sensitive application, such as voice, can work effectively across the
Internet.
•Cable modem or xDSL connections that utilize point-to-point connections to a corporate
intranet (not the Internet) may be utilized for the Citel application with perfectly
acceptable results.
•This is not unique to Citel solutions and, in fact, is universal to any voice solutions that
works over Internet Protocol.
•RVPoIP – Remote Voice Protocol over Internet Protocol: This is MCK’s patented voice
protocol that compresses and packetizes voice for transmission over an IP network
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Copyright 2007 Citel plc All Rights Reserved
The Situation: The Internet as a Non-Managed Network
At one time, it was widely anticipated that the growth and availability of xDSL intranet
services, and comparable cable services, would lead to management standards and
Quality of Service (QoS) that would enable advanced applications to be used over the
Internet. These access methods, widely marketed to home office users, were expected
to deliver corporate voice and data services to the home office or teleworker. The notion
was that standards for voice over data would follow to enable the development of these
next-generation networks that would carry all kinds of traffic.
The Reality
These eagerly awaited service offerings have not emerged and the marketplace is
confused regarding the implementation of IP telephony solutions in the home
environment. The Internet has continued to grow, but is primarily a data-centric and
unmanaged collection of disparate networks, all administered differently, with no real
quality of service standards emerging to prioritize one type of traffic over another. The
only thing that continues to unite the Internet is the use of a common protocol, Internet
Protocol. The Internet has failed to emerge into a network with broad-based Quality of
Service (QoS) standards
Internet Voice
This situation is not unique to Citel. Many of Citel’s competitors are still positioning IP
voice solutions as if they will work over the Internet. No different than Citel, these
companies’ products will experience the same issues, such as latency and jitter,
resulting in unacceptable voice quality. Latency (delay) and jitter (variable delay) are
network conditions that cause the voice packets to arrive late or out of order. To the
user, the conversation may cease temporarily or pause, causing the user to talk over
the other person’s conversation.
Placing voice over an IP network requires the reliable delivery of voice packets in a
consistently timely manner. Citel products typically transmit a voice packet every 8 – 40
milliseconds (ms). After transmission of the first packet, the other end takes it and starts
generating 8 –40 ms of voice for playback. The expectation is that the next packet will
be waiting upon completion of the first one. Citel products include a jitter buffer that has
the capability to delay the playback of the first packet, typically up to 100 ms, to improve
the likelihood that the next packet will arrive and be waiting for playback. The G.729a (8
Kbps) compression algorithm, included in Citel products, is the best there is for handling
lost (late) packets, filling in the elapsed 50 milliseconds with a continuation of the
previous pattern rather than a gap or white noise.
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Copyright 2007 Citel plc All Rights Reserved
Most of the time, quality high-speed Internet connections are able to deliver packets
within the 100 milliseconds jitter window, and the normal one-way delay of less than 100
ms generally allows us to stay within our total round-trip delay budget of under 500 ms.
Anything in excess of this and talk-over becomes garbled, with both parties talking over
one another’s conversation. Understanding the issues and consequences of using the
Internet as your transmission network is extremely important. For many customers,
DSL or Cable may very well provide an acceptable level of service. It may be the only
option that meets their cost constraints, and consistent audio quality may not be a big
issue.
Transmission Speeds
Transmission speed is very important to a successful IP telephony implementation. The
DSL service offered by many RBOCs is ADSL (Asymmetric Digital Subscriber Line) with
an uplink speed less than the downlink speed. Speeds of up to 384 Kbps can be
achieved when receiving (downloading), but the sending (uploading) speed is limited to
128 Kbps. Citel Extender products only need 23 Kbps per phone at G.729a. So, what’s
the problem?
The problem arises if the customer is sending and receiving data over the same link as
the voice. The voice will likely need to be prioritized over the data. Unfortunately,
problems occur due to the amount of time it takes to transmit a data packet at that
speed. The default Maximum Transmission Unit (MTU) size is 1500 bytes, and at 128K
it takes 96 milliseconds to send one packet of data. Citel transmits voice packets every
50 ms, so if the customer tries to send data 40 ms later, the link is free and the router
will send it immediately.
The problem in this scenario as there is no competing voice traffic for another 10 ms.
Transmission is not interrupted, so the next voice packet is delayed, as are the two after
it. For this reason, Citel recommends a minimum speed of 256K when sharing the link
with data. This allows the data packets to fit between the voice packets with only a
single packet ever being delayed. The MTU size can be reduced, but this may affect
the users ability to access Internet services. This problem is typically referred to as
fragmentation, and can be overcome by the use of a QoS router such as the Cisco
1760. Again, some users may be willing to live with this by agreeing to never send data
when they’re on the phone.
DSL may be satisfactory if the customer is using the same service provider at home and
the corporate office, and if the connections are large enough and never leaves the ISP
backbone. Customers have also had success with Cable modems where the home
office is only one hop away from the corporate broadband connection. Although both of
these scenarios can result in intermittent quality lapses, many customers are willing to
deal with intermittent poor quality in return for RVPoIP - Citel’s patented voice protocol -
using the low cost access of DSL and cable modems. However, keep in mind that Citel
will not endorse or support these types of implementations using the Internet as the
backbone.
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Copyright 2007 Citel plc All Rights Reserved
A Recommended Implementation
So what is the alternative to running Citel solutions over the Internet? Fortunately, there
are many good alternatives available. For example, voice over IP using traditional
networks, frame relay and ISDN, provides excellent business quality voice. In fact, the
two most effective implementations are over frame relay and ISDN Basic Rate Interface
(BRI). ISDN BRI is particularly attractive in areas where there is a fixed flat monthly rate
for usage (versus paying by packet).
Customer Solution: IP over Frame Relay
The Problem
A manufacturing company with 7 teleworkers scattered across the United States and Canada was looking
for a way to communicate more effectively with these “work at home” employees. They were seeking a
solution that would allow all employees to use a single communications system, to share common
applications such as voice messaging and to have access to any employee in the organization by dialing
4 digits. The availability of message waiting lamp indicators was also a hot button for them.
The Solution
The company looked to the Citel EXTender™ 4000 for IP and the EXTender PBXgateway™ to get the job
done. They started by installing Citel EXTender 4000s in each of the seven remote offices, along with a
QoS IP Ethernet router, such as the Cisco 1720. The company provisioned 128 Kbps frame relay
connections to each home office location, delivering enough bandwidth for each teleworker to access
corporate voice and data simultaneously. At the corporate office, the EXTender PBXgateway and Cisco
3620 IP Ethernet router were installed next to the PBX. A full T1 (1.54 Mbps) frame connection from the
central office provided the network. The result was a single seamless, easy to use corporate
communications system that provided voice and data access for the remote users.
Note: Citel has produced an Implementation Guide detailing the step-by-step process
for implementing the Citel EXTender 4000 for IP over a frame relay network connection.
Contact your Citel representative for more information.
Conclusion
While the Citel EXTender 4000 for IP is a great solution for the home office or
teleworker, it is not recommended for non-managed networks, such as the Internet.
Issues with Quality of Service will more likely lead to an unacceptable voice quality for
some customers. Higher quality implementation alternatives exist using the product over
networks (such as frame relay and ISDN BRI) where bandwidth can be controlled and
voice can be prioritized over data. MCK supports all of its products with a full range of
services, including installation, support, time and materials repair and advance
replacement.
For Further Assistance, Please Contact Citel Technical Support @ 1-877-248-3587
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