There are two basic types of telephony supported by Windows 2000:
- · Standard (or conventional) telephony, which is based on the Public Switched Telephone Network (PSTN) system.
- · IP telephony, which is audio, video, and data communications over computer networks using the TCP/IP protocols.
In this Daily Feature, I will discuss some of the problems that can occur when implementing IP telephony in Windows 2000 and how to troubleshoot common IP telephony issues.
Understanding IP telephony
Windows 2000 supports the Telephony Application Programmers Interface (TAPI), which allows computer communications to be integrated with telecommunications. The two types of telephony are integrated into one interface. For more information about TAPI, see the sidebar.
How IP telephony works
IP telephony uses packet switching, rather than the circuit switching technology used for traditional telecommunications. Information such as voice or video is broken into data packets for transmission, each of which contains IP headers that specify addressing and sequencing information.
TAPI service providers
Windows 2000 includes two TAPI service providers to support IP telephony:
- · H.323 service provider: For real-time audio, video, and data streaming where Quality of Service (QoS) guarantees are not required
- · Multicast conferencing service provider: For sending datagrams to groups of computers via group address, instead of sending to individual addresses
Want to know more about H.323 standards?
For an excellent tutorial on the H.323 standards and how they’re implemented, click here.
Components of IP telephony
To address problems with IP telephony connections, you must first understand a few key concepts, including the role of the Windows 2000 Active Directory, the role of the Site Server ILS service, the role of the MADCAP service, and the role of the H.323 gateway.
Role of Windows 2000 Active Directory
Active Directory is the master database used in Windows 2000 networks to store information about network objects including users, groups, and computers. TAPI uses the information in Active Directory when making IP telephony calls. This information includes the user names and IP addresses associated with user objects, allowing callers to place calls using names instead of IP addresses.
The Site Server ILS service can also perform this function (for example, if Active Directory is not installed on the network).
Role of the Site Server ILS service
The Site Server ILS service can publish mappings that associate user names with IP addresses for H.323 calls and publish multicast conferences on the network. It is used for locating users so other users can place calls to them, using NetMeeting and other conferencing software.
The location of the server(s) that are running Site Server ILS can be published in Active Directory, in which case TAPI will be able to find them.
Role of the MADCAP service
Multicast Address Dynamic Client Allocation Protocol (MADCAP) provides multicast IP addresses to MADCAP clients on the network.
Multicast addresses come from the Class D address range 126.96.36.199 to 188.8.131.52. This range is reserved for multicast communications.
Role of the H.323 gateway
An H.323 gateway connects an IP-based network to a PSTN network. Proxy servers, such as an ISA server that acts as a firewall between an internal network and the Internet, can run the H.323 gateway service to provide for telephony communications between the internal and external networks.
Troubleshooting IP telephony
Let’s look at some common problems that occur when using telephony applications and how you can correct or work around them.
Clients can’t connect to a call or conference
There are several possible reasons that a client might be unable to connect to a telephony call or conference.
- · If clients are unable to locate users or conferences to which they want to connect, this may be because there is no server running Site Server ILS or because the Site Server ILS server has not been published to Active Directory. Install the Site Server ILS service on a server on the network and/or publish the server to Active Directory using the ilscfg command.
- · If the Site Server ILS is installed and published but clients still cannot connect, this may be because there is no TCP/IP connection between the client and ILS server. Use the ping command to test connectivity.
- · Another possibility is that the client and ILS server are not in the same Windows 2000 domain. The ILS server and its clients must be members of the same domain.
Audio or video problems during calls or conferences
When clients are able to connect but have problems with audio and/or video during the call or conference, there are a couple of things you should consider.
If you are unable to hear audio, you should first make certain that the hardware (i.e., a sound card or video camera) is supported by Windows 2000. You can test the speakers and microphone using another application such as Sound Recorder (Start | Programs | Accessories | Entertainment | Sound Recorder).
Updating your device drivers may solve the problem. Check the Web site of the device vendor for the latest drivers.
If you are able to get audio or video but have problems or receive errors during the call, you should check the following:
- · Make sure the sound card or video device is not being used by another application. If it is, close the other application.
- · Check to see if the device is able to support full duplex operation and is configured to do so.
The quality of the audio/video may be improved by implementing QoS on the network.
QoS is a mechanism for guaranteeing a particular level of quality by ensuring adequate bandwidth for real-time applications. For a list of QoS resources, click here.
There are several problems that may arise with multicasting. If applications cannot obtain a multicast address from the MADCAP server, you should check TCP/IP connectivity between client and server and make sure that the two are in the same domain. You should also check the configuration of the MADCAP server. The multicast server (i.e., an ILS server) must be configured to support IP multicasting and manage the multicast group list.
Users may be unable to connect to multicast conferences for a variety of reasons. Check the security descriptor for the published conference. TAPI allows administrators to control which users can view and join conferences.
If the user can connect to the conference but is unable to see all participants, you should make sure that the routers and remote access servers are configured for multicast telephony. Multicast support should be enabled in both directions. The Internet Group Management Protocol (IGMP) must be installed and configured.
Windows 2000 includes the netsh andmrinfo commands, which can be used for gathering information about and troubleshooting multicast problems.
As high bandwidth technology becomes commonplace, IP telephony promises to play a major role in infrastructure decisions. Windows 2000 includes extensive support for IP telephony, which gives you the ability to transmit and receive voice, video, and data over an IP network in real time. In this Daily Feature, I have provided an overview of how IP telephony works, its major components, and how to address some common problems that occur when making connections to IP calls and conferences. Knowing the background of how IP telephony works is the first step any technician should take before beginning a troubleshooting operation.
Debra Littlejohn Shinder, MCSE, MVP is a technology consultant, trainer, and writer who has authored a number of books on computer operating systems, networking, and security. Deb is a tech editor, developmental editor, and contributor to over 20 additional books on subjects such as the Windows 2000 and Windows 2003 MCSE exams, CompTIA Security+ exam, and TruSecure's ICSA certification.