Healthy VoIP Nets�Part VI�Managing the Lower Layers

Our recent tutorials have explored a number of legacy voice network quality testing methods that can be applied to VoIP network management, including Subjective Evaluations such as the Mean Opinion Score (MOS); and Objective Testing and the E-Model and. We also considered a fairly new concept called Quality of Experience (QoE), in which the applications supporting the end users—such as video on demand—are also included in the overall network evaluation process.

However, many of these testing methodologies had their origins in research laboratories. While they may help you evaluate the capabilities of the VoIP network management system you are considering purchasing (and, of course, make for interesting reading), they may not necessarily help you with day-to-day network management functions, such as troubleshooting a VoIP network that is currently experiencing problems.

For that aspect, we need a more practical network management approach, which can be found in a brief review of the Open Systems Interconnection (OSI) model, first published in 1994. And for us engineering types, the model provides us with a systematic method of breaking down and organizing the many functions that comprise a VoIP network into smaller pieces, which are then easier to handle. Let’s briefly review that model, and look at some example VoIP management issues that can occur at each layer.

Physical Layer: maintains the physical and electrical connection between two endpoints, and may be implemented with twisted pair or fiber optic premises cable, a wireless LAN or cellular WAN link, or a host of other options including microwave, infrared, and satellite. The unit of information transmitted at the Physical layer is the bit.

Thus, if you have a single voice station, or a group of stations (such as a remote location) that you cannot reach, then an examination of the physical layer between these two points is a good starting point—the problem may be as simple as a disconnected cable or a loose connector. Physical layer tools, such as cable testers, will likely be the least expensive (and possibly most useful) items in you network management toolbox.

Data Link Layer: deals with the reliable connection and local addressing between adjacent nodes on the network, and is typically implemented on an Ethernet LAN or a T-1 WAN network interface card. This layer puts the bits from the Physical layer into a specific order, known as a frame. If noise or some other aberration occurs on the link, an error control mechanism within that Data Link layer frame (such as a checksum) may signal the problem.

However, just because an error occurs does not mean that you need to do something about it. For example, most VoIP frames contain 20-40 milliseconds of voice information. If only a few frames get clobbered, the humans may never notice the problem, until it gets more severe. A network monitoring system or protocol analyzer will detect those invalid frames, and give you an early warning of a problem (such as a noisy WAN link) that merits further investigation.

Network Layer: handles the routing and switching between two locations, and is almost invariably implemented using the Internet Protocol (the “IP” in VoIP). This layer also adds another layer of addressing (the IP address) so that your station can be uniquely identified on the global network. That IP address is contained within the Network layer unit of information, called the packet.

For VoIP networks, this addressing process may involve other systems. One example might be the Domain Name Service, or DNS, which translates between host names and IP addresses. Another might be Electronic Numbering, or ENUM, which translates between a telephone number (10 digits) and an IP address (32 bits for IPv4 or 128 bits for IPv6). So, if you find that you can’t call a specific region of your network, and the Physical connections have tested okay, then a scrambled address or addressing translation may be the culprit, and a protocol analyzer is going to be required to sort this one out.

Taken together, the Physical, Data Link and Network layers are often referred to as the Communications Subnetwork, as this part of the architecture is responsible for the transfer of the voice or data signals from one node (e.g. telephone) on the network to another.

Since this network may comprise a number of LAN and WAN connections, this delivery process can get quite involved. For example, with VoIP we are delivering packets that contain a digital representation of a voice conversation. That voice sample was originated by a human, passed through various processes, such as a codec, then transmitted down the LAN between the telephone and an IPBX, passed off to a router, possibly traversed a WAN link, may have passed through a number of intermediate routers, reached a destination IPBX, and hopefully ended up at the intended destination telephone.

While it is certainly possible to manage and/or test each of these network elements separately, access to all of the necessary test points could be challenging, to say the least. As a result, many networks use testing probes, which are deployed at strategic locations, and can report back to a central network management console on the health of that network segment.

But managing the lower layers is just part of the story. Our next tutorial will continue this discussion, and look at the network management functions for the upper layers of VoIP network architectures.

Copyright Acknowledgement: © 2008 DigiNet Corporation ®, All Rights Reserved

Author’s Biography
Mark A. Miller, P.E., is President of DigiNet Corporation®, a Denver-based consulting engineering firm. He is the author of many books on networking technologies, including Voice over IP Technologies, and Internet Technologies Handbook, both published by John Wiley & Sons.

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