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The probe comes in two pieces. The main unit is comprised of the CPU and interface cards. The second piece is a small tap that connects to the DS-3 lines. Three taps can be placed side by side in one unit of rack space. The main unit takes up three units of rack space; if you have several DS-3s to monitor, rack space can be eaten up quickly. The taps come with a 15-foot cable to link to the main unit, so they don't have to be placed right on top of each other. To install the taps, you must break the DS-3 connection for a short period of time, but once the taps are installed they operate in passive mode--power outages to the probe will not take the line down.

Capabilities and Limitations
I connected the probe to an Adtech AX/4000 Generator/Analyzer broadband test system because a live ATM connection was unavailable. This arrangement gave me full control of the traffic that was on the line and I could burst traffic as I pleased. I set up three PVCs' (permanent virtual circuits') worth of traffic: one that carried CBR (constant bit rate), another for VBR (variable bit rate) and the last for ABR (available bit rate). The probe is capable of looking at data with CBR, VBR, ABR and UBR (unspecified bit rate) QoS (quality of service). The probe also can look at just about any Layer 3 protocol you have on your network, including AppleTalk, DECnet, NetBIOS, NetWare, SNA and TCP/IP.

The DS3 ATM Probe is capable of monitoring only 20 PVCs, so larger installations would require several probes on the same line if statistics for all the PVCs were needed. This could be a very expensive proposition. If you have more than 20 PVCs, you still can collect statistics on the DS-3 and the traffic going over it, but not at the PVC level. SVC (switched virtual circuit) traffic also can be monitored for data such as voice calls, though the data contained within the cells cannot be broken down. Because the DS3 ATM Probe can monitor only pure data types, it will monitor only AAL-5 (ATM Adaptation Layer 5) traffic. With all the voice and video traffic that is starting to pass over ATM networks, it is a deficiency not to be able to decode AAL-1 or AAL-2 traffic. NetScout does such a good job decoding AAL-5 data it is frustrating that the probe can only count cells for other data types.

The probe does a complete SAR (segmentation and reassembly) analysis of every AAL-5 cell on the PVCs that it monitors. Because the unit acts as a tap to the line, no degradation in end-to-end throughput is seen while it is performing the internal SAR function. For my tests, I generated AAL-5 data from the AX/4000 with varying payloads and from different IP addresses. I connected a Sun Microsystems SPARC Ultra 10 via Ethernet to the probe and collected data with NetScout Manager Plus 5.7.1. The software pulled all the information off the unit and displayed all the graphs you would expect from a probe: all talkers, Top(n) talkers, all conversations and Top(n) conversations. The amount of traffic can be monitored on selected PVCs or on the DS-3 as a whole. Traffic can be broken down based on QoS or protocol. It can also be displayed based on NSAP addresses if you are running a pure ATM environment. IP data, for example, can be broken down even further to determine which port traffic is associated with a system. HTTP traffic can be separated from FTP or telnet traffic, and then associated not only to the requesting user but also to the server sending the data.

NetScout's DS3 ATM Probe provides all the information you need as far as user traffic on your DS-3. I wish, however, that traps for OAM, F4 and F5 cells could be set and displayed. Knowing who is sending what traffic is important, but seeing errors also is useful. The probe at least maintains CLP (cell loss priority) counters to see when a switch is being overloaded.

Smooth Configuration Process
Configuration was fairly easy. A simple connection via the console port to configure the IP address of the probe is all it takes to start using the probe. PVCs to be monitored can be configured from the console port or can be set up via NetScout Manager Plus or other software. If you want a little more control, you can configure sets of protocols for the unit to store statistics.

Unit management is handled by a direct connection to the serial port or through a connected modem. You can also initiate a telnet session through the Ethernet port. In addition, the probe is fully compliant with SNMP, ATM RMON, RMON and RMON2. DLSW (Data Link Switching) encapsulation for frame relay is also supported. Data can be extracted from the probe by using NetScout Manager Plus or other software written to access the MIBs.

The probe comes in two basic units, the difference being the amount of RAM installed. For my tests, 64 MB of RAM was fine. A unit with an additional 64 MB of RAM runs a little faster and stores more data.

Pricing Information:
NetScout DS3 ATM Probe (64 Mb) = $23,995.00
NetScout DS3 ATM Probe (128 Mb) = $24,995.00
NetScout Manager Plus = $8,995.00

Send your comments on this article to Darrin Woods at dwoods@nwc.com.