The Benefits of WDM & ATM

By Oded Agam, Director of Technical Services, RADCOM.
This article appeared in Telecom Business magazine, December 1998 and all rights belong to Telecom Business.

DWDM makes optimum use of facilities by allowing fiber-optic links to carry several channels simultaneously, providing transmission capabilities four to sixteen times those of traditional time division multiplexed (TDM) systems. Use of DWDM allows providers to carry IP, ATM and SONET over the optical layer. This unifying capability allows the carrier the flexibility to gradually respond to varying customer demands over one network. With DWDM, there is a need for fewer network elements and fewer facility sites, thus improving network reliability. DWDM allows bypassing of the SONET layer, which increases effective throughput and reliability and reduces cost.

Testing and troubleshooting of DWDM networks demand close attention to a number of limiting performance parameters.

If a carrier operates both ATM and SONET networks, the ATM signal does not have to be multiplexed up to the SONET rate to be carried on the DWDM network. Since the optical layer carries any type of signal without any additional multiplexing, carriers can quickly introduce ATM or IP without deploying an overlay network.

But DWDM is just the first step on the road to full optical networking. The concept of an all-optical network implies that the service provider will have optical access to traffic at various nodes in the network, much like the SONET layer. Optical wavelength add/drop (OWAD) offer the capability to add or drop wavelengths to/from a fiber, without requiring a SONET terminal. Cross-connect capability on the optical layer provides bandwidth management flexibility. OWAD offers flexibility and cost benefits to network providers, but also creates as many new problems in implementation, testing and maintenance.

Compared with repeater-based applications, a DWDM infrastructure increases the distances between network elements - reducing the initial network investments. The fiber optic amplifier in a DWDM system reduces costs by amplifying optical signals without converting them to electrical signals.

As IP grows in worldwide acceptance and becomes a de-facto standard, telecom networks will be seeing far more voice and data traffic over IP. But where will IP get its structure (e.g. protection and monitoring)? - IP over SONET, IP over ATM over SONET or IP over ATM over WDM. All of these methods introduce overhead to the transmission. The ultimate solution would be to take IP directly over WDM. This will converge to the optical network, providing scalability and cost-effectiveness. Some vendors (e.g., Ericsson) are placing protection directly into the optical layer, but until monitoring is available, ATM may be the glue between IP and the native WDM transport layer.

SONET is widely deployed; well-standardized technology and it will not vanish immediately. Existing Telcos can run SONET over WDM, but new players can simply use WDM only.

Optical attenuation is another fundamental concern in DWDM. The power of the optical signal decreases as it propagates through the fiber. Bit error ratio performance of an optical receiver is directly related to the optical power of the signal. With DWDM, optical power becomes a function of how many channels are transmitted on the fiber. More channels mean lower power per channel.

As 10 Gbps TDM becomes the standard rate for DWDM, carriers will perform wavelength-conditioning techniques in the DWDM system itself. One such technique is Forward Error Correction (FEC). FEC permits a significant increase in performance. Two basic types of FEC are utilized in networks today. The first, in-band FEC, encodes the FEC data into the unused portion of the SONET overhead. This provides some system improvement, but the space in the SONET frame is limited and thus the performance improvement is limited. The second and more powerful type of FEC is out-of-band FEC, which slightly increases the line rate when encoding the FEC data.

Another technique is the use of a "pilot light" on each channel to perform all-optical channel monitoring. By monitoring a light superimposed on each channel, network management systems can identify faults and insure connectivity and signal quality on each channel. This greatly simplifies troubleshooting the network. It is similar to the way test cells are used on specific Virtual Path/Channel in ATM.

• Cell Loss and Cell Loss Ratio (CLR): The difference between the number of transmitted and received test cells.
• Cell Sequence Integrity: Identifies out-of-sequence cells by comparing sequence numbers of received and transmitted test cells.
• Cell Error and Cell Error Ratio (CER): Received test cells with payload bit errors out of transmitted test cells.
• Cell Misinsertion and Cell Misinsertion Rate (CMR): Cells on one Virtual Circuit (VC) with payload information belonging to another VC. Testing performance includes verifying conformance to a Quality of Service agreement by measuring these parameters:
• Minimum Cell Transfer Delay (CTD): Minimum round-trip delay of test cells.
• Mean CTD: Average CTD.
• Jitter: The standard deviation of the CTD.
• Peak-2-Peak Cell Delay Variation (CDV): Difference between maximum and minimum CTD. · Peak Cell Rate (PCR): Maximum cell rate.
• Sustainable Cell Rate (SCR): Nominal cell transfer rate.
• Maximum Burst Size (MBS): Number of adjacent cells that were transmitted at the PCR.

The major new requirement in testing and monitoring of DWDM systems is the need to characterize and measure the different parameters as a function of wavelength. The fundamental measurements are:

• Signal-to-Noise Ratio: The best indicator of the overall performance of the channel.
• Channel power: Optical power in each channel. Verifies the equal distribution of power over the bandwidth of the optical amplifiers that are used.
• Channel center wavelength and spacing: Center wavelength of each channel. Detects drifts in the laser sources.
• Crosstalk: The level of undesired signal (noise plus contributions from other channels) in the passband of the tested channel.
• Total optical power: Negative effects of non-linear phenomena in the optical fiber depend on the total power carried. Care must be taken because optical power meters used today are optimized for the low power levels associated with single channel (+6dBm). WDM systems amplify several optical carriers simultaneously (+30dB). Therefore a known attenuator must be used.
• Chromatic Dispersion: Variat ion of the index of refraction of the fiber with wavelength. Needs to be controlled throughout the optical path.
• Polarization Mode Dispersion (PMD): Various polarization states of the optical signal propagate at different velocities. PMD affects the transmission quality by spreading signal pulses and raising the bit error rate (BER).

These parameters must be tested after installation because fibers can be crushed, twisted, bent or otherwise overstressed. Periodic testing is also a must because they may change with time, temperature, stress, and other environmental conditions.

About RADCOM
RADCOM is a leading network test equipment manufacturer. The company specializes in the design, manufacture, marketing and support of a line of high-quality, integrated, multi-technology test solutions for LANs, WANs and ATM. RADCOM's test and analysis equipment is used in the development and manufacturing of network equipment; the installation of networks; and the ongoing maintenance of operational networks to facilitate real-time identification, diagnosis, isolation and resolution of network problems.

RADCOM's sales network includes over 30 distributors in 35 countries worldwide and over 14 manufacturer's representatives across North America. Visit RADCOM's website at http://www.radcom.com.

About Oded Agam
Oded Agam has a BSEE degree from The Technion in Israel and a MSEE degree from the University of Tel-Aviv in Israel. Oded Agam is currently the National Accounts support for RADCOM in North America. Prior to that he was in the R&D department in RADCOM's headquarters which is located in Israel. At that position he served as an engineering team manager. Prior to that Oded Agam was with the Israeli Navy as a technical officer at various positions.

Oded Agam has broad knowledge of internetworking - ATM, Frame Relay, IP, Ethernet etc. He also has broad knowledge of testing networks; migrating to new networks and troubleshooting networks. Other expertise include hardware and software design of integrated systems.