Category Archives: DWDM Systems

DWDM Solutions for Arista 7500E Series Switches

Nowadays, the deployment of DWDM solution has been hotly debated in many enterprise networks, especially in the new Lay2 and Lay3 equipment like Arista 7500E series switches. For many enterprises, DWDM network solutions are undoubtedly the best choices of action, because they can provide a scalable and elastic solution for the enterprise that offered high bandwidth and data separation. This article will demonstrate DWDM solutions to Arista 7500E switches which are the foundation of two-tier open networking solutions for cloud data centers.

Analysis of DWDM System

DWDM (Dense Wavelength Division Multiplexing) is a technology allowing high throughput capacity over longer distances commonly ranging between 44-88 channels and transferring data rates from 100 Mbps up to 100 Gbps per wavelength. For intra-datacenter solutions, an endpoint connection often uses multimode (850 nm) for short ranges and single mode (1310 nm) for longer ranges. The DWDM node converts this local connection to a channelized frequency or wavelength, which is then multiplexed with other wavelength and transmitted over a single fiber connection.

A key advantage of DWDM is that it’s bitrate independent. DWDM-based networks can transmit data in IP, ATM, SONET, SDH and Ethernet. Therefore, DWDM systems can carry different types of traffic at different speeds over an optical channel. Voice transmission, email, video and multimedia data are just some examples of services which can be simultaneously transmitted in DWDM systems.

DWDM multi-channel Mux/Demux

Arista 7500E 100G DWDM Line Card

With full support for Layer2 and Layer3 protocols, Arista 7500E series switch is the ideal option for the network spine for two tier data centers applications. Arista 7500E especially provides the perfect resolution for high bandwidth Metro and long-haul DCI solutions with the 6-port DWDM line card. It has great advantage to migrate from existing 10G DWDM to 100G coherent line side modules. The 7500E series DWDM line card provides six 100G ports with coherent 100G tunable optics, which enables customers to connect directly into existing WDM MUX module without the need to add transceivers, which can save cost and space to a large extent. The coherent optics use C-band region wavelengths and offer a cost efficient solution for up to 96 channels of 100Gb over a single dark fiber pair.

Use Cases for Arista 7500E DWDM Card
    • Less Than 80 km Dark Fiber Connection
      For distance less than 80 km, Arista 7500E switch with DWDM line cards can directly terminate a dark fiber connection with a pair of passive DWDM Mux, thus achieving a point-to-point connection between two locations.

Dark Fiber Connection

  • Between 80 km and 150 km Connection
    For distance greater than 80 km but less than 150 km, losses occurred during the process of transmission should be considered. In order to boost the power level, an EDFA (Erbium Doped Fiber Amplifier) is used to gain flatness, noise level, and output power, which is typically capable of gains of 30 dB or more and output power of +17 dB or more. With the use of EDFA, the signal can be boosted into a certain power level, thus achieving distances of up to 150 km.

The Arista 7500E series DWDM solution offers a cost-effective solution for transporting scalable and massive volumes of traffic, and enhances the 7500E system by providing high performance 100G DWDM port density with the same rich features and dedicated secure encryption in compact and power-efficient systems. Enterprises can easily migrate existing metro and long-haul DWDM networks to add new 100G capacities, thus expanding Layer2 and Layer3 services.

Differences between CWDM System and DWDM System

As more bandwidth and faster data transmission rates over long distances are needed, users have to rely on more fiber optics. But laying more fibers will cost much. So WDM (Wave Division Multiplexing) technology appeared and solved this problem.

WDM is a technology which transmits multiple optical signals on a single fiber by using different wavelengths, or colors, of laser light to carry the different signals. By using bidirectional communications over a single fiber, network users can realize a multiplication effect in their available fiber’s capacity. The effect of expanding the capacity of the network without laying more fibers makes WDM systems so popular. The concept was first published in 1978. At the very beginning, only two signals could be combined by using WDM systems. But now more signals could be handled and transmission rates get faster.

Currently WDM systems are divided into two different types: CWDM (coarse wavelength division multiplexing) and DWDM (dense wavelength division multiplexing). CWDM systems typically have 8 channels with 8 wavelengths separated by 20 nm from 1470 nm to 1610 nm. But the channels could also be increased to 16. Typical DWDM systems use 40 channels and provide wavelengths up to 96 with 0.4nm spacing. The channel spacing decides the call of “coarse” and “dense”. There are many differences between these two systems.


Transmission Distance

CWDM systems can’t transmit data over distances as long as DWDM system because the wavelengths are not amplified. Usually CWDM can travel somewhere about 100 miles. Therefore CWDM is limited in its functionality over longer distances. While DWDM systems are set for long haul transmission by keeping the wavelengths tightly packed. They can transmit more data over longer distances with less interference than a CWDM system. So it’s suitable for data transmission over long distances.

Power Requirements

The power requirements for DWDM are significantly higher. For example, DWDM lasers are temperature-stabilized with Peltier coolers integrated into their module package. The cooler along with associated monitor and control circuitry consumes around 4 W per wavelength. But CWDM uses uncooled laser transmitter and it consumes about 0.5 W of power.


Two main factors like operating and hardware cost cause difference between CWDM and DWDM. The first example has been referred before. CWDM modulation laser is uncooled, but DWDM laser is cooling. Cooling laser using temperature tuning, uncooled laser adopts electronic tuning. The range of temperature distribution is non-uniform in a very wide wavelength, so the temperature tuning is very difficult to realize, which results in high cost. CWDM doesn’t have this problem. Second, for instance, DWDM transceivers are typically four or five times more expensive than CWDM counterparts. That’s mainly because of the lasers. Typical wavelength tolerances for DWDM lasers die are on the order of ±0.1 nm; whereas tolerances for CWDM lasers die are ±2-3 nm. Lower die yields increase the costs of DWDM lasers relative to CWDM lasers. From these two sides, CWDM system is cheaper than DWDM system.

Each kind of WDM system has its advantages and disadvantages. CWDM is very flexible and is good for campus LAN expansion. DWDM has large capacity and is ideal for long distance data transmission. CWDM and DWDM technology continue be to improved. The two have different functions and will complement not replace the other.