Optical Networking

Various optical link technologies are used in large enterprise infrastructures to assure some kind of connectivity. Some of the optical networking techniques include SONET/SDH, CWDM, DWDM, and RPR. We cover many networking technologies in our  CompTIA Network+ certification course.

Synchronous Optical Network and Synchronous Digital Hierarchy

Synchronous Optical Network (SONET) and Synchronous Digital Hierarchy (SDH) are usually considered similar, although there are some key differences between the two technologies. SONET is the North American high-speed transport standard for digital-based bandwidth and it is an ANSI standard. It provides incrementally increased rates that can be used on digital optical links. On the other hand, SDH is European and it is an ITU standard.

SONET and SDH are popular connectivity solutions that ISPs can offer based on customer needs. SONET/SDH uses Time Division Multiplexing (TDM) as the technique for framing the data and voice on a single wavelink through the optical fiber. Single digital streams are multiplexed over the optical fiber using lasers or LEDs. SONET typically uses fiber rings, although it is not exclusively based on a ring topology, and it allows transmission distances of 80 km, without having to use repeaters. Single-mode fiber can also obtain distances up to 80 km without a repeater.

Much of the long-haul fiber connections are actually SONET because it can use repeater technology across a number of ISP networks to boost the signals over long distances. Although not all of the SONET topologies will be ring-based, customers who need a reliable link should agree on a strict SLA with the ISP to ensure high reliability through a ring topology.

Some common Optical Carrier (OC) rates and the mapped bandwidth for each standard are shown below. Various OC standards correspond to various SONET and SDH standards. Synchronous Transport Signal (STS) is a SONET standard and Synchronous Transport Module (STM) is an SDH standard.

OC Standard	SONET Standard	SDH Standard	Capacity
OC-1	STS-1	STM-0	50 Mbps
OC-3	STS-3	STM-1	150 Mbps
OC-12	STS-12	STM-4	600 Mbps
OC-24	STS-24	–	1.2 Gbps
OC-48	STS-48	STM-16	2.4 Gbps
OC-192	STS-192	STM-64	9.6 Gbps
OC-768	STS-768	STM-256	38 Gbps
OC-3072	STS-3072	STM-1024	153 Gbps

Some of the considerations that must be taken into account when new SONET connections are being purchased include the following:

• Details about the transport usage (whether the link will be used for data or voice transport)
• Details about the topology (linear or ring-based)
• Details about single points of failure in the transport
• Customer needs
• Costs
• Implementation scenarios (e.g., multiple providers, multiple paths, etc.)
• The type of oversubscription offered by the ISP

Also useful to know is whether the bandwidth will be dedicated or shared with other users. If the services are from two ISPs, to achieve high availability and redundancy they may have different SONET implementations and may follow different paths.

ISPs usually have the same physical fiber paths (i.e., along gas pipes or public electrical paths). Even if dual ISPs are used, the physical fiber path is often the same and the failure risk does not decrease. If something happens to the pipes that have the fiber links attached, all of the ISPs that follow that specific path will suffer. The recommended scenario is having two ISPs with different physical cabling paths.

Coarse Wave Division Multiplexing and Dense Wave Division Multiplexing

Coarse Wave Division Multiplexing (CWDM) and Dense Wave Division Multiplexing (DWDM) are two different types of Wavelength Division Multiplexing (WDM). Both of these use a multiplexor (MUX) at the transmitter to put several optical signals on the fiber. A de-multiplexer (DEMUX) is installed at the receiver and will achieve the inverse operation. This concept is similar to a modem (modulator-demodulator).

CWDM transmits up to 16 channels, with each channel operating in a different wavelength. CWDM boosts the bandwidth of the existing GigabitEthernet optical infrastructure, without having to add new fibre-optic strands. CWDM has a wider spacing between the channels compared to the DWDM technology, so it is a much cheaper technology for transmitting multiple gigabit signals on a single fibre-optic strand. There is a great amount of support for this equipment from Cisco, which offers many Small Form-factor Pluggable (SFP) transceivers that can be used with CWDM links.

CWDM is often used by organizations on leased dark fiber topologies to boost the capacity from 1 to 8 or 16 Gbps over metropolitan area distances. The downside to CWDM is that it is not compatible with modern fiber amplifier technologies, such as Erbium Doped Fiber Amplifier (EDFA). EDFA is a method for amplifying light signals and it is making repeaters obsolete. CWDM is also used in cable television implementations.

DWDM is a core technology for optical transport networks that is similar to CWDM in many ways. However, with DWDM, the wavelengths are much tighter, so there are up to 160 channels as opposed to 16 channels with CWDM. This makes the transceivers and equipment much more expensive. Even though there are 160 channels, Cisco DWDM cards can support 32 different wavelengths. In addition, DWDM is compatible with EDFA, so it can reach longer distances when using this technology, which better supports Metropolitan Area Networks (MANs) and WAN applications. Using EDFA with DWDM technology can achieve distances of up to 120 km between amplifiers. DWDM is a high-speed Enterprise WAN and MAN connectivity service.

Figure 1 – DWDM Topology

 

Figure 1 above shows a sample topology of a DWDM optical network that connects three locations. This type of solution typically includes three components:

• Transponders: They receive the optical signal from a client, convert it into the electrical domain, and retransmit it using a laser.
• Multiplexers: They take the various signals and put them into a single-mode fiber. The multiplexer may support EDFA technology.
• Amplifiers: They provide powered amplification of the multi-wavelength optical signal.

Resilient Packet Ring

Resilient Packet Ring (RPR) is a Layer 2 transport architecture that offers packet transmission based on a dual-counter rotating ring topology.

RPR was standardized in 2004 under IEEE 802.17 and is a Layer 2 packet-based transport mechanism that can be offered by various ISPs. It is based on counter-rotating ring structures and using dual fibre-optic rings allows for a much higher level of packet survivability and availability. If one of the stations fails or the fiber is damaged, then data is transmitted over the other ring. RPR is a Layer 2 technology, so it can be used with either SONET/SDH or GigabitEthernet/10-GigabitEthernet at the Physical Layer. This supports Enterprise Network solutions, MANs, and transmission over long distances.

RPR is a proper solution for MANs with large organizations. A typical RPR topology contains the following components:

• The IP backbone network
• RPR supporting routers
• RPR ring, which is at the MAN Core and Distribution Layers

RPR can provide connectivity to campuses with different buildings or even different locations, where there are different business offices across multiple floors. RPR can also support VoIP gateways and provide data, multicasting, and video services.

RPR is a better solution than SONET or SDH when it comes to data traffic. SONET and SDH are capable of handling voice traffic because this type of traffic is consistent in terms of usage. However, data traffic is more bursty, especially when transferring large files, and RPR works better with this type of traffic behavior. RPR also works well in point-to-multipoint or multipoint-to-multipoint environments. It is good at leveraging QoS technologies to help various types of traffic.

RPR is excellent at providing oversubscription because of its bandwidth guarantees and it functions much like Ethernet in that it uses statistic multiplexing as opposed to TDM. RPR provides another valid solution in addition to some of the other mechanisms that operate over SONET and SDH.

Read the Cisco notes on optical networking.