The explosive growth of the Internet and Internet Protocol (IP) applications such as voice, video, and storage area networks has boosted bandwidth demands for corporations. With LAN network speeds ranging from 10 Mbps to 10 Gbps, and quality of service (QoS) playing an important role in the delivery of this data, there must be an alternative to traditional WAN and LAN services for connecting metropolitan-area networks (MANs).
Network connections that traditionally carried T1 and T3 speeds of data now require fiber channel, Enterprise System Connection (ESCON), gigabit Ethernet, and 10 gigabit Ethernet to satisfy demand. The increased demand coupled with the advances in optical technology has dramatically increased capacity and reduced cost, making it attractive for service providers to offer fiber-based network services for the metro market.
Fiber-based metropolitan networks address business needs in three areas:
Data networking and migration—. Optical networking technologies offer many data speeds and connections that allow the support of a variety of networking technologies, such as IP, synchronous optical network (SONET), Asynchronous Transfer Mode (ATM), and time-division multiplexing (TDM). Networks can consolidate multiple wavelengths of traffic onto a single fiber to provide multiservice transport and facilitate the migration from traditional electrical networking technologies onto a common optical transport.
Disaster recovery and business continuance—. Having a backup data center with backup storage is a primary consideration for most large businesses today. Metro optical networks provide fast, campus-to-campus transport with redundancy. Real-time disaster-recovery solutions, such as synchronous mirroring, guarantee that mission-critical data is securely and remotely mirrored to avoid any data loss in the event of a disaster.
Storage consolidation—. Network attached storage (NAS), when integrated with storage area networking (SAN) applications, provides IP-based storage consolidation and file sharing. You can use multiple file storage access methods (such as Network File System [NFS] and CIFS) against the same storage farm to share unique data with multiple users and applications. Metro optical networks facilitate not only the implementation of storage, but also the extension of storage beyond a single data center.
The three primary optical technologies employed today are
SONET and synchronous digital hierarchy (SDH)
Dense wavelength division multiplexing (DWDM)
Dynamic packet transport (DPT)
All three technologies provide the conversion of electrical signals into light and back again. Fiber Optic Transmission Systems (FOTS) do the conversion. Fiber-optic signals are not susceptible to electrical interference. The signals can transmit over long distances and send more information than traditional electrical transports. The combination of these benefits provides lower costs than traditional data electrical transport mechanisms.
Service providers have offered SONET services for some time. Benefits of SONET networks include high-speed network services that meet voice-transport requirements as well as survivability and availability needs. SONET network speeds currently range from 51.84 Mbps to 9953.28 Mbps.
You can connect SONET nodes in the following ways:
Point to point
Linear—Each device connects to the device before and after it, with up to 16 devices total.
Unidirectional path-switched ring (UPSR)—All traffic is homed to a central location.
Two fiber bidirectional line-switched ring (2F BLSR)—Traffic is local to each set of neighbors, and bandwidth is reusable.
Four fiber bidirectional line-switched ring (4F BLSR)—Same as 2F BLSR except with multiple rings for diversity.
DWDM is based on the premise that optical signals of differing wavelengths do not interfere with each other. Wavelength-division multiplexing (WDM) differs from time-division multiplexing technologies (such as SONET) in the following way:
TDM employs a single wavelength across a fiber. Data is divided into channels so that multiple channels can travel across a single fiber.
WDM employs multiple wavelengths (lambdas) per fiber, which allows multiple channels per fiber (up to 160). Each lambda can include multiple TDM channels.
DWDM offers scalability over traditional TDM technologies. Because data can travel considerably farther across DWDM than traditional TDM (120 km versus 40 km), you need fewer repeaters. DWDM also allows for higher capacity across long-haul fibers as well as quick provisioning in metro networks.
Metro DWDM needs to be cheap and simple to install and manage. It must also be independent of bit rate and protocol as a transport and provide 16–32 channels per fiber. DWDM nodes attach to each other in a ring pattern using optical add-drop multiplexers, which add and drop traffic at each remote site, and all traffic is homed to a central site.
DPT uses SONET/SDH framing and employs intelligent protection switching in the event of fiber facility or node failure or signal degradation. DPT uses a bidirectional, counter-rotating ring structure for metro applications and a star structure with a central switching device for service PoP backbones.
DPT facilitates the bridging of dark fiber, WDM, and SONET networks. On a campus-ring application, DPT interconnects buildings, data centers, and WAN services. It allows the extension of real-time applications as well as multisite distributed virtual private networks (VPNs).
DPT metro loop rings allow the delivery of voice, video, and Internet connectivity to businesses and high-rise residential buildings.
At-A-Glance—Metro Optical
Companies, universities, and government organizations often have several campuses in proximity to one another. In addition to the high bandwidth requirements for application sharing and communication between campuses, networks also need to support applications such as the following:
Business resilience (disaster recovery)
Storage consolidation (centralized data)
Distributed workplace (resources throughout the network)
Fiber-optic networks can carry large amounts of multiple types of services simultaneously. They can also provide connectivity between LANs, access to WANs, and consolidation of storage area networking/network attached storage (SAN/NAS) applications.
</division><division> <title>What Are the Problems to Solve?</title>
Optical networking uses pulses of light to transmit data over fiber-optic cables. These pulses of light are subject to degradation as they travel down the fiber, but in general, the deterioration is less than that of copper, so fiber signals can travel much further.
Metro fiber optics networks have a range of 20 to 250 km. You can deploy them as point-to-point, ring, and mesh topologies. The four main protocols are Optical Ethernet, course wavelength division multiplexing (CWDM), dense wavelength division multiplexing (DWDM), and synchronous optical network (SONET).
</division><division> <title>Optical Ethernet</title>Gigabit Ethernet is the simplest and least expensive form of optical transport. Optical Ethernet uses a device called a Gigabit Interface Converter (GBIC), which plugs into a switch port and converts an Ethernet stream to an optical signal. Ethernet can leverage the growing service-provider metro Ethernet infrastructure or dark fiber.
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At-A-Glance—CWDM, DWDM, and SONET
CWDM uses wavelength-specific pairs of GBICs to combine up to eight optical signals onto a single fiber.
Each switch pair is fitted with one or more pairs of GBICs. Each GBIC pair is tuned to a specific frequency, which allows the switch to add in (multiplex) or pluck out (demultiplex) a single beam of light (data stream).
You can deploy CWDM as ring or point to point. One major drawback to CWDM is that it cannot be amplified, which limits the distance. The rule-of-thumb maximum distance is 80 km for point to point or a ring circumference of 30 km.
</division><division> <title>DWDM</title>DWDM uses the same multiplexing scheme as CWDM. DWDM signals, however, are spaced more closely together, allowing DWDM systems to multiplex up to 32 signals on a single fiber.
DWDM signals can be amplified, making the system ideal for backup data centers or larger (more geographically dispersed) campuses. With amplification, DWDM signals can transmit up to 250 km.
</division><division> <title>SONET</title>SONET is a Layer 1 technology that supports the high transmission rates (155 Mps to 10 Gps) needed in metro applications.
SONET serves as a transport for other technologies such as Ethernet and Asynchronous Transfer Mode (ATM). Service providers commonly use it for transport (metro and long haul). SONET also has extensive operation, administrative, maintenance, and provisioning (OAMP) capabilities, allowing precise fault detection and rapid (50 ms) failover.
</division>At-A-Glance—Fiber, Design, and Multiplexing
The two types of optic fiber are multimode and single-mode.
With multimode fiber, light propagates in the form of multiple wavelengths, each taking a slightly “different” path. Multimode fiber appears primarily in systems with short transmission distances (under 2 km).
In single-mode fiber, light can propagate in only one mode. Single-mode fiber usually appears in long-distance and high-bandwidth applications.
</division><division> <title>Key Design Criteria</title>
Optical signals are subject to deterioration as they travel down fibers in the form of attenuation, dispersion and nonlinearities, chromatic distortion, and polarization mode distortion. These factors limit the distance and bandwidth of optic signals:
Attenuation—. Loss of power over distance. In some cases, amplifiers can boost power.
Dispersion and nonlinearities—. Distance and speed erode signal clarity.
Chromatic distortion—. Spreading of the signal over distance. This spreading can cause signals to interfere with each other.
Polarization mode distortion—. At 10 Gb rates and higher, signals tend to broaden as they travel down the fiber, causing intersignal interference.
Multiplexing is the process of combining multiple signals over a single wire, fiber, or link. Time-division multiplexing (TDM) brings in lower-speed signals assigns them time slots, and places them into a higher-speed serial output. The receiving end reconstructs the signals.
One of the properties of light is that light waves of different wavelengths do not interfere with one another within a medium. Because of this property, each individual wavelength of light can represent a different channel of information. Combining light pulses of different wavelengths, many channels can transmit across a single fiber simultaneously. This process is wave-division multiplexing (WDM).
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