Router Overview - Optical Routers and Technologies
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In a conventional internetwork, information would be transmitted across great distances using twisted-pair copper wire, across a WAN or even a LAN. As useful and utilitarian as twisted-pair cabling and an electrical network have been, fiber-optics allow information to be transferred at immensely higher rates. In the past, when computers shared only brief conversations across the miles, electrical networks could handle the load. But now, as information is shared as it has never been shared before, there is a clear need for an upgrade in network capacities.
Comparing the bit rates in electrical networks to optical networks is like putting Woody Allen in a prison yard fist fight with Mike Tyson—there’s just no comparison. The greatest thing that optical networking has going for it is raw speed.
Common WAN links that move across electrical networks are T-1 (1.544 Mbps) and T-3 (45 Mbps). On the LAN front, things get a little better. Most organizations use 10 or 100 Mbps Ethernet. The top-of-the-line Ethernet clocks in at 10 Gbps. However, once fi-ber-optics get into the race, look out.
At their slowest, fiber-optic networks speed along much faster than a T-1 or a T-3. Once fiber shifts out of first gear, there ceases to be a comparison. When discussing optical networking speeds, you’ll hear the terminology change from T-1 or T-3 to OC. OC stands for opticalcarrier.OC takes over where T leaves off. Once the optical carrier gets involved, speeds not only reach 1 Gbps but even leave 1 Gbps in the rearview mirror. Table 4-1 shows how optical networking line speeds increase.
As you can see, the speed rates in optical networks (not to mention their development) are increasing at an amazing velocity. Thanks to dense wavelength division multiplexing (DWDM)optical bandwidth will only increase, because more than one stream of data can be introduced on a single run of fiber. More on that in a moment.
Optical Technologies There are two prevalent technologies in the world of optical routing: SONET and DWDM. SONET is the oldest and most popular technology, while DWDM is somewhat of a new kid on the block, but supports capacities much greater than SONET. Let’s examine these technologies in a little more depth.
SONET -- The most basic and popular architecture for an optical network is the Synchronous Optical Network (SONET).
SONET is a standard for optical telecommunications transport developed by the Exchange Carriers Standards Association (ECSA) for the American National Standards Institute (ANSI), the body that sets industry standards in the U.S. for telecommunications and other industries. The comprehensive SONET standard is expected to provide the transport infrastructure for worldwide telecommunications for at least the next two or three decades.
NOTE: In Europe, SONET is known by another acronym, SDH, which is short for Synchronous Digital Hierarchy.
SONET is so speedy that you could transmit an entire 650MB CD-ROM from New York to Seattle in less than one second. Not only is SONET fast, but it’s also rather versatile. Voice calls from one office to another can be multiplexed along with data and fired out across the same fiber. Further, because of the generous bandwidth SONET affords, compression and encapsulation into Internet Protocol (IP) packets is unnecessary. For comparison’s sake, a single OC-3 connection can carry more than 2,000 simultaneous voice calls. Further, all types of data can be multiplexed alongside the calls.
SONET offers a top-end bandwidth of OC-192 (9.952 Gbps) and can carry a diverse range of information. In addition to high speeds, SONET features bit-error rates of one error in 10 billion bits. Compare this with copper transmission methods that have bit-error rates of one error in 1 million bits.
DWDM -- In its beginning, SONET delivered bandwidth that was previously unimaginable. At the time, delivering OC-3 levels (155.52 Mbps) provided more bandwidth than anyone knew what to do with. Of course, those were in the mid-1980s, a decade before the Internet and high-bandwidth applications. Technology kept delivering faster and faster optical carriers. After OC-3, there were OC-12, OC-48, and beyond.
OC-48 (2.5 Gbps) is a popular speed for SONET; however, the next level, OC-192 (10 Gbps) is about the best SONET will be able to deliver. Sure, ten years ago no one knew what a gigabit was, but now we do and we can’t get enough of them. The problem is that 10 Gbps is about SONET’s limit. The solution is to jump to DWDM.
DWDM is a technique in which multiple signals can traverse a single strand of optical fiber. The lasers used in optical networking can be tuned to different wavelengths (think of them as different colors). As such, it is possible to put multiple colors on a single fiber. When the receiving router sees the various colors, it knows which colors to separate out for which data streams, as shown in Figure 4-4.
Cisco’s Optical Offerings
Cisco utilizes both SONET and DWDM with its optical routers. For example, the Cisco ONS 15808 optical router supports DWDM technology. This carrier class router supports speeds between 2.5 and 10 Gbps and is capable of transmitting up to 2,000 kilometers. The router is able to be upgraded so that 160 channels are transmitted across the fiber with speeds up to 40 Gbps.
Cisco also provides a certain level of modularity with its devices. Rather than make a few models with a predetermined number of ports set up for Gigabit Ethernet and another amount dedicated to SONET or DWDM, the company has developed cards and modules that can be plugged into a router, making it customizable. That is, you can decide to load the router with SONET modules, DWDM modules, or any combination of optical and electrical you please.
 This chapter is from Cisco: A Beginner's Guide, by Velte and Velte (McGraw-Hill/Osborne, 2004, ISBN: 0072256354). Check it out at your favorite bookstore today.
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