Various multiplexing technologies, such as Space Division, Time Division, and Wavelength Division Multiplexing, are helping facilitate the advancement of network speeds on fiber-optic cabling. Below, we will examine each of these technologies.
Space Division Multiplexing.
Also known as parallel fibers or parallel optics, space division multiplexing is used to add one (or more-than-one) lane by simply adding one (or more-than-one) optical fiber into the composite link. By lane, we're referring to another physical fiber strand. Today in the industry we're seeing various examples of this technique being used, like where 40Gbps is being delivered over multi-mode fiber by 40G SR4, using four fibers, or lanes. That means four lanes in one direction and four lanes in the other. Incidentally, the 4 on end of SR4 denotes four lanes of 10Gbps each.
10 lanes of 10Gbps (SR10) is the norm for the 100Gbps solution. In addition, there's another generation of 100G that uses four lanes to deliver 100G, boosting the lane rate to 25Gbps. Therefore, we're seeing improvements in parallel optic techniques and time division multiplexing to realize the goal of higher speeds.
By increasing from four lanes in either direction up to 16 or 24 lanes, it is quite possible to achieve speeds of 200Gbps, 400Gbps and even more; but there're practical limits. Where possible, a four-lane solution is quite clearly more practical than a 24-lane solution. This is so, because 16 or 24 lanes drives additional cost into the cabling system, thus offering a reduced return.
Time Division Multiplexing.
Basically, time division multiplexing is a method of using smaller increments of time to transmit more data and multiplexing reduced data rate signals into a higher speed compound signal. This results in lower speed electrical signals being interwoven in time and broadcast out on a speedier composite lane. This means that the resulting data rate would be many times higher than the individual rates going in.
We see examples today where, by using these parallel electrical signals coalesced in a multiplexer and serialized over fiber, Ethernet rates have been achieved. For example, there're four-lane options for 10Gpbs Ethernet, with each lane being at a quarter rate of 2.5Gbps. Today, 25Gbps for Ethernet is the top speed per lane; and we know that 50Gbps lane rates are currently being developed for the future.
These higher rates require multifaceted and complex code schemes to push more bits through with each symbol. These figures indicate that speed limits are reaching upper limit, thus requiring different techniques to increase the composite lane speed.
Wavelength Division Multiplexing.
Wavelength division multiplexing means concurrent signalling across multiple lanes, separated by different colors, or wavelengths, of light, multiplexed both into and out of a single fiber. The transmitting wavelength band is split into segments, with each segment available to be used as a communication channel.
Many channels can be squeezed into a small spectrum. Conventional versions used for single-mode, long-haul systems are called coarse wave division multiplexing, or dense wave division multiplexing. Today, in multi-mode systems, we're already beginning to see techniques in short wavelength division multiplexing.
With short wavelength division multiplexing, in order to increase lanes within one strand of optical fiber, wavelengths are utilized in the reduced-cost short-wavelength range of approximately 850nm.
We're already seeing examples in today's market, like Cisco's 40 BD, or Bi-Di. With Bi-Di (bi-directional), signals in each optical fiber strand are transmitted in both directions, using two different wavelengths to differentiate between reflections that may occur. By using 20Gbps per wavelength in both fibers and using a duplex LC connector, they are able to achieve 40Gbps via the 2-core fiber channel.
Multiplexing technologies such as Space Division, Time Division, Wavelength Division are helping facilitate high speeds over fiber optic cabling for distributed antenna systems (DAS).
Share this post