Fiber Optic Essentials

130 Fiber Optic Essentials

6.1 WDM Design Considerations

There are many important characteristics to consider when designing a

DWDM system. One of the most obvious design points is the largest

total number of channels (largest total amount of data) supported over the

multiplexed fiber optic network. Typically one wavelength is required

to support a data stream; duplex data streams may require two different

wavelengths in each direction or may use the same wavelength for bi-

directional transmission. As we will discuss shortly, additional channel

capacity may be added to the network using a combination of WDM and

other features, including time division multiplexing (TDM) and wave-

length reuse. Wavelength reuse refers to the product’s ability to reuse

the same wavelength channel for communication between multiple loca-

tions; this increases the number of channels in the network. For example,

consider a ring with three different locations A, B, and C. Without wave-

length reuse, the network would require one wavelength to communicate

between sites A and C, and another wavelength to communicate between

sites B and C, or two wavelength channels total. With wavelength reuse,

the first and second sites may communicate over one wavelength, then the

second and third sites may reuse the same wavelength to communicate

rather than requiring a new wavelength. Thus, the second wavelength is

now available to carry other traffic in the network. Wavelength reuse is

desirable because it allows the product to increase the number of physical

locations or data channels supported on a ring without increasing the

number of wavelengths required; a tradeoff is that systems with wave-

length reuse cannot offer protection switching on the reused channels.

TDM is another way in which some WDM products increase the number

of channels on the network. Multiple data streams share a common fiber

path by dividing it into time slots, which are then interleaved onto the

fiber. TDM acts as a front-end for WDM by combining several low data

rate channels into a higher data rate channel; since the higher data rate

channel only requires a single wavelength of the WDM, this method

provides for increased numbers of low speed channels. As an example,

if the maximum data rate on a WDM channel is 1 Gbit/s then it should

be possible to TDM up to four channels over this wavelength, each with

a bit rate of 200 Mbit/s, and still have some margin for channel over-

head and other features. This is also sometimes known as wavefill, or

sub-rate multiplexing (SRM). The TDM function may be offered as part

of a separate product, such as a data switch, that interoperates with the

wavelength multiplexer; preferably, it would be integrated into the WDM

6. Wavelength Multiplexing 131

Table 6.2 Common SONET networking hierarchy

Signal Bit rate Voice slots

DS0 64 kbps 1 DS0

DS1 1.544 Mbps 24 DS0

DS2 6.312 Mbps 96 DS0

DS3 44.736 Mbps 28 DS0

design. Some products only support TDM for selected telecommunica-

tions protocols such as SONET; in fact, this telecom protocol is designed

to function in a TDM-only network, and can easily be concatenated at

successively faster data rates. The basic data rates and notation associated

with SONET networks are shown in Table 6.2.

Note that the original unit used in multiplexing voice traffic is 64 kbps,

which represents one phone call. Within North America, 24 of these

units are TDM multiplexed into a signal with an aggregate speed of

1.544 Mbit/s for transmission over T1 lines. Outside of North America, 32

signals are TDM multiplexed into a 2.048 Mbit/s signal for transmission

over E1 lines.

Optical SONET/SDH

carrier signal Bit rate Capacity

OC-1 STS-1 51.84 Mbps 28 DS1s or 1 DS3

OC-3 STS-3/STM-1 155.54 Mbps 84 DS1 or 3 DS3

OC-12 STS-12/STM-4 622.08 Mbps 336 DS1 or 12 DS3

OC-48 STS-48/STM-16 2488.32 Mbps 1344 DS1 or 48 DS3

OC-192 STS-192/STM-64 9953.28 Mbps 5379 DS1 or 192 DS3

Using virtual tributaries, 28 DS1 signals can be mapped into the STS-1

payload, or multiplexed to DS3 with an M13 multiplexer and fit directly

into STS-1.

However, since WDM technology can be made protocol independent,

it is desirable for the TDM to also be bit rate and protocol independent,

or at least be able to accommodate other than SONET-based protocols.

This is sometimes referred to as being “frequency agile.” Note that while

a pure TDM network requires that the maximum bit rate continue to

increase in order to support more traffic, a WDM network does not

require the individual channel bit rates to increase. Depending on the