Broadband Networks

In the current electronic networking world there are principally three transport services available to users/customers in various forms. These are

·        Circuit services

·        Plain old telephone service (POTS),

·        ISDN (BRI, PRI),

·        PDH, (T1/E1, T3/E3),

·        Synchronous digital hierarchy (SDH),

·        Datagram services

·        Internet Protocol (IP)

·        Internetwork packet exchange (IPX)

·        Virtual circuit services

·        Frame relay

·        Asynchronous transfer mode (ATM).

Figure 1 - Service layers of a broadband network with possible position of an optical network.

Networks provide services in many different ways. In small and private networks where a dedicated infrastructure can be provisioned, the network can be optimized for the specific service requirements of the users. However, when public infrastructures are needed to connect a variety of private networks or users together, it is difficult for a single networking technology to provide all the required services. Multiple service requirements are generally provided via a layered architecture such as that shown in Figure 1 (so called overlay networks). Also shown is the possible position of an optical network (ON) layer in an overall broadband network (BBN) architecture. The figure indicates that an ON could support packet service directly, through an ATM layer or an ATM-over-SONET/SDH architecture, among other choices, such as IP over SONET/SDH.

Today, the bottom layer of most BBNs consists of single-wavelength point-to-point optical fiber links, and the bandwidth available on these fibers using existing transmission technologies is being consumed by network growth.

Three optical transmission technologies have the capability to significantly increase optical link bandwidths:

·        WDM

·        Short-pulse optical time division multiplexing (OTDM).

·        Optical code division multiplexing OCDM

WDM transmission systems divide the fiber bandwidth into a large number of optical wavebands, each of which carry information at electronic rates (~ 10 Gb/s).

OTDM systems divide the fiber into a smaller number of broader wavebands (possibly only one) and use short pulses to transmit at very high rates (> 100 Gb/s) per waveband. These pulse streams are then optically demultiplexed down to slower electronic rates for switching and distribution to users.

Whilst WDM and OTDM techniques partition the available spectrum and time to different users, respectively, OCDM techniques multiplex users simultaneously andasynchronously (or synchronously) across the same spectrum and timeslot through a unique code.

WDM transmission systems are currently the most promising for BBNs because WDM technology is far more mature than OTDM and OCDM. Therefore we ignore OCDM in the sequel, since it is still at research stage.

However, OTDM enables very-high-speed optical logic which can be used to provide enhanced digital services and functions such as packet routing, signal regeneration, forward error correction, bit error rate (BER) measurements for network management, stream encryption, and so forth.

More detailed description of these technologies is presented in the next chapter. Here, we focus on optical networking and services

Figure 2 - Service layer taxonomy.