Page 685 - Introduction to Information Optics

12.1. Background 669

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Fig. 12.1. DWDM transmission system configuration.

It took about 20 years for the data rate of TDM fiber-optic systems to
increase from 44 Mb/s in 1976 to 10 Gb/s in 1996. While the next TDM hurdle,
a 40-Gb/s commercial system, is expected to be surmounted by 2001, DWDM
systems have evolved from less than 10 channels in 1995 to more than 100
channels in 2000, delivering > 1 Tb/s capacity. Having secured its success in
transmission capacity boosting, the next challenge for WDM is to help
improve communication network architecture so that the unprecedented
bandwidth of optical fiber can be accessed easily by data traffic.
By the end of the 20th century, there were two logical communication
network infrastructures: a circuit-switched network that carries voice traffic,
and a packet-switched network that carries data traffic. Physically, both
circuit-switched and packet-switched networks run on the same telecommuni-
cation networks. The core network infrastructure is optimized for the circuit-
switched voice traffic because voice traffic came first and there was much more
voice traffic at the beginning. Voice transmission started from a single voice
circuit per copper wire pair on open-wire pairs in 1910s. Voice signals were
digitized, multiplexed, and transmitted from one central office to another
through the use of channel banks based on TDM technology. Tl, which stands
for Trunk Level 1, is a digital transmission link that has a total signaling speed
of 1.544 Mb/s consisting of 24 voice channels at 64 kb/s per channel plus
multiplexing overhead. It is the standard for digital transmission in North
America. Tl service has a variety of applications. These include the trunking
of voice and data services from the customer's premises to the central office or
long-distance point of presence (POP), Internet access, access to frame relay