Page 285 - Optical Communications Essentials

Optical Link Design

Optical Link Design 275

TABLE 16.5. Spreadsheet for Calculating a LAN Link Rise-Time Budget
Component Rise time Rise-time budget
Allowed rise-time budget t sys 0.7/B NRZ 7.0ns
LED transmitter rise time 2.0ns
Pulse spread from modal dispersion 180ps
Chromatic dispersion in fiber 240ps
Receiver rise time 3.5ns
System rise time [Eq. (16.5)] 4.0ns

Since the Gigabit Ethernet signal uses an NRZ format, the rise time needs to be less
than 0.7/(1000Mbps) 0.70ns. Here the rise-time criterion is just satisfied, so that
the maximum link length is 220m for Gigabit Ethernet operating at 850nm on 62.5-µm
fibers. The maximum length for Gigabit Ethernet running on 50-µm fibers is 550m
due to the higher bandwidth of these fibers.

16.3.3. SONET link rise time
The following components are used in the 2.5-Gbps SONET link in Sec. 16.2.3:

■ A laser transmitter with a 0.1-ns rise time and a 1.0-nm spectral width
■ An APD receiver with a front-end bandwidth B RX 2500MHz
■ A G.655 single-mode fiber with D CD 4ps/(nm km) and D PMD 0.1 ps/ k m
at 1310nm

Then t CD 0.12ns, t PMD 0.001ns, and t RX 0.14ns, so that the total rise
time is
2
2
2
2
t sys (t TX t CD t PMD t RX ) 1/2
2
2
2
2 1/2
[(0.10) (0.12) (0.001) (0.14) ] 0.21ns
Since the SONET signal uses an NRZ format, the rise time needs to be less than
0.7/(2500Mbps) 0.28ns, so there is enough rise-time margin in this case.
16.4. Line Coding
In designing a communication link, an important consideration is the format of
the transmitted digital signal. This is significant because the receiver must be
able to extract precise timing information from the incoming signal. The three
main purposes of timing are

■ To allow the signal to be sampled by the receiver at the time the signal-to-
noise ratio is a maximum

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