9.4
FIBER-OPTIC COMMUNICATION LINKS
231
9.3.8.3
Access Techniques
. Inbound refers to traffic from VSAT(s) to hub, and
outbound refers to traffic from hub to VSAT(s). The outbound link is commonly a time-
division multiplex (TDM) serial bit stream, often 56 kbps, and some high-capacity systems
reach 1.544 Mbps or 2.048 Mbps. The inbound links can take on any one of a number
of flavors, typically 9600 bps.
More frequently, VSAT systems support interactive data transactions, which are very
short in duration. Thus, we can expect bursty operation from a remote VSAT terminal. One
application is to deliver, in near real time, point-of-sale (POS) information, forwarding
it to headquarters where the VSAT hub is located. Efficiency of bandwidth use is not a
primary motivating factor in system design. Thus for the interactive VSAT data network
environment, low delay, simplicity of implementation, and robust operation are generally
of importance than the bandwidth efficiency achieved.
Message access on any shared system can be of three types: (1) fixed assigned,
(2) contention (random access), or (3) reservation (controlled access). There are hybrid
schemes between contention and reservation.
In the fixed assigned multiple access, VSAT protocols are SCPC/FDMA, CDMA (a
spread spectrum technique), and TDMA.
17
All three are comparatively inefficient in the
bursty environment with hundreds of thousands of potential users.
9.4
FIBER-OPTIC COMMUNICATION LINKS
9.4.1
Applications
Fiber optics as a transmission medium has a comparatively unlimited bandwidth. It has
excellent attenuation properties, as low as 0.25 dB/km. A major advantage fiber has when
compared with coaxial cable is that no equalization is necessary. Also, repeater separation
is on the order of 10100 times that of coaxial cable for equal transmission bandwidths.
Other advantages are:
z
Electromagnetic immunity
z
Ground loop elimination
z
Security
z
Small size and lightweight
z
Expansion capabilities requiring change out of electronics only, in most cases
z
No licensing required
Fiber has analog transmission application, particularly for video/TV. However, for this
discussion we will be considering only digital applications, principally as a PCM highway
or "bearer."
Fiber-optic transmission is used for links under 1 ft in length all the way up to and
including transoceanic undersea cable. In fact, all transoceanic cables currently being
installed and planned for the future are based on fiber optics.
Fiber-optic technology was developed by physicists and, following the convention of
optics, wavelength rather than frequency is used to denote the position of light emission
in the electromagnetic spectrum. The fiber optics of today uses three wavelength bands:
(1) around 800 nm (nanometers), (2) 1300 nm, and (3) 1600 nm or near-visible infrared.
This is illustrated in Figure 9.22.
17
SCPC stands for single channel per carrier.