A baseband LAN is a point-to-point

A baseband LAN is a point-to-point or point-to-multipoint network. Two transmission
problems arise as a result. The first deals with signal level and signal-to-noise (S/N)
ratio, and the second deals with standing waves. Each access on a common medium
must have sufficient signal level and S/N such that copied signals have a BER in the
11.5 OVERVIEW OF ANSI/IEEE LAN PROTOCOLS 295
range of 1×10
−8
to 1×10
−12
. If the medium is fairly long in extension and there are
many accesses, the signal level must be high for a transmitting access to reach its most
distant destination. The medium is lossy, particularly at the higher bit rates, and each
access tap has an insertion loss. This leads to very high signal levels. These may be rich
in harmonics and spurious emissions, degrading bit error rate. On the other hand, with
insufficient level, the S/N ratio degrades, which will also degrade error performance. A
good level balance must be achieved for all users. Every multipoint connectivity must be
examined. The number of multipoint connectivities can be expressed byn(n−1),where
nis the number of accesses. If, on a particular LAN, 100 accesses are planned, there are
9900 possible connectivities to be analyzed to carry out signal level balance. One way to
simplify the job is to segment the network, placing a regenerative repeater (or bridge) at
each boundary. This reduces the signal balance job to realizable proportions and ensures
that a clean signal of proper level is available at each access tap. For baseband LANs,
50-
coaxial cable is favored over the more common 75-
cable. The lower-impedance
cable is less prone to signal reflections from access taps and provides better protection
against low-frequency interference.