To ensure that data transfers reliably, industry standards dictate the shapes of pulses that represent bits. The standards include masks that define the tolerances of timing and amplitude of the pulses. Telecom equipment receivers must properly interpret pulses that fall within the mask limits, and transmitters must drive the signals along a medium so they arrive within tolerance. The entire pulse must fall within the mask for it to comply with a standard. Later, they use the masks in tests to verify that a product complies with industry standards, and they may use the masks in pass-fail tests of production products.
Masks define the amplitude, risetime, falltime, and jitter for pulses in telecom networks.
Some digital scopes include mask options that let you measure the pulses. The mask is typical of that for other electrical signals such as T1, T3, and DS3 that travel over coaxial and twisted-pair wires. Binary 1's may have two masks with inverse polarities. The polarity depends on the type of bit encoding and on the bit pattern used in the transmission system. This mask is typical of that for electrical signals that travel over coaxial and twisted-pair wires.