Channel performance measures

The performance of a channel is judged by how fast, how reliably and how promptly it carries data. The main measures are:

1. Bandwidth

The range of frequencies the channel passes, in Hz (analog sense). In the digital sense it is the maximum bit rate of the link, in bps. Higher bandwidth allows a higher data rate (Nyquist: C = 2B log₂L).

2. Throughput

The actual number of bits successfully delivered per second. It is always less than or equal to the bandwidth because of delays, errors, retransmissions and protocol overhead. Example: a 10 Mbps link may give only 6 Mbps throughput.

3. Latency (delay)

The time for a message to travel from source to destination:

Latency = propagation time + transmission time + queuing time + processing time

• Propagation time = distance / propagation speed
• Transmission time = message size / bandwidth

4. Jitter

The variation in delay of successive packets. If packets arrive at uneven intervals, real-time voice and video break up. Buffers at the receiver reduce jitter.

5. Bit error rate (BER)

BER = number of erroneous bits / total bits transmitted. It depends on the signal-to-noise ratio and the modulation used. Example: 1 error in 10⁶ bits gives BER = 10⁻⁶.

6. Others

• Bandwidth-delay product = bandwidth × round-trip delay: the number of bits that can be "in flight" on the link.
• Signal-to-noise ratio (SNR): signal power / noise power; sets the Shannon capacity.