Data Communications and

NetworksNetworksChapter 1: Performance

Pham Quang Thai pqthai.hcmut@gmail.com

1

Bandwidth

ThroughputBit error

rate

Performance

Latency (Delay)

Bandwidth-Delay

Product

Bandwidth and throughput

In networking, we use the term bandwidth in

two contexts.

Bandwidth (Hz) : the range of frequencies in a

composite signal or the range of frequencies that composite signal or the range of frequencies that

a channel can pass.

Bandwidth (bps): the speed of bit transmission in

a channel or link.

Throughput: the actual speed of bit

transmission in a channel or link

Bandwidth and throughput

Throughput

Bandwidth

Example

An average of 12,000 frames per minute with

each frame carrying an average of 10,000 bits

is send through a channel with 10 Mbps

bandwidth. What is the throughput of this bandwidth. What is the throughput of this

network?

Latency (delay)

Propagation time: time for the signal to travel

from one point to another

Transmission time: time for the whole

message to be sentmessage to be sent

Total latency (delay): time for the whole

message to be sent and travel through the

medium

Example

The bandwidth of the network is 100 Mbps. The distance between the sender and the receiver is 12000 km. Signal travels at 2.4 108 m/s in copper.

1. Compare the propagation time and the transmission time for a 2.5-kbyte message (an e-mail)

2. Compare the propagation time and the transmission time for a 500-Mbyte message (a video)

The bandwidth-delay product defines

the number of bits that can fill the link

Bit error rate: the percentage of bits that

have errors relative to the total number of

bits received

The Q function

mSNR

=

Example

1. A 2-level signal is transmitted through channel with Gaussian

distributed noise. The means of lower level (bit 0) and

higher level (bit 1) are E and E, respectively. The noises

standard deviation =N0E/2. The probabilities of bit 0 and 1 are equal. Find the BER.1 are equal. Find the BER.

2. The signal in the following figure is sent through a channel

with Gaussian distributed noise. The noises standard

deviation =N0E/2. Find the BER.

BER, eyediagram and error vector magnitude

Pb: BER, L: no. of levels, Eb/N0: SNR, M: no. of M-arys levels