Performance IssuesSupplement

CECS 474 Computer Network Interoperability

Notes for Douglas E. Comer, Computer Networks and Internets (5th Edition)

Tracy Bradley Maples, Ph.D.Computer Engineering & Computer ScienceCal ifornia State University, Long Beach

Performance Networks need high performance (or high performance per unit cost). The old computer adage, “Get it right and then make it fast,” may not apply. Networks must be designed at the outset for speed.

Bandwidth Definition 1. Bandwidth is the measure of the capacity of a transmission system. It is the range

(or band) of frequencies used on the transmission medium. Bandwidth is typically measured in Hertz.

2. Bandwidth is the maximum number of bits that can be transmitted in a certain amount of time over a particular medium. This is the data transfer rate or transmission rate of the system.

 We use definition 2 in computer networks.

Bandwidth (Cont’d)

Question: If the transmission rate is 10 million bits/sec (Mbps), how long does it take to transfer 1 bit?  

Answer: 0.1 μsec to transmit each bit

You can also think of each bit on a network as being a pulse of some width.

  The more sophisticated the transmission/receiving technology, the narrower each bit can become. Other factors (e.g., software) affect the throughput as well.

How is this calculated?

How long to transfer 5 bits?

What if the transmission rate

is 4 Gbps?

Bandwidth vs. Throughput vs. Effective Throughput

Recall…

Defn: Bandwidth is the maximum number of bits that can be transmitted in a given amount of time over a particular medium. This is the data transfer rate or transmission rate of the system. Usually, described in bits/sec (or bps).

Consider…

Defn: Network throughput (or effective throughput) is the measured number of bits that can be transmitted over a particular medium in a given amount of time. Usually, described in bits/sec (or bps).

The throughput is the maximum number of bits/sec an application can expect to receive.

Bandwidth >= Effective Throughput For applications, we can describe throughput as the “bandwidth requirements of an application.”

Latency Defn: Latency (or delay or end-to-end delay) is the amount of time is takes for a single bit to propagate from one end of a network to another. Latency is measured in terms of time. Defn: Round Trip Time (RTT) is the time it takes for a bit to travel from sender to receiver and back again.  There are three components that form the latency:

1. Propagation delay

2. Transmission Time

3. Queueing & Processing Delays

Latency (cont’d)

1. Propagation delay

We calculate this using the speed-of-light propagation delay:

• in a vacuum, 3.0 * 108 meters/sec• in a cable, 2.3 *108 meters/sec• in fiber, 2.0 * 108 meters/sec

This value is a function of the distances and the speed-of-light delay.

2. Transmission Time

This is the amount of time it takes to transmit the data onto the transmission media. This value is a function of the bandwidth and the packet size.

3. Queueing & Processing Delay 

This is the time the data spends in being processed and waiting for its turn (queueing) to be transmitted. This value is almost impossible to calculate.

Latency (cont’d)  

Latency = Propagation Delay + Transmit Time + Queueing & Processing Delay= Tp+ Tx+ Tq

 Tp (Propagation Delay) = (Distance across link)/(Speed-of-light delay) Tx (Transmit Time) = (Size of date )/(Throughput)

Tq (Queueing & Processing Delay) = This is hard to measure so a statistically generated value or a constant is used. where 

Distance = length of the wire over which the data will travel (usually meters/sec) Speed-of-light = effective speed of light over the channel Size = size of the packet (usually bits) Throughput = #bits/(unit time) at which the packet is transmitted (usually bits/sec

Latency (cont’d)

Latency is limited by physics. In particular, it is limited by the speed of light.

 Example: How long does it take for a bit to propagate across the continental US?

3000 mile propagation delay in fiber (approximate width of the United States)

= 24 ms latency

“You cannae change the laws of physics.”

-- Mr. Scott, Star Trek

How is this calculated?

1. Circuit-switched networks provide service by setting up a total path of connected links from the origin to the destination host. A control message is first sent to setup a path from the origin to the destination. (A return signal informs the origin that data transmission may proceed.) Once data transmission starts, all channels in the path are used simultaneously, and the entire path remains allocated to the transmission (whether or not it is in use).

2. Packet-switchednetworks decompose messages into small pieces called packets.

These packets are each numbered and make their way through the net in a store-and-forward fashion.

Links are considered busy only when they are currently transmitting packets

Recall: Two types of Switched Networks

Circuit & Packet Switching Performance Issues

Header overhead (i.e., the amount of "extra" information that must be sent along with the data to ensure proper transmission)

For large amounts of data:

circuit switching <= packet switching

Transmission delay (i.e., the amount of time it takes data from the time it enters the network until it arrives at its' destination) 

For short and bursty messages:packet switching has the lowest delay.

For long, continuous streams of data:circuit switching has the lowest delay.

Time

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Time

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Circuit Switching Network Time Diagram

Time

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Time

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Packet Switching Network Time Diagram