Network-Layer Performance Factors
Network-Layer Performance Factors
The performance of a packet-switched network is mainly measured using three factors:
- Delay
- Throughput
- Packet Loss
These factors determine how quickly and efficiently data can be transferred from a source to a destination.
1. Delay
Delay is the time taken by a packet to travel from the source to the destination.
A packet does not reach the destination instantaneously. As it travels through links and routers, it experiences different types of delays.
The four types of delay are:
- Transmission delay
- Propagation delay
- Processing delay
- Queuing delay
1.1 Transmission Delay
Transmission delay is the time required to place all the bits of a packet onto the transmission link.
It depends on:
- Packet length
- Transmission rate of the link
Formula
where:
- Packet length is in bits
- Transmission rate is in bits/second
Example
Suppose:
- Packet size = 10,000 bits
- Transmission rate = 100 Mbps
Then:
100×10^6 =100 μs
Important observation
A larger packet produces a larger transmission delay.
A higher transmission rate produces a smaller transmission delay.
2. Propagation Delay
Propagation delay is the time required for a bit to travel through the physical transmission medium from one point to another.
It depends mainly on:
- Distance between the two points
- Propagation speed of the medium
Formula
Example
Suppose:
- Distance = 2000 m
- Propagation speed = m/s
Then:
2000/ 2×10^8=10 μs
Important observation
Propagation delay depends on the physical distance and propagation speed, not on the packet size.
3. Processing Delay
Processing delay is the time required by a router or destination host to process a packet.
At a router, processing includes activities such as:
- Receiving the packet from the input port
- Removing/checking the header
- Performing error detection
- Determining where the packet should go
- Delivering it to the appropriate output port
At the destination, processing involves delivering the packet to the appropriate upper-layer protocol.
Therefore:
Processing delay = time required to process a packet at a router or destination host.
Processing delay can vary from packet to packet, but it is normally considered using an average value.
4. Queuing Delay
A router cannot always transmit a packet immediately.
Packets waiting to be processed or transmitted are stored in queues.
Queuing delay is the amount of time a packet waits in the input and output queues of a router.
Incoming packets ↓ Input Queue ↓ Processing ↓ Output Queue ↓ Transmission
A packet may experience:
- Input queuing delay – waiting in the input queue.
- Output queuing delay – waiting in the output queue.
Important point
Queuing delay depends heavily on the network traffic/load.
When network traffic is low:
Queuing delay is usually small.
When network traffic is high:
Queues become longer and queuing delay increases.
If a queue becomes full, packets may be discarded, resulting in packet loss.
5. Total Delay
A packet traveling from source to destination may pass through several routers.
If there are n routers, there are n + 1 links.
Assuming equal delays at the different components, the textbook gives:
where:
- Dtr = transmission delay
- Dpg = propagation delay
- Dpr = processing delay
- Dqu = queuing delay
- n = number of routers
Why are there different terms?
With n routers:
- There are n + 1 transmission delays
- There are n + 1 propagation delays
- There are n + 1 processing delays
- There are n queuing delays
Therefore:
Source → R1 → R2 → ... → Rn → Destination
contains n + 1 links, but only n routers where queuing delay occurs.
6. Throughput
Throughput is the number of bits that pass through a particular point in the network in one second.
It represents the actual data transmission rate achieved by the communication path.
Example
Suppose a path consists of three links:
| Link | Transmission Rate |
|---|---|
| Link 1 | 200 Kbps |
| Link 2 | 100 Kbps |
| Link 3 | 150 Kbps |
The overall throughput is:
Throughput=min(200,100,150)=100 Kbps
The 100 Kbps link is the bottleneck.
7. Bottleneck
The bottleneck is the link with the lowest transmission rate along a path.
Think of the network as a series of pipes:
Source │ │ 200 Kbps ↓ Link 1 │ │ 100 Kbps ← Bottleneck ↓ Link 2 │ │ 150 Kbps ↓ Destination
Even though some links can carry data faster, the slowest link limits the overall throughput.
For a path containing n links:
Throughput=min{TR1,TR2,…,TRn}
8. Throughput in the Internet
In the Internet, communication generally involves:
Source Access Network ↓ Internet Backbone ↓ Destination Access Network
The Internet backbone normally has a very high transmission rate.
Therefore, the throughput is often determined by the slower of the two access networks.
Example
Suppose:
- Server connection = 100 Mbps
- User connection = 40 Kbps
- Backbone = very high speed
Then:
So:
Throughput=40 Kbps
The user's connection is the bottleneck.
9. Effect of Shared Links
A link between two routers may be shared by several data flows.
For example:
Source 1 ──┐ Source 2 ──┼──→ Shared Link ──→ Destinations Source 3 ──┘
If the transmission rate of the shared link is 600 Kbps and the link is shared among three flows, the effective rate available to an individual flow may be much lower.
The textbook gives the example where a main link's effective transmission rate for the throughput calculation is 200 Kbps because the link is shared among three paths.
Thus, when calculating throughput, we must consider sharing of links by multiple flows.
10. Packet Loss
Packet loss occurs when a packet is discarded before reaching its destination.
One major reason for packet loss is buffer overflow at routers.
A router uses buffers to temporarily store packets waiting for processing or transmission.
Packets ↓ Input Buffer ┌───────────────┐ │ P1 P2 P3 P4 P5│ └───────────────┘ ↓ Processing
The buffer has a limited capacity.
If packets arrive faster than the router can process or transmit them:
Arrival rate > Processing/Transmission rate
the queue grows.
Eventually:
Buffer becomes full ↓ New packet arrives ↓ Packet is discarded ↓ Packet Loss
11. Effect of Packet Loss
Packet loss can significantly affect network performance.
If a packet is lost, it may need to be retransmitted by an upper-layer protocol such as TCP.
This can produce additional traffic:
Packet sent ↓ Packet lost ↓ Retransmission ↓ Additional traffic ↓ More congestion ↓ Possible additional packet loss
Thus, packet loss can contribute to a cycle of congestion and further packet loss.
The textbook notes that queuing theory is used to study and help prevent queue overflow and packet loss.
12. Summary of Network Performance Factors
| Factor | Meaning | Mainly depends on |
|---|---|---|
| Transmission delay | Time to put packet bits onto the link | Packet size and transmission rate |
| Propagation delay | Time for bits to travel through the medium | Distance and propagation speed |
| Processing delay | Time to process a packet | Router/host processing |
| Queuing delay | Time packet waits in queues | Network traffic/load |
| Throughput | Actual data rate achieved | Bottleneck link and shared capacity |
| Packet loss | Packets discarded during transmission | Buffer capacity and traffic load |
A useful way to remember
Network performance = Delay + Throughput + Packet Loss
And delay itself consists of:
Transmission + Propagation + Processing + Queuing
While throughput is generally limited by:
The bottleneck (minimum transmission rate) along the path.
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