The idea
The speed printed on a link and the speed you actually get are rarely the same number. QoS needs measurements, and the lecture’s first two are about how much data really gets through, then how much of that is actually useful.
How it works
The four metrics
The lecture lists four QoS metrics: throughput and goodput, packet loss, delay and jitter. Delay and jitter are defined in QoS and flow characteristics, and this page covers throughput, goodput and loss.
How it works
Throughput versus bandwidth
The terms bandwidth and throughput are not the same.
- Bandwidth is the maximum rate at which data can be moved over a given link.
- Throughput, sometimes called effective bandwidth, is the actual speed over a link or circuit.
Throughput is less than the potential bandwidth. The lecture’s example is a
link with a bandwidth of 1 Mbps whose throughput may be 200 kbps if the
devices connected to the link can only handle 200 kbps.
Worked example
The lecture's throughput example
Bandwidth: the link can carry at most
1 Mbps=1000 kbps.Bottleneck: the connected devices handle only
200 kbps.Throughput: the actual speed is
200 kbps.Ratio:
200 / 1000=0.2, so only one fifth of the capacity is used.
AnswerThroughput is 200 kbps, one fifth of the bandwidth.
How it works
Goodput
Goodput is the application-level throughput. It is the number of useful, correct bits per unit of time from a source to a destination, excluding protocol overhead such as headers and excluding retransmitted data packets.
Goodput is less than throughput because of:
- protocol overhead
- transport-layer flow control and congestion avoidance
- retransmission of lost or corrupt packets
| Bandwidth | Throughput | Goodput | |
|---|---|---|---|
| What it measures | Maximum rate a link can move data | Actual speed over the link | Useful correct bits per unit time at application level |
| Relation | Largest | Less than bandwidth | Less than throughput |
| Counts headers and retransmissions | Not applicable | Yes | No |
How it works
Packet loss
Packet loss means some packets fail to reach the destination. The lecture lists these causes:
- signal degradation
- channel congestion
- corrupted packets rejected in transit
- faulty networking hardware or routing configurations
Protocols such as TCP recover lost packets. The lecture also says packet loss does not always indicate a problem, because some loss is acceptable.
Where marks get lost
Throughput is not the sender's rate
A common wrong answer to the 1 Mbps / 200 kbps question is 1 Mbps, because the link is rated at that. Throughput is measured at what actually passes, so the slowest part sets it.
In the exam
- Bandwidth is maximum, throughput is actual (effective bandwidth).
- Goodput needs three words: application level, useful, excluding overhead and retransmissions.
- Reasons goodput is below throughput: protocol overhead, flow and congestion control, retransmission.
- Loss causes: signal degradation, congestion, corrupted packets rejected, faulty hardware or routing. TCP is the named recovery protocol.
Check yourself
- Metrics are throughput and goodput, packet loss, delay and jitter.
- Throughput is the actual speed. The lecture’s example is 200 kbps on a 1 Mbps link.
- Goodput is throughput minus overhead and retransmissions.
- Loss has several causes and a small amount is acceptable.