Delay, Loss & Throughput Flashcards
1
Q
Delay Overview
A
- Delay is a measure of the time taken for a packet to travel across the network
- Measured in fractions of a second
- Usually milliseconds – i.e., thousandths of a second
2
Q
Types of Delay
A
- There are four main types of delay:
- Processing Delay
- Queuing Delay
- Transmission Delay
- Propagation Delay
3
Q
Processing Delay
A
- Time taken for a device to examine a packet’s header and decide where to direct the packet
- May include a check of bit-level errors (as caused during
transmission) - Typically, this delay is very small
- Usually microseconds – i.e., millionths of a second
- Can vary depending on how busy the device is
4
Q
Queuing Delay
A
- Once a packet has been processed, it will join a queue
– It waits here to leave the device
– It will not be sent until it reaches the head of the queue - Queuing Delay is the time spent waiting in the queue before a packet is transmitted
- The length of the queue, and thus the delay, is dependent on the congestion level of the node/router
- N.B. A queue only develops if the packet arrival rate to link (temporarily) exceeds output link capacity
5
Q
Transmission Delay
A
- Packets are transmitted once they reach the head of the queue…
- Networks are Store-and-Forward
- An entire packet must be received before it’s forwarded
- Transmission Delay is the amount of time required to push (transmit) all of the packet’s bits onto the link
- Transmission Delay = L/R
- L: packet length (bits)
- R: link bandwidth (bps)
6
Q
Propagation Delay
A
- Once a bit has been pushed onto a link, it needs to propagate to the next device
- Each link has an associated Propagation Delay
- Measured as the time needed to get over the link from one end to another
- Propagation delay is dependent on the physical type of the link*
- for wireless communications – a little less than the speed of light
- Propagation Delay = d/s
- d: length of physical link
- s: link’s propagation speed (~2x108 m/sec)
7
Q
Transmission vs Propagation Delay
A
- A subtle difference, but important!
- Transmission Delay is the time required to push a packet out
– It’s a function of the packet’s length and the transmission rate of the link interface
– Nothing to do with the distance between the two devices - Propagation Delay is the time taken for a bit to propagate from one device to the next
– Function of the link technology and the distance between two devices
– Nothing to do with the packet’s length or the transmission rate of the link interface
8
Q
Convoy Analogy
A
- Car ~ bit; convoy ~ packet
- Cars “propagate” at 100 km/hr
- Toll booth takes 12 sec to service
a car (bit transmission time) - Q: How long until convoy is lined
up before 2nd toll booth? - Time to “push” entire convoy through toll
booth onto highway = 12*10 = 120 sec - Time for last car to propagate from 1st to
2nd toll both: 100km/(100km/hr) = 1 hr - A: 62 minutes
9
Q
- Suppose cars now “propagate” faster - at 1000 km/hr
- And that toll booths now take one min to service a car
- Q: Will cars arrive at the 2nd booth before all cars have been serviced at the1st?
A
A: Yes! after 7 min, first car arrives at the 2nd booth; three cars are still at the 1st booth
– First car spends 1 minute at toll booth, and then 1/10 of an hour = 6 mins propagating through the link until arriving at 2nd booth
– At this point, car 8 is still at 1st booth (with 2 cars waiting behind)
10
Q
Nodal Delay
A
- Total of all previously-mentioned types of delay
- Measured per node (i.e., per each device in a network)
11
Q
End-to-End Delay and RTT
A
- End-to-End Delay is an often-used measurement that is the total of all nodal delays, from one host to another
- May include numerous nodes
- Varies over time as the various component sources of delay increase and decrease
- Round Trip Time (RTT): End-to-End Delay measured in both directions
- From one to host to another, and then back again
- Doesn’t necessarily have to use the same path/route!
12
Q
Measuring Delay
Traceroute
A
- What does “real” Internet delay look like?
- The traceroute program measures delay from the source to each router on the path to a destination
- Records RTT (there and back)
- For each router, the source:
– Sends three packets (probes) to a router that lies on the path to the destination
– Routers return packets to the sender
– Sender measures lag between transmission and reply
13
Q
Throughput Overview
A
- Throughput is the rate (bits/timeunit) at which bits are transferred from a sender to a receiver
– Instantaneous throughput: rate at given point in time
– Average throughput: rate over a period of time
– Peak throughput: highest instantaneous throughput rate seen so far - Throughput is often restricted by a single-point bottleneck
- Some protocols can “throttle” themselves, and reduce their own rate
– This avoids stressing bottleneck, but at the cost of lower rates
14
Q
bottleneck link
A
link on the end-end path that constrains end-end throughput
15
Q
Throughput (Internet Scenario)
A
- Per-connection end-end throughput: min(Rc,Rs,R/10)
- In practice: Rc or Rs is often the bottleneck
19
Q
Queuing Delay and Packet Loss
A
- Packet Loss has a close relationship with Queuing Delay
- When packets arrive at a rate greater than the maximum supported throughput, the router’s queue starts to fill up
20
Q
Queuing Delay and Packet Loss 2
A
- When a queue becomes full, additional packets cannot be received
- As a result, they are discarded and lost
- Lost packets may be retransmitted by the previous router, or by the source host, or not at all
