Lecture 2: Delays

What are the different forms of delays?

Processing, queing, transmission, propogation.

Propogation Delay is?

The time it takes for a bit to propogate from one end of a link to the other.
In this course assume speed s = 3×10⁸ m/s (speed of light). Link length is d.
p_prop = d/s

Transmission Delay is?

The time required to push all bits of a packet onto the physical link. It depends purely on the packet length (L) and the link rate (R):
d_trans = L/R

Queuing Delay is?

The time a packet has to wait to be transmitted onto a link.

A packet can be transmitted on a link only if there is no other packet currently being transmitted on that link. The queuing delay depends on the number and size of earlier-arriving packets that are queued and waiting for transmission onto the link.

Processing Delay is?

The time required to examine the packet’s header, determine where to direct the packet, check for bit-level errors, and more.

Is in the order of microseconds (or less) in modern routers.

In this course d_proc → 0

Store-and-Forward transmission is?

Transmission where routers must receive a packet in its entirety before transmitting its first but onto the outbound link.

Traffic Intensity (I) is parameterized how?

I = L * a / R

L: packet length
a: arrival rate (packets/second)
R: link bandwith (bits/second)

Queuing Behavior depending on I

I ≈ 0: very small or negligible queuing delay
I → 1: delay increases exponentially
I > 1: Arriving rate exceeds transmission capacity; the queue grows indefinitely, resulting in buffer overflow and packet loss.

Ping measures?

Round-trip-time (RTT) ≈ 2*(d_proc + d_queue + d_prop)

Link capacity is?

The maximum achievable data rate R [bit/s] of a communication link.

Throughput is?

The rate of successful data delivery over a communication channel [bit/s].
Instantaneous: rate at given point in time.
Average: rate over range of time.

Goodput is?

The rate of data passed to the application layer [bit/s] (i.e., application-level throughput)

Jitter is?

variation in the end-to-end delay across consecutive packets. High jitter degrades real-time applications like VoIP or video calls.