This document discusses various time-related concepts and units used in computer networking. It defines units like milliseconds, microseconds, nanoseconds, and others. It also defines concepts like asynchronous vs synchronous transmission, round-trip time, jitter, bandwidth, propagation delay, queuing delay, and more. Finally, it discusses measuring time on computers and comparing time scales between processors, networks, and interactive devices.

1. Nomenclature with
some digressions
CS 3873
Fall 2015
Richard Murphy

2. Units and scales
 Be familiar with the units & scales involved
 Time
 Milliseconds : 10-3 (or ms)
 Microseconds : 10-6 (or μs)
 Nanoseconds : 10-9 (or ns)
 Units
 MB = megabytes (unit of storage)
 Mb = megabits (unit of transmission)
 Time is the key to operation & measurement

3. Time nomenclature
 Asynchronous
 No time relationship between two end-points
 Synchronous
 Clocks at each end-point are synchronized
 Isosynchronous
 Synchronization is achieved by start and stop bits in
the data sent between end-points
 Self-clocking
 Example: Manchester encoding – clock embedded
with the data

4. Time nomenclature
 RTT or Round-trip Time
 The time to send a datagram from one entity to another and then
back (the two one-way trip times are not necessary equal)
 Bit time – see bandwidth (also called “wire time”)
 Jitter
 The variation in periodicity of packet or bit arrival (usually due to
network congestion or routing changes)
 NTP
 Network Time Protocol – A protocol to synchronize a computer
clock to a external time source (see RFCs 778, 1119, & 1305)

5. Bandwidth
 Stated as megabits per second or mb/s or mb
sec-1
 The bit time is 1/bandwidth
 Bandwidth measures capacity not speed
 Bytes are the unit of storage, not bandwidth
and are usually written with B, while bits are
written with b

6. Delays
 Propagation delay
 Function of distance and the type of media
 Queuing delay
 Mostly a function of traffic through the forwarding
device, queuing discipline, & available memory
 Processing delay
 Complicated due to routing/switching architecture,
hardware/software processing, packet inspection,
filtering, and so on

7. Delays
 Transmission delay
 A function of bandwidth (sometimes called the
wire time)
 The time it takes to put a unit of data completely
onto the transmission media ( 1
𝑏𝑎𝑛𝑑𝑤𝑖𝑑𝑡ℎ
𝑥 𝑏𝑖𝑡𝑠 𝑝𝑒𝑟 𝑢𝑛𝑖𝑡 )
 Latency
 End-to-end total delay

8. Time revisited
 Measuring time on computers
 Time intervals (how long does it take)
 Absolute time (time stamps)
 Granularity
 Drift
 Jiffies – time between two successive clock ticks
on a computer (LINUX)
 See LINUX man page on LEARN/Blackboard

9. About time and other things
 Variance of times for a repeated operation is
many times more important than the average
time (we will see this with the TCP RTT
estimator)
 Time measurements can be synchronous to
some external source or relative to the entity
making the measurements
 It is important to remember all of these terms
and how to use them

10. Computer vs Network Time Scales
 Distinct Time Scales
 Processor
 Network
 Interactive devices
 What this means
 Network is slow from
the CPU viewpoint
 Network times may
or may not affect the
user experience
1 ns 1 μs 1 ms 1 s
Time Scale ~1 GHz CPU
Transmission delay
Prop delay SwRI-
UT Austin
Integer add
Screen refresh
Log scale

11. Events on a human timescale

12. More definitions
 Utilization
 Channel utilization is the percentage of capacity
used to transfer actual data (derived from rate of
sending, overhead bits, acknowledgement
datagrams)
 Throughput
 Rate of successful message delivery over a
channel (retransmissions reduce throughput)

13. And a few concerning
statistics
 Inter-arrival time
 Time between successive arrivals of messages at
a network device; usually these times are drawn
from some distribution function (example:
Poisson)
 Heavy-tail – example: Pareto distribution
 Self-similar
 Statistics look the same over different time or
spatial scales

14. Pareto Density Functions

15. Message nomenclature I like to
use
 Physical layer: bits/symbols
 Link layer: frame
 Network layer: datagram
 Transport layer: segment
 Application layer: message
 Layer independent: protocol data unit (PDU)