Computer networks course

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)