Chapter 2
The Physical Layer
1
Outline & Objectives
1. Transmission media
2. Data switching
3. Data multiplexing
2
Transmission Media
1. Twisted pair
2. Coaxial cable
3. Fiber optics
4. Relevant parameters
3
Twisted Pair
Category 3 UTP.
•
•
•
•
•
•
Category 5 UTP.
It is the oldest and still most common transmission medium.
A pair consists of two insulated copper wires (about 1 mm thick each). Its
most common application is the telephone system.
Twisted pairs can run several km without amplification, but for longer
distances, repeaters are needed.
It can transmit either analog or digital information.
The bandwidth depends on the thickness of the wire and the distance
traveled. A few Mbps can be achieved for a few km.
Main advantages: adequate performance and low cost.
4
Coaxial Cable
•
Coaxial cable has better shielding and greater bandwidth than twisted
pairs, so it can span longer distances at higher speed.
•
It can be used for both analog and digital transmission, and is widely
used for LANs and cable TV.
5
Fiber Optics
•
Optical fibers transmit data via light, rather than pulses of electricity like using
copper wires. The result is a very reliable and super fast connection that
has 26,000 times more transmission capacity than twisted-pair cables.
•
Fiber optics are used for long-distance transmission in network backbones,
high-speed LANs, and high-speed Internet access.
•
An optical transmission system has three key components: the light source, the
transmission medium, and the detector. Conventionally, a pulse of light
indicates a 1 bit and the absence of light indicates a 0 bit.
•
Optical fibers are made of glass, which, in turn, is made from sand, a material
available in unlimited amounts.
•
Due to the low attenuation, repeaters are needed only about every 50 km on
long lines, versus about every 5 km for copper. It is thin and lightweight.
6
Relevant parameters
1. Propagation speed:
The speed signals travel over wires
2. Propagation delay:
Distance / propagation speed
3. Transmission time:
Data size / bit rate
4. Packet
delivery time: Transmission time + propagation delay
5. Processing
delay: The packet processing time at receiver side
6. Round trip time:
2 * packet delivery time + processing time
7
Data Switching
1. Circuit switching and packet switching
2. Comparisons
8
Circuit Switching and Packet Switching
(a)
Circuit
Switching:
(b)
Packet
Switching.
Circuit switching: Each switching office has a number of incoming & outgoing lines. Before any data
can be sent, an end-to-end path with physical connections needs to be set up. Once the connection
is set up, the only delay is propagation time and there is no danger of congestion.
Packet switching: Packets are sent once available – no need to set up a dedicated path in advance.
Routers use store-and-forward transmission to send each packet to the destination. No fixed path
– different packets can follow different paths and arrive out of order.
9
Comparisons
12
Data Multiplexing
1. Classification
2. Illustration
3. Code division multiple access
11
Data Multiplexing: Classification
Physical channels are very valuable, so it is worthwhile to multiplex
many logical channels over a single physical channel. There are
three typical multiplexing schemes:
1.
2.
3.
FDM (Frequency Division Multiplexing): the frequency spectrum is divided
among the logical channels, with each user having exclusive possession of
his frequency band. FDM requires analog circuitry.
TDM (Time Division Multiplexing): the users take turns, each one
periodically getting the entire bandwidth for a little burst of time. TDM can
be handled entirely by digital electronics, so it has become far more
widespread and is mostly used in interoffice trunks for digital data.
CDM (Code Division Multiplexing): narrowband signal spreads out over a
wider frequency band, allowing multiple signals from different users to
share the same frequency band. It is commonly called CDMA (Code Division
Multiple Access).
12
Data Multiplexing: Illustration
10
Code Division Multiple Access (1)

Each bit time is subdivided into m short intervals called chips. Typically, there are
64 or 128 chips per bit.

Each station is assigned a unique m-bit code called a chip sequence
To transmit a 1 bit, a station sends its chip sequence. To transmit a 0 bit, a station
sends the negation of its chip sequence.


All chip sequences are pair-wise orthogonal, i.e. the normalized inner product of
any two distinct chip sequences is 0, S • T = 0.
-1-1-1-1+1
+1-1-1+1
+1+1
+1+1
+1-1-1
* * * * * * * *
-1-1-1-1-1-1+1
+1+1
+1-1-1+1
+1+1
+1
Binary chip sequences


+1
+1+1
+1-1-1-1-1+1
+1-1-1+1
+1-1-1
--------------------------------------------------------88
00
Normalized inner product
If S • T = 0, then S • ¬T = 0.
S • S = 1, and S • ¬S = -1.
14
Code Division Multiple Access (2)
A
B
-1 -1 -1 +1 +1 -1 +1 +1
-1 -1 -1 +1 +1 -1 +1 +1
-1-1-1-1+1
+1-1-1+1
+1+1
+1+1
+1-1-1
C
-1 +1 -1 +1 +1 +1 -1 -1
-1 +1 -1 +1 +1 +1 -1 -1
D
-1 +1 -1 -1 -1 -1 +1 -1
-1 +1 -1 -1 -1 -1 +1 -1
Bit time A B C D
1
1
S1=(-1 +1 -1 +1 +1 +1 -1 -1)
2
1 1
S2=(-2 0 0 0 +2 +2 0 -2)
3
1 0
S3=( 0 0 -2 +2 0 -2 0 +2)
4
1 0 1
S4=(-1 +1 -3 +3 +1 -1 -1 +1)
5
1 1 1 1 S5=(-4 0 -2 0 +2 0 +2 -2)
6
1 1 0 1 S6=(-2 -2 0 -2 0 -2 +4 0)
What did C transmit?
S1 • C = (1+1+1+1+1+1+1+1)/8 = 1
S2 • C = (2+0+0+0+2+2+0+2)/8 = 1
S3 • C = (0+0+2+2+0 -2+0 -2)/8 = 0
S4 • C = (1+1+3+3+1 -1+1 -1)/8 = 1
S5 • C = (4+0+2+0+2+0 -2+2)/8 = 1
S6 • C = (2 -2+0 -2+0 -2 -4+0)/8 = -1
Received Signals
Why it works?
1
1
1
1
0
S6 · C
= (A + B + ¬C + D) · C
= A · C + B · C + ¬C · C + D · C
= 0 + 0 -1 + 0
= -1
15
Summary
1. What are the widely used network transmission
media, and what are their pro & con?
2. What are key parameters in data transmission?
3. What are the two typical data switching schemes,
and what are their differences?
4. What are the three typical data multiplexing
schemes, and what are differences among them?
16