Transcript Document

Data and Computer
Chapter 1 – Data Communications,
Data Networks, and the Internet
Ninth Edition
by William Stallings
Data Communications, Data
Networks, and the Internet
“The fundamental problem of
communication is that of reproducing at
one point either exactly or approximately a
message selected at another point”
- The Mathematical Theory of
Claude Shannon
The term telecommunication means communication at a
distance. The word data refers to information presented in
whatever form is agreed upon by the parties creating and
using the data. Data communications are the exchange of
data between two devices via some form of transmission
medium such as a wire cable.
Figure 1.1 Five components of data communication
Figure 1.2 Data flow (simplex, half-duplex, and full-duplex)
A network is a set of devices (often referred to as nodes)
connected by communication links. A node can be a
computer, printer, or any other device capable of sending
and/or receiving data generated by other nodes on the
Figure 1.3 Types of connections: point-to-point and multipoint
Technological Advancement
Driving Forces
growth at
a high &
• Development of
new services
• Advances in
Changes in Networking
* Emergence of high-speed LANs
* Corporate WAN needs
* Digital electronics
Communications Model
Communications Tasks
Transmission system utilization Addressing
Signal generation
Message formatting
Exchange management
Error detection and correction
Network management
Flow control
Transmission Lines
The basic building block of
any communications facility
is the transmission line.
The business manager is
concerned with a facility
providing the
required capacity,
with acceptable reliability,
at minimum cost.
Two mediums currently driving
the evolution of data communications
transmission are:
Advances in technology have led to greatly
increased capacity and the concept of
integration, allowing equipment and
networks to work simultaneously.
Network Hardware
Classifying networks based on their scale:
 Local
Area Networks
 Metropolitan Area Networks
 Wide Area Networks
 Wireless Networks
 Home Networks
 Internetworks
Network Hardware
LANs and WANs
There are two broad categories
of networks:
Local Area Networks (LAN)
Wide Area Networks (WAN)
Wide Area Networks (WANs)
 Span
a large geographical area
 Require
 Rely
the crossing of public right-of-ways
in part on common carrier circuits
 Typically
consist of a number of
interconnected switching nodes
Wide Area Networks
Alternative technologies used include:
Circuit switching
Packet switching
Frame relay
Asynchronous Transfer Mode (ATM)
Circuit Switching
 Uses
a dedicated communications path
 Connected sequence of physical links
between nodes
 Logical channel dedicated on each link
 Rapid transmission
 The most common example of circuit
switching is the telephone network
Network Hardware
Wide Area Networks
Packet Switching
 Data
are sent out in a sequence of small
chunks called packets
 Packets are passed from node to node
along a path leading from source to
 Packet-switching networks are commonly
used for terminal-to-terminal computer and
computer-to-computer communications
Asynchronous Transfer Mode
 Referred
to as cell relay
 Culmination of circuit switching and packet
 Uses fixed-length packets called cells
 Works in range of 10’s and 100’s of Mbps
and in the Gbps range
 Data rate on each channel dynamically set
on demand
Local Area Networks (LAN)
Metropolitan Area Networks
Network Hardware
Wireless Networks
Categories of wireless networks:
 System interconnection
 Wireless LANs
 Wireless WANs
Network Hardware
Wireless Networks
 System
Bluetooth a short-range wireless network. Allows
system components together, digital cameras,
headsets, scanners, and other devices to connect to a
computer by merely being brought within range.
 Wireless
Every computer has a radio modem and antenna with
which it can communicate with other systems.
Standard for wireless LANs: IEEE 802.11, which most
systems implement and which is becoming very
Network Hardware
Wireless Networks
WANs (cont.)
Wireless LANs an operate at rates up
to about 50 Mbps over distances of
tens of meters. While cellular systems
(Wireless WANs) operate below 1
Mbps, but the distance between the
base station and the computer or
telephone is measured in kilometers
rather than in meters.
High-bandwidth wide area wireless
networks are also being developed
(IEEE 802.16).
The Channel Allocation
To allocate a single broadcast channel among
competing users, we can use:
• Static Channel Allocation in LANs and MANs
• Dynamic Channel Allocation in LANs and MANs
Static Channel Allocation in LANs
and MANs
Frequency Division Multiplexing (FDM) is an
example of static channel allocation where the
bandwidth is divided among a number of N
When there is only a small and constant number
of users, each of which has a heavy (buffered)
load of traffic (e.g., carriers' switching offices),
FDM is a simple and efficient allocation
However, when the number of senders is large
and continuously varying or the traffic is bursty,
FDM presents some problems.
Multiple Access Protocols
Carrier Sense Multiple Access Protocols
Collision-Free Protocols
Limited-Contention Protocols
Wavelength Division Multiple Access Protocols
Wireless LAN Protocols
Evolution of random-access methods
ALOHA network
Procedure for ALOHA protocol
The basic idea of an ALOHA system is simple: let users transmit whenever they
have data to be sent. There will be collisions, of course, and the colliding
frames will be damaged. If the frame was destroyed, the sender just waits a
random amount of time and sends it again.
How the channel know that there is a collision:
- Due to the feedback property of broadcasting, a sender can always find out
whether its frame was destroyed by listening to the channel, the same way
other users do. With a LAN, the feedback is immediate; with a satellite, there
is a delay of 270 msec before the sender knows if the transmission was
- If listening while transmitting is not possible for some reason,
acknowledgements are needed.
Pure ALOHA (2)
In pure ALOHA, frames are transmitted at
completely arbitrary times.
The throughput of ALOHA systems is maximized
Other protocols
Slotted ALOHA: It assumed the time is divided into discrete intervals.
The station can send at the beginning of the next time interval whenever it have data
ready after the start of the current time interval.
1- Persistent CSMA: When a station has data to send, it first listens to the channel to
see if anyone else is transmitting at that moment.
if the channel is idle, it start transmission.
If the channel is busy, the station waits until it becomes idle. When the station
detects an idle channel, it transmits a frame. If a collision occurs, the station waits a
random amount of time and starts all over again.
Nonpersistent CSMA: same as 1-persistent except that the station does not continually
sense the channel when it finds it busy, rather it waits a random period of time and
then sense the channel again. When the channel becomes idle it transmit.
p-Persistent CSMA: same as Nonpersistent CSMA but the station transmit with
probability p when the channel is idle.
Persistence strategies
CSMA/CD procedure
CSMA/CA procedure
Wireless LAN Protocols
A system of notebook computers that
communicate by radio can be regarded as a
wireless LAN
A common configuration for a wireless LAN is
an office building with base stations (also
called access points) strategically placed
around the building.
All the base stations are wired together using
copper or fiber.
A simplifying assumption that all radio
transmitters have some fixed range will be
Wireless LAN Protocols (2)
A naive approach to using a wireless LAN
might be to try CSMA: just listen for other
transmissions and only transmit if no one
else is doing so.
The trouble is, this protocol is not really
appropriate because what matters is
interference at the receiver, not at the
A wireless LAN. (a) A transmitting. (b) B transmitting.
When A is transmitting to B (previous figure part a)
If C senses the medium, it will not hear A because
A is out of range, and thus falsely conclude that it
can transmit to B.
If C does start transmitting, it will interfere at B,
wiping out the frame from A.
The problem of a station not being able to detect a
potential competitor for the medium because the
competitor is too far away is called the hidden
station problem.
When B transmitting to A (previous figure part b)
If C senses the medium, it will hear an ongoing
transmission and falsely conclude that it may not
send to D, when in fact such a transmission would
cause bad reception only in the zone between B
and C, where neither of the intended receivers is
This is called the exposed station problem.
Hidden station problem
The CTS frame in CSMA/CA handshake can
prevent collision from
a hidden station.
Use of handshaking to prevent hidden station problem
Exposed station problem
Use of handshaking in exposed station problem
The problem is that before starting a transmission, a
station really wants to know whether there is
activity around the receiver.
An early protocol designed for wireless LANs is
MACA (Multiple Access with Collision Avoidance)
(Karn, 1990).
The basic idea behind it is for the sender to
stimulate the receiver into outputting a short frame,
so stations nearby can detect this transmission and
avoid transmitting for the duration of the upcoming
(large) data frame.
Let us now consider how A sends a frame to B.
- A starts by sending an RTS (Request To Send)
frame to B. This short frame (30 bytes) contains
the length of the data frame that will eventually
- Then B replies with a CTS (Clear to Send) frame.
The CTS frame contains the data length (copied
from the RTS frame). Upon receipt of the CTS
frame, A begins transmission.
Wireless LAN Protocols (7)
The MACA protocol. (a) A sending an
RTS to B.
(b) B responding with a CTS to A.
A routing table can be either static or dynamic. A static
table is one with manual entries. A dynamic table is one
that is updated automatically when there is a change
somewhere in the Internet. A routing protocol is a
combination of rules and procedures that lets routers in
the Internet inform each other of changes.
Topics discussed in this section:
Intra- and Interdomain Routing
Distance Vector Routing and RIP
Link State Routing and OSPF
Path Vector Routing and BGP
Figure Autonomous systems
Figure 22.13 Popular routing protocols
Figure 22.14 Distance vector routing tables
Figure 22.15 Initialization of tables in distance vector routing
In distance vector routing, each node shares its routing table
with its
immediate neighbors periodically and when there is a change.
Figure Updating in distance vector routing
Figure Two-node instability
 Trends
challenging data communications:
• traffic growth
• development of new services
• advances in technology
 Transmission
• fiber optic
• wireless
 Network
 Internet
• evolved from the ARPANET
• TCP/IP foundation