Transcript Lec 01

Data Communications
Chapter 1 – Data Communications,
Data Networks, and the Internet
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 Communication,
Claude Shannon
A Communications Model
Key Communications Tasks
Transmission system utilization Addressing
Interfacing
Routing
Signal generation
Recovery
Synchronization
Message formatting
Exchange management
Security
Error detection and correction
Network management
Flow control
Data Communications Model
Transmission Medium
 selection
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internal use entirely up to business
long-distance links made by carrier
 rapid
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is a basic choice
technology advances affects choice
fiber optic – high capacity, getting cheaper
Wireless - mobility
 transmission
cost is still high
 hence interest in efficiency improvements
– multiplexing and compression
Networking
 growth
of number & power of computers is
driving need for interconnection
 also seeing rapid integration of voice,
data, image & video technologies
 two broad categories of communications
networks:
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Local Area Network (LAN)
Wide Area Network (WAN)
Wide Area Networks
 span
a large geographical area
 rely in part on common carrier circuits
 alternative WAN technologies used
include:
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circuit switching
packet switching
frame relay
Asynchronous Transfer Mode (ATM)
Circuit Switching
 uses
a dedicated communications path
established for duration of communication
 comprising a sequence of physical links
 with a dedicated logical channel
 eg. telephone network
Packet Switching
 data
sent out in sequence
 small chunks (packets) of data at a time
 packets passed from node to node
between source and destination
 used for terminal to computer and
computer to computer communications
Frame Relay
 packet
switching systems have large
overheads to compensate for errors
 modern systems are more reliable
 errors can be caught in end system
 Frame Relay provides higher speeds
 with most error control overhead removed
Asynchronous Transfer Mode
 ATM
- evolution of frame relay
 fixed packet (called cell) length
 with little overhead for error control
 anything from 10Mbps to Gbps
 constant data rate using packet switching
technique with multiple virtual circuits
Local Area Networks
 smaller

scope
Building or small campus
 usually
owned by same organization as
the attached devices
 data rates much higher within
 switched LANs, eg Ethernet
 wireless LANs
Metropolitan Area Networks
 MAN
 middle
ground between LAN and WAN
 private or public network
 high speed
 large area – city or metro
Types of connections: point-to-point and multipoint
1.15
Categories of topology
1.16
A fully connected mesh topology (five devices)
1.17
A star topology connecting four stations
1.18
A bus topology connecting three stations
1.19
A ring topology connecting six stations
1.20
A hybrid topology: a star backbone with three bus networks
1.21
The Internet
 Internet
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evolved from ARPANET
first operational packet network
applied to tactical radio & satellite nets also
had a need for interoperability
led to standardized TCP/IP protocols
Internet Elements
Internet Elements
 The
purpose of the Internet, of course, is
to interconnect end systems, called hosts;
these include PCs, workstations, servers,
mainframes, and so on. Most hosts that
use the Internet are connected to a
network, such as a local area network
or a wide area network (WAN).
 These
networks are in turn connected by
routers. Each router attaches to two or
more networks. Some hosts, such as
mainframes or servers, connect directly to
a router rather than through a network.
 the
Internet operates as follows. A host
may send data to another host anywhere
on the Internet. The source host breaks
the data to be sent into a sequence of
packets, called IP datagrams or IP
packets.
 Each packet includes a unique numeric
address of the destination host. This
address is referred to as an IP address,
because the address is carried in an IP
packet.
 Based
on this destination address, each
packet travels through a series of routers
and networks from source to destination.
Each router, as it receives a packet,
makes a routing decision and forwards the
packet along its way to the destination.
Internet Architecture
 The
Internet today is made up of
thousands of overlapping hierarchical
networks.
 Individual hosts and LANs are connected
to an Internet service provider (ISP)
through a point of presence (POP).
 The
connection is made in a series of
 steps starting with the customer
premises equipment (CPE). The CPE is
the communications equipment located
onsite with the host.
 the CPE was traditionally a 56-kbps
modem.
Summary
 introduced
data communications needs
 communications model
 defined data communications
 overview of networks
 introduce Internet