Transcript COA2011PKP-6 - coapkp-ukm

Computer Organization
and Architecture
Lecture 6: Networks and Data
Communications
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Data Communications
A simple view
 Data - messages to be shared between sender and receiver
 Communications channel that can capably and reliably transport
messages
 Protocols establish accurate and appropriate meaning to the messages
that are understood by both senders and receivers
 Physical connection that is independent of the messaging
◦ message sharing “connection” between applications at the sender
and the receiver
◦ physical connection with signaling that represents the messages being
transported
 Examples
◦ POTS - plain old telephone service
◦ Web servers and Web browsers
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HTTP Request and Response
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Model of a Communication Channel
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Messages
Communication between cooperating applications at each
end node
 Can take many forms such as data, a program, a file, or
multimedia
 Represented digitally
 Data is described as a byte stream because
communications are predominantly serial
 Limitation as a communication tool is the varying message
length
◦ Long messages could tie up a communication channel
indefinitely creating problems for other messages that
share that channel
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Packets
A group of related packets make up a single message
 Consist of data encapsulated by the packet header
which contains information about the packet
 Used to solve problems of channel availability and
maximum utilization
 Equivalent to an envelope that contains pages of data
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Packet Header
Also known as the preamble
 Contains
◦ Description of the packet
◦ Destination address of receiver
◦ Source address of sender
◦ Information about the data being sent
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Advantages of Packets
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Simplifies operations and increases communications
efficiency
Reasonable unit for routing of data
Alternative to dedicating a channel for the entire length of
the message
Packets from several sources can share a single channel
Each sender/receiver pair appears to have a channel to itself
Receiving computer can process an entire block of data
instead of a character or byte at a time
Simplifies synchronization of the sending and receiving
systems by providing clear start and stop points
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Channel Characteristics (1)
Communication channel
◦ The path for the message between two communicating
nodes
◦ May include intermediate nodes that forward packets to the
next node
◦ Interfaces at each end of the connection may be different
 Links
◦ A segment of a communication channel
 Bandwidth
◦ Bit rate of overall channel
 Medium
◦ Guided – communications limited to a specific path
◦ Unguided – communications not limited to a specific path
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A Multi-Link Channel
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Channel Characteristics (2)
Data transmission directionality
◦ Simplex – messages are carried only in one direction
◦ Half-duplex – messages are carried in both directions but
only one direction at a time
◦ Full duplex – messages are simultaneously carried in both
directions
 Number of connections
◦ Point-to-point
◦ Multipoint
 Digital vs. Analog
 End node interfaces
◦ Wired or wireless Ethernet
◦ Bluetooth, WiMax, DSL or cable link, modem, etc.
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Packet Routing
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Circuit switching
◦ Dedicated channel between source and destination for
duration of connection
Virtual circuit
◦ A channel path that is used to send packets between
two end nodes
◦ Intermediate nodes may be shared with other channel
paths
Packet switching (datagram switching)
◦ Each packet is routed from node to node independently
based on various criteria
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Packet Routing
End-to-end channel with many
possible paths through intermediate
nodes
Connecting End
Points through
Links and
Networks
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Virtual Circuits in a Network
Packet Routing
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Routers
◦ Specialized devices used to interconnect network and
pass packets from one network to another
◦ Operation (see following slide)
 When packet arrives at input port
 Processor decides where packet is to be directed
 A switch is set to direct the packet to the correct
output port
Gateways
◦ Same as routers but connect dissimilar networks
together
◦ Convert packet headers for the dissimilar networks
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Router Block Diagram
NIC: network
interface controller
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Network Overview
Communication Models
 TCP/IP
 OSI
 Addressing
 Network Topology
 Types of Networks
 Local Area Networks
 Backbone Networks
 Metropolitan Area Networks
 Wide Area Networks
 Internet Backbones and the Internet
 Piconets
 Standards
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Communication Model
Implemented as a hierarchical protocol stack
 Each layer of the stack at the sender node contributes
information that is used by the corresponding peer layer at the
receiver node
 Different protocols for the different aspects of communication
 Separating tasks and including well defined interfaces between the
tasks
◦ Adds flexibility
◦ Simplifies design of protocols
◦ Permits modification or substitution of protocols without
affecting unrelated tasks
◦ Permits a system to select only the protocols needed for a
particular application
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TCP/IP
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Transmission Control Protocol/Internet Protocol
Based on five protocol layers, although layers 1 and 2 are not actually
specified in the standard. However, the TCP/IP model recognizes the
existence of these layers as a necessity.
The TCP/IP protocol suite encompasses an integrated suite of
numerous protocols that work together and guide all aspects of
communication.
Layer 5
Layer 4
Layer 3
Layer 2
Layer 1
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Operation of TCP/IP Model
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Application Layer (Layer 5)
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Layer where message is created
Includes any application that provides software that can
communicate with the network layer
Sockets
◦ Originated with BSD UNIX
◦ Provide the interface between the application layer
and transport layer
◦ Used by applications to initiate connections and to
send messages through the network
◦ A means for adding new protocols and keeping the
network facilities current in their offerings
◦ Example: SCSI over IP
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SCSI over IP
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Transport Layer (Layer 4)
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Provides services that support reliable end-to-end
communications
Generates the final address of the destination
Responsible for all end-to-end communication facilities
Packetization of the message, breaking up of the message
into packets of reasonable size takes place at this level
Three different protocols
◦ TCP
◦ UDP
◦ SCTP
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Transport Layer Protocols
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TCP (Transmission Control Protocol)
◦ Reliable delivery service
◦ Sending and receiving TCP each create a socket
◦ Control packets are used to create a full duplex connection between
the sockets
◦ A single TCP service can create multiple connections that operate
simultaneously by creating additional sockets as needed
◦ Routing is the responsibility of the network layer (layer 3)
UDP (User Datagram Protocol)
◦ Unreliable, connectionless service
◦ No acknowledgment of receipt by receiving node
◦ Example: streaming video
SCTP (Stream Control Transmission Protocol)
◦ Similar to TCP but with improved fault tolerance and ability to
transport multiple messages through the same connection
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Logical Connection View of TCP
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Network Layer (Layer 3)
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The TCP/IP network layer is also called the internetworking
layer or the IP layer
Responsible for the addressing and routing of packets to
their proper and final destination
Unreliable, connectionless, packet switching service
Does not guarantee delivery nor check for errors
Routers and gateways are sometimes referred to as level 3
switches to indicate the level at which routing takes place
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Network Layer (cont.)
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Communications within a local network:
◦ No routing is required because nodes are directly
addressable
◦ Physical addresses for corresponding IP addresses are
looked up in a table
◦ IP appends a header with the physical address and
passes the datagram to the data link layer (layer 2)
Communications sent outside of the local network
◦ At each intermediate node, the network layer
removes the current node address and determines
the next node address
◦ The new address is added to the packet and passed
to the data link layer (layer 2)
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Data Link Layer (Layer 2)
Responsible for the reliable transmission and delivery of
packets between two adjacent nodes
 Packets at this layer are called frames
 Often divided into the following two sublayers:
◦ Software logical link control sublayer
 Error correction, flow control, retransmission, packet
reconstruction and IP datagram/frame conversions
 Numbers frames and reorders received frames to
recreate the original message
 Rarely used
◦ Hardware medium-access control sublayer
 Defines procedures for access the channel and detecting
errors
 Responsible for services such as data encoding, collision
handling, synchronization, and multiplexing
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Physical Layer (Layer 1)
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Layer at which communication actually takes place
consisting of a bare stream of bits
Primarily implemented in hardware by a network
interface controller (NIC)
Physical access protocol includes
◦ Definition of the medium
◦ Signaling method, signal parameters, carrier
frequencies, lengths of pulses, synchronization and
timing issues
◦ Method used to physically connect the computer to
the medium
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Passing a Message Through an Intermediate
Node
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OSI Model
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Open Systems Interconnection Reference Model was
created by the International Standards Organization (ISO)
Although a conceptually important model, OSI is not
widely accepted or used for actual communication
Contains seven layers instead of five
The application layer in the TCP/IP model is essentially
represented by three layers in the OSI model
◦ Application layer
◦ Presentation layer
◦ Session layer
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Comparison of OSI and TCP/IP
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OSI Presentation Layer
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Responsible for presenting data at the destination with
the same meaning and appearance as it would have at
the source
Provides common data conversions and transformations
that allow systems with different standards to
communicate
Includes services such as data compression and
restoration, encryption and decryption, data
reformatting, ASCII-Unicode conversion, etc.
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OSI Session Layer
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Establishes a dialogue between two cooperating applications
or processes at the ends of the communication link
Responsible for
◦ Establishing the session between the applications
◦ Controlling the dialogue
◦ Terminating the session
Examples
◦ Remote login
◦ Print spooling to remote printer
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TCP/IP Addressing (1)
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User friendly addresses
◦ URL – www.youtube.com
◦ Email – [email protected]
◦ Printer name on the network
Domain name
◦ Standard global domain name system provides global
scope for user friendly addresses
◦ Hierarchical system for name creation and registration
◦ Tools for locating and identifying specific names
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TCP/IP Addressing (2)
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Port Addresses (port numbers)
◦ Transport layer uses to identify the application that is
to receive the message
◦ 16 bits in length
◦ Example: port 80 is commonly used for Web services
◦ First 1024 numbers are called well-known ports
because they are standard addresses specified for
most common applications
◦ User defined port numbers are also available to
applications
◦ For example, the following Web service uses the userdefined port of 8080
http://www.somewhere.org:8080/hiddenServer/index.
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Well-Known Port Addresses
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TCP/IP Addressing (3)
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IP addresses
◦ Logical addresses
◦ IPv4
 32-bit addresses arranged as 4 octets, delimited by dots
 Each octet is written as a decimal number between 0 and 255
 Example: 208.80.152.2 (Wikipedia’s IP address)
◦ IPv6
 Intended to eventually supplant IPv4 to provide additional IP
addresses
 128-bit addresses arranged as 8 groups of four-digit hexadecimal
numbers separated by colons
 Leading zeros and zero values in one or more consecutive
groups may be eliminated
 Example: 6E:2A20::35C:66C0:0:5500 is the same as
006E:2A20:0000:0000:035C:66C0:0000:5500
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TCP/IP Addressing (4)
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Domain name translation
◦ Translate a user friendly address into an IP address and port address
for the transport layer
◦ Utilizes a global domain name directory service
Address resolution protocol (network layer)
◦ Translates IP addresses into physical addresses
MAC (medium-access control) address
◦ Most common type of physical address
◦ Every manufactured device that may connect to a network anywhere
in the world is supplied with a permanent, unique MAC address
◦ Format consists of 48 bits arranged as 6 two-digit hexadecimal
numbers separated by colons
◦ Example: 00:C0:9F:6C:F9:D0
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Different Addresses Used in a Network
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Network Topology
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Fundamental layout of a network
Describes the path or paths between any two points in the
network
Affects availability, speed and traffic congestion of the network
Logical topology – operational relationship between the various
network components
Physical topology – actual layout of the network wiring
Automobile Traffic
Scenarios
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Four Network Topologies
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Mesh Topology
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Multiple paths between end nodes
Failure of an individual intermediate node will slow but not
stop the network as long as an alternative path is available
Large networks that use switches and routers are typically
partial mesh networks
Full mesh network
◦ Direct point-to-point channel connecting every pair of
nodes
◦ Impractical due to the large number of connections
needed
◦ Number of connections = nodes x (nodes – 1) / 2
◦ 500 computer nodes would require 125,000
interconnecting cables!
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Five-Node Full Mesh Network
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Bus Topology
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Similar to multipoint buses
Each node is tapped into the bus along the bus
To communicate, each node “broadcasts” a message that travels
along the bus
Every node on the bus receives the message but it is ignored by
all nodes except the one whose node matches the delivery
address in the message
Transmission from any stations travels entire medium (both
directions)
Termination required at ends of bus to prevent the signal from
echoing
Branches can be added to a bus, expanding it into a tree but
messages are still broadcast throughout the entire tree
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Bus Network Implementation
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Only requires a single pair of wires from one end of the
network space to the other
◦ Easiest to wire of the network topologies
◦ Low cost
Traffic congestion is a major issue
Rarely used in designs of new networks except for
wireless networks
Because of the unguided nature of radio waves, wireless
networks require some form of bus topology
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Star Topology
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Primarily used for local area networks and sometimes
used to connect satellite offices to a central office
All nodes are connected point-to-point to a central device
Nodes communicate through the central device
Switching in the central device connects pairs of nodes
together to allow them to communicate directly
Central device can steer data from one node to another
as required
Most modern switches allow multiple pairs to
communicate simultaneously
Failure of central device causes entire network to go
down
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Ring Topology
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Point-to-point connection from each node to the next
Last node is connected back to the first to form a closed
ring
Each node retransmits the signal that it receives from the
previous node in the ring
Packets are placed on the loop at a node, and travel from
node to node until the desired node is reached
Although the ring is inherently unidirectional, it is possible
to build a bidirectional ring network
Popular in the past because they provided a controlled way
in which to guarantee network performance
◦ Legacy token-ring local area networks
Used in some FDDI fiber optic backbone and metropolitan
area networks
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Local Area Networks (LAN)
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A network that connections computers and other
supporting devices over a relatively small localized area
Typically ranging in size from a single room to multiple
buildings in close range of each other
Most of the computers are personal computers or
workstations
Routers and perhaps gateways are used to connect the
LAN to other networks
Creating separate LANs for different departments or for
different business functions is done to minimize extraneous
traffic on the network
Most modern LANs are based on one of the Ethernet
protocol standards
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Common Ethernet Standards
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Typical Home Network
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Ethernet Hubs
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Based on bus topology
A passive central connection device used to simplify
wiring and maintenance
Physical layer device where all of the connections are
tied together inside the hub
Signals are broadcast to every device connected to the
hub
Uses the CSMA/CD medium access control protocol
Use of hubs is declining because switches often provide
better performance
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Ethernet Switches
Logically a star topology, not a bus topology
 Able to set up a direction connection between any two
nodes
 Multiple pairs of nodes can communicate at the full
bandwidth
 Prevalent method for wired local area networks
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Hub vs. Switch Based Ethernet
Logically a bus and can
be viewed as a zerolength bus
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Logically and
physically a star
topology
Wireless Ethernet (WiFi)
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Radio-based compatible extension to the Ethernet
standard
Central access point is similar to a hub but is an active
node
Central access point transmits and receives radio waves
to communicate with the nodes
Radio space must be shared between the nodes
Does not use the CSMA-CD protocol because it is
possible for units to be far away that although they can
communicate with the access point, they cannot detect
one another
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Wireless Ethernet Characteristics
* Unofficial as of June 2008
** Possible future theoretical maximum data rate of 600 Mbps
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Wireless Mesh Network
Mesh points operate at the medium-access
control layer and do not require wiring
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Backbone Networks
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Also called tiered Ethernet
Ties together LANs and provides access to external networks
like the Internet
Chief motivation is to improve overall performance of a larger
network by creating separate networks for groups of users who
primarily communicate with one another
Communicate between the LANs is enabled only when necessary
Overall range of the network can be extended beyond the limits
of a single LAN
Can be viewed as a large LAN where each node is itself a LAN
Intranets – an organizational network where user interfaces and
applications are primarily based on Web services
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Backbone Network
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Metropolitan Area Networks
A network larger in geographical scope than a LAN but
within a range of less than 30 miles or 50 km
 Often there is a desire to create network links to link
locations that would require running wires through someone
else’s property.
◦ Requires services from a service provider or public carrier
◦ Begins to resemble a WAN
◦ Edge connection – a connection at an access point on the
customer’s premises that connects to a provider
 Campus area network (CAN)
◦ Network type between a LAN and a MAN
◦ Number of interconnected buildings clustered together
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Metropolitan Area Network
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Wide Area Networks (WAN)
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Facilitate communications between users and applications over large
geographical distances
Distinguishing feature is the extensive reliance on service providers to
provide the required connectivity between nodes
The carrier network is sometimes represented as a collection of private
virtual networks
Primary reasons for WANs
◦ Organization requires data communication links between widely
spread facilities and between an organization and its external contacts
◦ Organization requires fast access to the Internet, either as a
consumer or as a provider of Internet services, or both
Extranet
◦ A connection between a business and its business partners that
usually uses the Internet as a medium for its activities
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Two Real-World WANs
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Wide Area Network Carrier Options
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Internet Backbones and the Internet
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Internet Service Providers (ISPs)
Internet backbone
◦ High speed fiber optic networks that carry traffic between major
cities throughout the world
◦ Speed ranges from 45 to 625 Gbps with faster backbones in the
future
◦ Created to speed network traffic that would otherwise require many
slow hops to the final destination
◦ No official central backbone and no official guidance for its
development
Network access points
◦ Interchanges between the backbones
Local ISPs receive their service from regional ISPs who, in turn, receive
their service from national ISPs
Most regional ISPs also interconnect among themselves
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Comparison of Internet and Highway
Architecture
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Piconets
Also known as personal area networks (PAN)
 Created for the personal use of an individual
 Generally have ranges of 30 feet or less which is
sufficient to permit an individual to interconnect
personal computing devices
 Connections between different cooperating users are
possible but rare
 Bluetooth is the primary medium for PANs
 Example: interconnection between a cell phone, handsfree speaker and car radio
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Standards Organizations
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ISO (International Standards Organization)
◦ > 17,000 standards including the OSI Reference model
IEEE (Institute for Electrical and Electronics Engineers
◦ Ethernet standards – Ethernet (802.3), Wi-Fi (802.11), Bluetooth
(802.15) and WiMax (802.16)
IETF (Internet Engineering Task Force)
◦ Internet standards based on RFCs (request for comments)
ICANN
◦ Internet Corporation for Assigned Names and Numbers
◦ IP address allocation, domain name registration, protocol parameter
assignment
◦ Management of domain name and root server systems
IANA (Internet Assigned Numbers Authority
◦ Registers application layer port numbers and specific parameter values
used in Internet protocol headers
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Chapter Example
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User sitting at a computer types a URL that contains a
domain name into a web browser
First, HTTP client obtains the IP address of the Web
server
Then HTTP client initiates the process with a request
to the TCP socket to establish a logical connection with
the HTTP server at the destination site
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Domain Names and DNS Services
Domain Names
◦ Hierarchical system of network address identifiers used
throughout the Internet and on local area networks,
intranets and extranets
◦ Created so users would not have to memorize IP
addresses
 Domain Name System (DNS)
◦ Domain name resolution – translates domain names
into IP addresses
◦ Uses a massive distributed database containing a
directory system of servers
◦ Each entry contains a domain name and an associated IP
address
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Domain Name System (DNS)
DNS Server
Hierarchy
The Elements of a Domain
Name
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Top Domain Name Registrations
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Domain Name Resolution
Transport Layer
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TCP protocol
◦ Sends a packet to TCP at the destination site,
requesting a connection
◦ Handshaking – back and forth series of requests and
acknowledgments
◦ If handshaking negotiations are successful, a
connection is opened
◦ Connection is logically full-duplex
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Three-Way TCP Connection Handshake
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TCP Segment Format
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Network Layer
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IP protocol
◦ Responsible for relaying packets from the source end
node to the destination end node through
intermediate nodes
◦ Performed using datagram packet switching and
logical IP addresses
◦ Best-attempt unreliable service
◦ Size of datagram ranges from 20 to 65,536 bytes
◦ Header size between 20 and 60 bytes
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IPv4 Addresses
Registered and allocated by ICANN
 32 bits long divided into 4 octets
 Assigned in blocks of contiguous addresses
◦ Number of addresses is a power of two
 Divided into three levels
◦ Network address
◦ Subnetworks (subnets)
◦ Hosts (nodes)
 Masks
◦ Used to separate the different parts of the address
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IPv4 Datagram Format
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IP Addresses
IP Block
Addresses
IP Hierarchy and
Subnet Mask
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Reseved IP Addresses
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DHCP
Two methods to distribute IP addresses more efficiently:
1. Use of private network IP addresses behind a router
 The router must readdress traffic passing between the Internet and
the local network
 Management of readdressing becomes difficult with large networks
2.
Dynamic Host Configuration Protocol (DHCP)
◦ Maintain a bank of available IP addresses and assign them dynamically to
computers for use when the computers are attached to the network
◦ Method often used by large organizations, DSL and cable providers
◦ DHCP client on computer or network device broadcasts a query to
locate the DHCP server
◦ DHCP server responds with a lease which includes an IP address,
domain name of network, IP address of DNS server, subnet mask, IP
address of gateway and other configuration parameters
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Operation of IP
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Two major functions
◦ Routes datagrams from node to node until they reach their
destination node
◦ Translates IP addresses to physical addresses before it passes the
packets to the data link later for delivery
Address Resolution Protocol (ARP)
◦ Implemented at the network layer
◦ Translation of IP address to physical address at each intermediate
node until destination is reached
◦ A broadcast of the IP address is sent to every node on the network.
The matching node responds with a physical address
◦ Physical address (MAC address in the case of Ethernet) is sent in
frame to the data link layer
◦ At final destination, the packet is passed up to the transport layer for
deployment to the application layer
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Data Link Layer
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Layer responsible for transmitting a packet from one
node to the next node
Node access defined by the medium access control
(MAC) protocol
◦ Steer data to its destination
◦ Detect errors
◦ Prevent collisions
Ethernet (CSMA-CD)
◦ Predominant medium-access protocol for local area
networks
◦ Standard Ethernet packet is a frame (see next slide)
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Ethernet Frame
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Hub-Based Ethernet
Simple means of wiring a bussed Ethernet together
 Logically still a bus network
 CSMA-CD
 Collision
◦ Occurs when multiple nodes access the network in
such a way that their messages become mixed and
garbled
 Network propagation delay
◦ Amount of time that it takes for one packet to get
from one end of the network to the other
 Adequate for networks with light traffic
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Switched Ethernet
Permits point-to-point connection of any pair of nodes
 Multiple pairs can be connected simultaneously
 Possible to connect nodes in full-duplex mode
 Each pair of connections operates at the maximum bit
rate of the network
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Why can’t there be any collisions in a switched Ethernet
network?
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Quality of Service (QoS)
Methods to reserve and prioritize channel capacity to
favor packets that require special treatment
2. Service guarantees from contract carrier services that
specify particular levels of throughput, delay and jitter
◦ Jitter – variation in delay from packet to packet
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Differentiated service (DiffServ)
◦ 8-bit (DS) field in IP header
◦ Set by the application at the sender or by the first
node
◦ Diffserv capable nodes such as routers can then
prioritize and route packets based on the packet class
1.
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Network Security Categories
1.
2.
3.
4.
5.
Intrusion
◦ Keeping network and system resources free from
intruders
Confidentiality
◦ Keeping the content of data private
Authentication
◦ Verifying the identity of a source of data being received
Data integrity and non-repudiation
◦ Protecting the content of data communication against
changes and verifying the source of the message
Assuring network availability and access control
◦ Keep network resources operational and restricting access
to those permitted to use them
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Network Security

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Network intrusions
◦ Packet sniffers read data in a packet as it passes through a network
◦ Probing attacks to uncover IP address / port numbers that accept
data packets
Physical and Logical Restriction
◦ Limit access to wiring and network equipment
◦ Firewall
◦ Private networks
Encryption
◦ Symmetric key cryptography
 Both key used for encryption and decryption
 Both sender and receiver use the same key which makes security
difficult
◦ Public key cryptography
 Two different keys are used for encryption and decryption
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Alternative Protocols to TCP/IP

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MPLS (Multi-Protocol Label Switching)
◦ Creates a virtual circuit over packet switched networks
to improve forwarding speed of datagrams
ATM (Asynchronous Transfer Mode)
◦ Partial-mesh network technology in which data passes
through the network in cells (53-byte packets)
SONET (Synchronous Optical Network) and
SDH (Synchronous Digital Hierarchy)
◦ Protocol that uses fiber optic to create wide area
networks with very high bit rates over long distances
Frame Relay
◦ Slow, wide area network standard
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Communication Channel
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Communication Channels: Many Ways to
Implement
Signal: specific data transmitted
 Diagram shows a multi-link channel connecting a computer and
a wireless laptop
◦ Physically: signal passes through different channel forms
including audio, digital, light, radio
◦ Converters between separate channel links

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Communication Channel

Characterized by
◦ Signaling transmission method
◦ Bandwidth: amount of data transmitted in a fixed
amount of time
◦ Direction(s) in which signal can flow
◦ Noise, attenuation, and distortion characteristics
◦ Time delay and time jitter
◦ Medium used
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Signaling Transmission Method
Choice depends on medium and signal characteristics
 Analog
◦ Signal takes on a continuous range of values
 Discrete
◦ Signal takes on only finite, countable set of values
 Digital
◦ Binary discrete signal
◦ Frequently preferred because less susceptible to noise and
interference
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Channel Organization


Point to point channels
◦ Simplex: channel passes data in one direction only
◦ Half-duplex: transmits data one direction at a time
(walkie-talkie)
◦ Full-duplex: transmits data in both directions
simultaneously (telephone)
Multipoint: broadcasts messages to all connected
receivers
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Multiplexing


Carrying multiple messages over a channel
simultaneously
◦ TDM (time division multiplexing)
 Example: packet switching on the Internet
 Use: digital channels
◦ FDM (frequency division multiplexing)
 Example: Cable TV
 Analog channels
Synchronized switches or filters separate different data
signals at receiving end
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Signaling Technology


Signal carriers
◦ Electrical voltage
◦ Electromagnetic radio wave
◦ Switched light
Data represented by changes in the signal as a function
of time
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Communicating between Digital and Analog



Ideally conversion should be reversible
Limitations
◦ Noise: interference from sources like radio waves,
electrical wires, and bad connections that alter the data
◦ Attenuation: normal reduction in signal strength during
transmission caused by the transmission medium
◦ Distortion: alteration in the data signal caused by the
communication channel
◦ Ability to perfectly represent analog data in digital form
Consequences
◦ Error correction required to compensate for
transmission limitations
◦ Small information loss results from converting analog to
digital
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Analog Signals
Wireless networking
 Most telephones
 Satellites
 Microwave communications
 Radio and sound
◦ Radio waves can be converted to electrical signals for
use with wire media for mixed digital and analog data
 Example: Cable TV with digital Internet feed

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Sine Wave (1)
Common natural occurrence
 Basic unit of analog transmission
◦ Amplitude: wave height or power
◦ Period: amount of time to trace one complete cycle of
the wave
◦ Wavelength : distance spanned by a sine wave in space
◦ Frequency: cycles per second, i.e., number of times sine
wave repeated per second
 1 Hertz = 1 cycle/sec
◦ Unit of bandwidth for analog device

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Sine Wave (2)
f = 1/T
f is the frequency of the sine wave and where T is
the period measured in seconds
λ=c/f
λ is the wavelength of the sine wave and c is the
speed of light
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Circle and the Sine Wave

Points on a sine wave frequently designated in degrees
◦ v = A sin[Θ] where A is the maximum amplitude and
Θ is the angle in the diagram
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Phase-Shifted Sine Waves

Difference, measured
in degrees, from a
reference sine wave
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Waveform Representation
All can be represented as the sum of sine waves of
different frequencies, phases, and amplitudes
 Spectrum: frequencies that make up a signal
 Bandwidth: range of frequencies passed by the channel
with a small amount of attenuation
 Filtering: controlling the channel bandwidth to prevent
interference from other signals

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Creating a Square Wave from Sine Waves
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Signal Frequencies


Sound waves: approximately 20 Hz to 20 KHz
◦ Stereo systems: 20-20,000 Hz for high fidelity
◦ Phones: 0-4000 Hz for voice but limits speed
Electromagnetic radio waves: 60 Hz to 300 GHz
◦ AM radio: 550 KHz to 1.6 MHz
 20 KHz bandwidth centered around dial frequency
of the station
◦ FM radio: 88 MHz to 108 MHz
 100 KHz bandwidth per station
◦ TV: 54 MHz to 700 MHz
 >4.5 MHz bandwidth per channel
◦ Cell phones, Wi-Fi wireless networks: 800 MHz to
5.2Ghz
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Signal Frequencies
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Sine Waves as Carriers



A single pure tone consists of a sine wave
◦ The orchestral note middle A is a 440-Hz sine wave
To represent the signal modulate one of the three
characteristics – amplitude, frequency, phase
◦ Example: AM or amplitude modulated radio station at
1100 KHz modulates amplitude of the 1100 KHz sine
wave carrier
Demodulator or detector restores original waveform
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Amplitude Modulations
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Modulating Digital Signals


Two possible values: 0 and 1
3 techniques
◦ ASK: amplitude shift keying
 Represents data by holding the frequency constant
while varying the amplitude
◦ FSK: frequency shift keying
 Represents data by holding the amplitude constant
while varying the frequency
◦ PSK: phase shift keying
 Represents data by an instantaneous shift in the
phase or a switching between two signals of
different phases
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Modulating Digital Signals
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Frequency Division Multiplexing
Optical form of frequency division multiplexing (FDM) is
known as wavelength division multiplexing (WDM)
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Attenuation



Function of the nature of the transmission medium and
the physical length of the channel
More difficult to separate the signal from noise at higher
transmission speeds
◦ Signal-to-noise ratio:
 Strength of the signal in relation to power of the
noise
 Measure at the receiving end
Amplifiers: restore original strength of the signal (but also
amplifies noise)
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Effects of Attenuation

Channel fading and phase shifts vary with the frequency
of the signal
◦ Example: If the signal consists of sine waves of
frequencies f1 and f2 from different parts of the
spectrum, the output of the channel will be distorted
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Synchronizing Digital Signals
Synchronizing digital signals difficult
 Asynchronous transmission
◦ Clear start and stop signals
◦ Small number of bits, usually one byte
◦ Use: low-speed modems, Ethernet frames
 Synchronous transmission
◦ Continuous digital signal
◦ Use: high-speed modems and point-to-point methods

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Reception Errors


Timing mismatch between sending and receiving
computers
Inability to distinguish groups of 1’s or 0’s
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Block and Manchester Encoding
Block Encoding
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Manchester Encoding
A-to-D Conversion
Digital signals used to represent analog waveforms
 Examples:
◦ CDs, DVDs
◦ Direct satellite TV,
◦ VOIP
◦ Telephone voice mail
◦ Streaming video
 A-to-D Pulse Code Modulation

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A-to-D: Pulse Code Modulation
1.
Analog waveform sampled at regular time intervals
◦ Maximum amplitude divided into intervals
 Example: 256 levels requires 8 bits/sample
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A-to-D: Pulse Code Modulation
2.
Sample values converted into corresponding
number value
◦ Information lost in conversion
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A-to-D: Pulse Code Modulation
3.
Number reduced to binary equivalent
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Digital Signal Quality
Subject to noise, attenuation, distortion like analog
 Signal quality less affected because only necessary to
distinguish 2 levels
 Repeaters
◦ Recreate signals at intervals
◦ Use: transmit signals over long distances
 Error correction techniques available

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Time Division Multiplexing

TDM - multiple signals share channel
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Bandwidth
Digital signals: sum of sine waves of different frequencies
 Higher frequencies: higher data rates
 Channel with wider bandwidth has higher data rates
 Data rates usually measured in bits per second

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Modems
 Modem (modulator/demodulator)
 Convert digital signals to analog and back
 Use: home to service provider via phone line or cable
 Speed: baud rate or bits per second (bps)
DSL
KT6213
Transmission Media
Means used to carry signal
 Characterized by
◦ Physical properties
Bandwidth
◦ Signaling method(s)
Sensitivity to noise
 Guided media: confine signal physically to some kind of
cable
 Unguided media: broadcast openly
 Signal-to-noise ratio
◦ Higher ratio for given bandwidth increases data
capacity of the channel

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Electrical Media



Require complete circuit
◦ 2 wires: one to carry the signal, second as a return to
complete the circuit
Wired media or just wire
◦ Inexpensive and easy to use
Signals carried as changing electrical voltage or current
KT6213
Types of Cable: Copper


Coaxial cable
◦ Wire surrounded by insulation
◦ Copper shield around insulation
 Acts as signal return
 Shields from external noise
◦ High bandwidth: 100 Mbps
 Example: analog cable TV with FDM for dozens of
channels at 6 MHz
Twisted pair
◦ Most local area networks; phone lines in buildings
◦ More susceptible to noise than coaxial cable
◦ Used for shorter distances and slower signals
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Types of Cable: Fiber Optic
Fiber optic cable
◦ Consists of glass fiber thinner than human hair
◦ Uses light to carry signals
◦ Laser or light-emitting diode produces signal
◦ Cladding: plastic sheath to protect fibers
 Advantages
◦ Light waves: high frequency means high bandwidth
◦ Less susceptible to interference and tampering
◦ Lighter than copper cable
 Disadvantages
◦ Difficult to use, especially for multipoint connections

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Electromagnetic Waves



Microwaves
◦ Frequencies below light but above 1 GHz
Unguided medium
◦ Tightly focused for point-to-point use
◦ Highly susceptible to interference
Applications
◦ Large-scale Internet backbone channels
◦ Direct satellite-to-home TV
◦ IEEE 802.11 Wi-Fi
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Wireless Networking



Wi-Fi (wireless Ethernet)
◦ Short-range, local area networking
WiMAX, cellular telephone technology
◦ Competing versions of longer range wireless
networking
Bluetooth
◦ Personal level networking
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Wi-Fi


Access point
◦ Hub for wireless devices
◦ Router between wireless and wired devices
◦ Forwards packet to destination station
CSMA-CA
◦ Collision avoidance, not collision detection!
KT6213
Wi-Fi Network Configuration
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