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Lecture 6: Networks and Data Communications

Computer Organization Computer Organization and Architectureand Architecture

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Data CommunicationsData CommunicationsA 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 ResponseHTTP Request and Response

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Model of a Communication ChannelModel of a Communication Channel

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MessagesMessagesCommunication between cooperating applications

at each end nodeCan take many forms such as data, a program, a

file, or multimediaRepresented digitallyData is described as a byte stream because

communications are predominantly serialLimitation 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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PacketsPackets

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 HeaderPacket Header

Also known as the preambleContains

◦ Description of the packet◦ Destination address of receiver◦ Source address of sender◦ Information about the data being sent

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Advantages of PacketsAdvantages of Packets

Simplifies operations and increases communications efficiency

Reasonable unit for routing of dataAlternative to dedicating a channel for the entire

length of the messagePackets from several sources can share a single

channelEach sender/receiver pair appears to have a

channel to itselfReceiving computer can process an entire block of

data instead of a character or byte at a timeSimplifies synchronization of the sending and

receiving systems by providing clear start and stop points

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Channel Characteristics (1)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 ChannelA Multi-Link Channel

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Channel Characteristics (2)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 RoutingPacket Routing

Circuit switching◦ Dedicated channel between source and

destination for duration of connectionVirtual 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 RoutingPacket RoutingEnd-to-end channel with many possible paths through intermediate nodes

Virtual Circuits in a Network

Connecting End Points through Links and Networks

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Packet RoutingPacket Routing

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 portGateways

◦ Same as routers but connect dissimilar networks together

◦ Convert packet headers for the dissimilar networks

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Router Block DiagramRouter Block Diagram

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Network OverviewNetwork 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 ModelCommunication 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/IPTCP/IP 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 ModelOperation of TCP/IP Model

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Application Layer (Layer 5)Application Layer (Layer 5)Layer where message is created Includes any application that provides software

that can communicate with the network layerSockets

◦ 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 IPSCSI over IP

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Transport Layer (Layer 4)Transport Layer (Layer 4)

Provides services that support reliable end-to-end communications

Generates the final address of the destinationResponsible for all end-to-end communication

facilitiesPacketization 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 ProtocolsTransport Layer Protocols 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 TCPLogical Connection View of TCP

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Network Layer (Layer 3)Network Layer (Layer 3)

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.)Network Layer (cont.)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)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)Physical Layer (Layer 1)

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 Passing a Message Through an Intermediate NodeIntermediate Node

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OSI ModelOSI Model

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 fiveThe 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/IPComparison of OSI and TCP/IP

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OSI Presentation LayerOSI Presentation Layer

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 LayerOSI Session Layer

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)TCP/IP Addressing (1)User friendly addresses

◦ URL – www.youtube.com◦ Email – somebody@yahoo.com◦ 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)TCP/IP Addressing (2)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 user-defined port of 8080http://www.somewhere.org:8080/hiddenServer/index.html

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Well-Known Port AddressesWell-Known Port Addresses

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TCP/IP Addressing (3)TCP/IP Addressing (3) 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:66C0:0000:550

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TCP/IP Addressing (4)TCP/IP Addressing (4) 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 Different Addresses Used in a NetworkNetwork

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Network TopologyNetwork Topology 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 TopologiesFour Network Topologies

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Mesh TopologyMesh TopologyMultiple paths between end nodesFailure 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 NetworkFive-Node Full Mesh Network

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Bus TopologyBus Topology Similar to multipoint buses in chapter 7 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 ImplementationBus Network Implementation

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 issueRarely used in designs of new networks except

for wireless networksBecause of the unguided nature of radio

waves, wireless networks require some form of bus topology

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Star TopologyStar TopologyPrimarily used for local area networks and sometimes

used to connect satellite offices to a central officeAll nodes are connected point-to-point to a central

deviceNodes communicate through the central deviceSwitching 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 TopologyRing TopologyPoint-to-point connection from each node to the nextLast node is connected back to the first to form a

closed ringEach node retransmits the signal that it receives from

the previous node in the ringPackets are placed on the loop at a node, and travel

from node to node until the desired node is reachedAlthough the ring is inherently unidirectional, it is

possible to build a bidirectional ring networkPopular 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)Local Area Networks (LAN)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 StandardsCommon Ethernet Standards

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Typical Home NetworkTypical Home Network

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Ethernet HubsEthernet HubsBased on bus topologyA passive central connection device used to

simplify wiring and maintenancePhysical layer device where all of the

connections are tied together inside the hubSignals are broadcast to every device

connected to the hubUses the CSMA/CD medium access control

protocolUse of hubs is declining because switches

often provide better performance

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Ethernet SwitchesEthernet SwitchesLogically a star topology, not a bus topologyAble to set up a direction connection

between any two nodesMultiple pairs of nodes can communicate at

the full bandwidthPrevalent method for wired local area

networks

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Hub vs. Switch Based EthernetHub vs. Switch Based Ethernet

Logically a bus and can be viewed as a zero-length bus

Logically and physically a star topology

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Wireless Ethernet (WiFi)Wireless Ethernet (WiFi)

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 CharacteristicsWireless Ethernet Characteristics

* Unofficial as of June 2008** Possible future theoretical maximum data rate of 600 Mbps

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Wireless Mesh NetworkWireless Mesh Network

Mesh points operate at the medium-access control layer and do not require wiring

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Backbone NetworksBackbone Networks 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 NetworkBackbone Network

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Metropolitan Area NetworksMetropolitan Area NetworksA network larger in geographical scope than a LAN but

within a range of less than 30 miles or 50 kmOften 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 providerCampus area network (CAN)

◦ Network type between a LAN and a MAN◦ Number of interconnected buildings clustered together

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Metropolitan Area NetworkMetropolitan Area Network

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Wide Area Networks (WAN)Wide Area Networks (WAN) 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 WANsTwo Real-World WANs

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Wide Area Network Carrier OptionsWide Area Network Carrier Options

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Internet Backbones and the InternetInternet Backbones and the Internet 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 Comparison of Internet and Highway ArchitectureArchitecture

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PiconetsPiconets

Also known as personal area networks (PAN)Created for the personal use of an individualGenerally 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 PANsExample: interconnection between a cell

phone, hands-free speaker and car radio

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Standards OrganizationsStandards Organizations 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 ExampleChapter Example

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 ServicesDomain Names and DNS ServicesDomain 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)Domain Name System (DNS)

DNS Server Hierarchy

The Elements of a Domain Name

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Top Domain Name RegistrationsTop Domain Name Registrations

Domain Name ResolutionDomain Name Resolution

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Transport LayerTransport Layer

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 Three-Way TCP Connection HandshakeHandshake

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TCP Segment FormatTCP Segment Format

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Network LayerNetwork Layer

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 AddressesIPv4 Addresses

Registered and allocated by ICANN32 bits long divided into 4 octetsAssigned in blocks of contiguous addresses

◦ Number of addresses is a power of twoDivided 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 FormatIPv4 Datagram Format

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IP AddressesIP Addresses

IP Block Addresses

IP Hierarchy and Subnet Mask

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Reseved IP AddressesReseved IP Addresses

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DHCPDHCPTwo 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 IPOperation of IP 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 LayerData Link Layer

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 FrameEthernet Frame

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Hub-Based EthernetHub-Based Ethernet

Simple means of wiring a bussed Ethernet together

Logically still a bus networkCSMA-CDCollision

◦ 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 otherAdequate for networks with light traffic

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Switched EthernetSwitched Ethernet

Permits point-to-point connection of any pair of nodes

Multiple pairs can be connected simultaneously

Possible to connect nodes in full-duplex modeEach pair of connections operates at the

maximum bit rate of the network

Why can’t there be any collisions in a switched Ethernet network?

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Quality of Service (QoS)Quality of Service (QoS)

1. 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 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

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Network Security CategoriesNetwork Security Categories1. Intrusion

◦ Keeping network and system resources free from intruders

2. Confidentiality◦ Keeping the content of data private

3. Authentication◦ Verifying the identity of a source of data being

received

4. Data integrity and non-repudiation◦ Protecting the content of data communication against

changes and verifying the source of the message

5. Assuring network availability and access control◦ Keep network resources operational and restricting

access to those permitted to use them

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Network SecurityNetwork Security 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/IPAlternative Protocols to TCP/IPMPLS (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 distancesFrame Relay

◦ Slow, wide area network standard

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Communication ChannelCommunication Channel

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Communication Channels: Many Communication Channels: Many Ways to ImplementWays 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 ChannelCommunication 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 MethodSignaling Transmission MethodChoice 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 OrganizationChannel 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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MultiplexingMultiplexing

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 TechnologySignaling 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 Communicating between Digital and Analog Analog Ideally conversion should be reversibleLimitations

◦ 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

KT6144 / KT6213

Analog SignalsAnalog Signals

Wireless networkingMost telephonesSatellitesMicrowave communicationsRadio 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

KT6144 / KT6213

Sine Wave (1)Sine Wave (1)

Common natural occurrenceBasic 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

KT6144 / KT6213

Sine Wave (2)Sine Wave (2)

λ = c / f λ is the wavelength of the sine wave and c is the speed of light

f = 1/T f is the frequency of the sine wave and where T is the period measured in seconds

KT6144 / KT6213

Circle and the Sine WaveCircle 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

KT6144 / KT6213

Phase-Shifted Sine WavesPhase-Shifted Sine Waves

Difference, measured in degrees, from a reference sine wave

KT6144 / KT6213

Waveform RepresentationWaveform RepresentationAll can be represented as the sum of sine

waves of different frequencies, phases, and amplitudes

Spectrum: frequencies that make up a signalBandwidth: range of frequencies passed by

the channel with a small amount of attenuation

Filtering: controlling the channel bandwidth to prevent interference from other signals

KT6144 / KT6213

Creating a Square Wave from Sine Creating a Square Wave from Sine WavesWaves

KT6144 / KT6213

Signal FrequenciesSignal FrequenciesSound 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

KT6144 / KT6213

Signal FrequenciesSignal Frequencies

KT6144 / KT6213

Sine Waves as CarriersSine 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

KT6144 / KT6213

Amplitude ModulationsAmplitude Modulations

KT6144 / KT6213

Modulating Digital SignalsModulating Digital SignalsTwo possible values: 0 and 13 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

KT6144 / KT6213

Modulating Digital SignalsModulating Digital Signals

KT6144 / KT6213

Frequency Division MultiplexingFrequency Division Multiplexing

Optical form of frequency division multiplexing (FDM) is known as wavelength division multiplexing (WDM)

KT6144 / KT6213

AttenuationAttenuationFunction 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 endAmplifiers: restore original strength of the

signal (but also amplifies noise)

KT6144 / KT6213

Effects of AttenuationEffects 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

KT6144 / KT6213

Synchronizing Digital SignalsSynchronizing Digital Signals

Synchronizing digital signals difficultAsynchronous 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

KT6144 / KT6213

Reception ErrorsReception Errors

Timing mismatch between sending and receiving computers

Inability to distinguish groups of 1’s or 0’s

KT6144 / KT6213

Block and Manchester EncodingBlock and Manchester EncodingBlock Encoding Manchester Encoding

KT6144 / KT6213

A-to-D ConversionA-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

KT6144 / KT6213

A-to-D: Pulse Code ModulationA-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

KT6144 / KT6213

A-to-D: Pulse Code ModulationA-to-D: Pulse Code Modulation

2. Sample values converted into corresponding number value

◦ Information lost in conversion

KT6144 / KT6213

A-to-D: Pulse Code ModulationA-to-D: Pulse Code Modulation

3. Number reduced to binary equivalent

KT6144 / KT6213

Digital Signal QualityDigital 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

KT6144 / KT6213

Time Division MultiplexingTime Division Multiplexing

TDM - multiple signals share channel

KT6144 / KT6213

BandwidthBandwidth

Digital signals: sum of sine waves of different frequencies

Higher frequencies: higher data ratesChannel with wider bandwidth has higher

data rates Data rates usually measured in bits per

second

KT6144 / KT6213

ModemsModems

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

KT6144 / KT6213

Transmission MediaTransmission Media

Means used to carry signalCharacterized by

◦ Physical properties Bandwidth◦ Signaling method(s) Sensitivity to

noiseGuided media: confine signal physically to

some kind of cableUnguided media: broadcast openlySignal-to-noise ratio

◦ Higher ratio for given bandwidth increases data capacity of the channel

KT6144 / KT6213

Electrical MediaElectrical Media

Require complete circuit ◦ 2 wires: one to carry the signal, second as

a return to complete the circuitWired media or just wire

◦ Inexpensive and easy to useSignals carried as changing electrical voltage

or current

KT6144 / KT6213

Types of Cable: CopperTypes 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 MHzTwisted pair

◦ Most local area networks; phone lines in buildings

◦ More susceptible to noise than coaxial cable◦ Used for shorter distances and slower signals

KT6144 / KT6213

Types of Cable: Fiber OpticTypes 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

KT6144 / KT6213

Electromagnetic WavesElectromagnetic 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

KT6144 / KT6213

Wireless NetworkingWireless Networking

Wi-Fi (wireless Ethernet)◦ Short-range, local area networking

WiMAX, cellular telephone technology◦ Competing versions of longer range

wireless networkingBluetooth

◦ Personal level networking

KT6144 / KT6213

Wi-FiWi-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!

KT6144 / KT6213

Wi-Fi Network ConfigurationWi-Fi Network Configuration