Communications Networks

Chapter 2

Communications Networks

IntroductionIntroduction

• TopicsTopics– Telephony NetworksTelephony Networks– OSI Reference ModelOSI Reference Model– The InternetThe Internet– Asynchronous Transfer Mode NetworksAsynchronous Transfer Mode Networks– Networking ComponentsNetworking Components– Network TopologiesNetwork Topologies

IntroductionIntroduction

• A network is a group of computers A network is a group of computers that can communicate with each that can communicate with each other so they can share information other so they can share information

• When computers can communicate When computers can communicate with each other they can share with each other they can share resourcesresources

• When a server provides a resource When a server provides a resource for a client to access, this is referred for a client to access, this is referred to as a shared resource to as a shared resource

Telephony NetworksTelephony Networks

• IP telephony networks make better IP telephony networks make better use of available bandwidthuse of available bandwidth

• VoIP network carries voice traffic VoIP network carries voice traffic cheaper than a switched circuit cheaper than a switched circuit telephone network telephone network

Telephony NetworksTelephony Networks

• In a PSTN, a dedicated end-to-end In a PSTN, a dedicated end-to-end circuit is allocated for each call circuit is allocated for each call

• In a VoIP network, data is much more In a VoIP network, data is much more compressed and carried in packetscompressed and carried in packets

The OSI ModelThe OSI Model

• An architecture that allows the An architecture that allows the devices of different manufacturers to devices of different manufacturers to work together to communicate with work together to communicate with different operating systems different operating systems

• This architecture determines how This architecture determines how hardware, software, topologies and hardware, software, topologies and protocols exist on the network and protocols exist on the network and how they operate how they operate


• Physical layerPhysical layer– Defines mechanical, functional, Defines mechanical, functional,

procedural and electrical aspects of procedural and electrical aspects of networking networking

- Includes connectors, circuits, voltage Includes connectors, circuits, voltage levels and grounding levels and grounding


• Data Link layerData Link layer– Converts data from upper layers into Converts data from upper layers into

logical packages or frameslogical packages or frames– Converts logical frames into raw bits Converts logical frames into raw bits

that are transmitted by the Physical that are transmitted by the Physical layerlayer


• Network layerNetwork layer– Provides connectivity and path selection Provides connectivity and path selection

between two systems between two systems – routing and addressing layerrouting and addressing layer


• Transport layerTransport layer– Provides a virtual end-to-end connection Provides a virtual end-to-end connection

so that data transferred between two so that data transferred between two hosts will arrive without errors and in hosts will arrive without errors and in the correct orderthe correct order


• Session layerSession layer– Allows two applications on different Allows two applications on different

computers to establish dialog controlcomputers to establish dialog control– Regulates which side transmitsRegulates which side transmits– Determines the time and length of the Determines the time and length of the

transmissiontransmission


• Presentation layerPresentation layer– Translates data from the Application Translates data from the Application

layer into an intermediary format layer into an intermediary format – Provides services such as data Provides services such as data

encryption, and compresses data encryption, and compresses data


• Application layerApplication layer– Provides application programs access to Provides application programs access to

print and file services to ensure that print and file services to ensure that effective communication with other effective communication with other application programs is possible application programs is possible

Data EncapsulationData Encapsulation

The InternetThe Internet

• The Internet was originally called The Internet was originally called ARPANET ARPANET – Developed by the Department of Defense to Developed by the Department of Defense to

provide a way to connect networks provide a way to connect networks

• Internet is a network of interconnected, Internet is a network of interconnected, yet independent networks yet independent networks

• The language of the Internet is TCP/IP The language of the Internet is TCP/IP

Asynchronous Transfer Asynchronous Transfer Mode (ATM) Networks Mode (ATM) Networks • ATM uses connection-oriented ATM uses connection-oriented

switches to permit senders and switches to permit senders and receivers to communicate by receivers to communicate by establishing a dedicated circuit establishing a dedicated circuit – Data travels in fixed 53-byte cells Data travels in fixed 53-byte cells – Five bytes are used for header Five bytes are used for header

information and 48 bytes are used for information and 48 bytes are used for data data

– Data transfer rate can reach up to 9,953 Data transfer rate can reach up to 9,953 Mbps Mbps

Networking Components Networking Components

• Baseband uses a digital transmission Baseband uses a digital transmission pulse at a single fixed frequency pulse at a single fixed frequency – Entire bandwidth of the cable is used to Entire bandwidth of the cable is used to

transmit one data signal transmit one data signal – Limits any cable strand to either half Limits any cable strand to either half

duplex or full duplex duplex or full duplex

Networking ComponentsNetworking Components

• Broadband uses analog transmission Broadband uses analog transmission over a continuous range of values over a continuous range of values – Travels one way only in optical waves Travels one way only in optical waves – Necessary to have two channels, one for Necessary to have two channels, one for

receiving and one for sending datareceiving and one for sending data– More than one transmission can operate More than one transmission can operate

on a single cable on a single cable


• Media: Cables and Wireless Media: Cables and Wireless – Coaxial cable was the first type of cable Coaxial cable was the first type of cable

used to network computers used to network computers – Coaxial cables are made of a thick Coaxial cables are made of a thick

copper core with an outer metallic shield copper core with an outer metallic shield used to reduce external interference used to reduce external interference

– Twisted pair cable comes in seven Twisted pair cable comes in seven different categories. different categories.


• Media: Cables and WirelessMedia: Cables and Wireless– Twisted-pair cabling is either unshielded Twisted-pair cabling is either unshielded

(UTP) or shielded (STP) (UTP) or shielded (STP) – Fiber was designed for transmissions at Fiber was designed for transmissions at

higher speeds over longer distances higher speeds over longer distances – Fiber uses light pulses for signal Fiber uses light pulses for signal

transmission, making it immune to RFI, transmission, making it immune to RFI, EMI, and eavesdropping EMI, and eavesdropping


• Media: Cables and WirelessMedia: Cables and Wireless– Wireless network refers to technology Wireless network refers to technology

that allows two or more computers to that allows two or more computers to communicate using standard network communicate using standard network protocols, but without network cabling protocols, but without network cabling

– Wireless networking hardware requires Wireless networking hardware requires the use of technology that deals with the use of technology that deals with data transmission over radio frequencies data transmission over radio frequencies


• Media: Cables and WirelessMedia: Cables and Wireless– Most widely used wireless standard is Most widely used wireless standard is

the IEEE 802.11 standardthe IEEE 802.11 standard– The IEEE standards for wireless are The IEEE standards for wireless are

802.11a and 802.11b 802.11a and 802.11b


• A hub is a multiport repeater that A hub is a multiport repeater that retransmits a signal on all ports retransmits a signal on all ports – Operates at Layer 1 of the OSI model Operates at Layer 1 of the OSI model – Can connect segments or a network Can connect segments or a network – Cannot segment a network Cannot segment a network


• A bridge can connect two different A bridge can connect two different types of topologies types of topologies – Does not understand anything above Does not understand anything above

the Data Link layerthe Data Link layer– Moves data more rapidly Moves data more rapidly – Takes longer to transmit because it Takes longer to transmit because it

analyzes each packet analyzes each packet


• Switches operate at the Data Link Switches operate at the Data Link layer of the OSI model layer of the OSI model – Packet forwarding decisions are based Packet forwarding decisions are based

on MAC addresses on MAC addresses – Determines from a physical address Determines from a physical address

(MAC address) which device a packet is (MAC address) which device a packet is intended for and switches it out toward intended for and switches it out toward that device that device


• Routers operate at the Network layer Routers operate at the Network layer of the OSI modelof the OSI model– Forwards information to its destination Forwards information to its destination

on the network or the Internet on the network or the Internet – Routers maintain tables that are Routers maintain tables that are

checked each time a packet needs to be checked each time a packet needs to be redirected from one interface to another redirected from one interface to another

Networking TopologiesNetworking Topologies

• All devices on the network compete for All devices on the network compete for access on a single shared piece of mediaaccess on a single shared piece of media

• Only one device can transmit or talk on Only one device can transmit or talk on the media at a time while all others must the media at a time while all others must listenlisten

• When more than one device When more than one device simultaneously tries to talk, there is simultaneously tries to talk, there is competition for access to the media competition for access to the media resulting in a collision of information resulting in a collision of information


• BusBus topology:topology:– Consists of computers connected by a Consists of computers connected by a

single cable called a backbone single cable called a backbone – All the computers share in its capacity All the computers share in its capacity – Simplest method for connecting Simplest method for connecting

computers computers – 10Base2 or10Base5 cable is used10Base2 or10Base5 cable is used– The more devices, the slower the network The more devices, the slower the network


• Ring topology:Ring topology:– Consists of each computer, connects Consists of each computer, connects

directly to the next one in line, forming directly to the next one in line, forming a circle a circle

– Data travels in a clockwise direction and Data travels in a clockwise direction and each machine accepts the information each machine accepts the information intended for it intended for it

– Passes on the information that is for Passes on the information that is for other machines other machines


• Ring topology:Ring topology:– Uses a token, which is actually a small Uses a token, which is actually a small

packet, to send information packet, to send information – Every computer in the ring is Every computer in the ring is

responsible for either passing the token responsible for either passing the token or creating a new one or creating a new one


• Peer-to-peerPeer-to-peer network: network: – All machines are equalAll machines are equal– Each can act as a server and a client Each can act as a server and a client – There is no central control over shared There is no central control over shared

resources resources – Individual users decide what to share and Individual users decide what to share and

with whom with whom – Less secure than a server based network Less secure than a server based network


• Star topology:Star topology:– Computers are connected to a Computers are connected to a

centralized hub by a cable segment centralized hub by a cable segment – Require more cabling than ring or bus Require more cabling than ring or bus

topologies topologies – One computer connection goes down, it One computer connection goes down, it

does not affect the rest of the network does not affect the rest of the network – Much easier to move computers around Much easier to move computers around

or connect them to other networksor connect them to other networks


• MeshMesh topology:topology:– All devices are connected to each other All devices are connected to each other

more than once to create fault tolerance more than once to create fault tolerance – A single device or cable failure will not A single device or cable failure will not

affect the performance affect the performance – More expensive More expensive – Requires more hardware and cabling Requires more hardware and cabling


•Star bus topology:Star bus topology:– Computers are connected to hubs in a Computers are connected to hubs in a

star formation and then the hubs are star formation and then the hubs are connected via bus topology connected via bus topology

– More expensive to implement More expensive to implement – Longer distances can be covered Longer distances can be covered – Networks can more easily be isolated Networks can more easily be isolated


• Star ringStar ring topology:topology:– Data is sent in a circular motion around Data is sent in a circular motion around

the star the star – Eliminates the single point of failure that Eliminates the single point of failure that

happens in a ring topology happens in a ring topology – Uses token passing data transmission Uses token passing data transmission

with the physical layout of a star with the physical layout of a star

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Communications Networks