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Computer Network Fundamentals (IT 221) أساسيات شبكات الحاسبLecture 1:• Computer network: collection of autonomous computers

interconnected by a single technology.

• Uses of computer networks: 1. Data communication: - communication: sharing info, can be local(face to face) or remote(over distance).- data communication: exchange of data between two devices via transmission

medium(wire cable).- communicating devices: made up of HW (physical equipments) and SW.The effectiveness depends on:A. Delivery: system must deliver data to correct destination.B. Accuracy: the system must deliver data accurately.C. Timeliness: system must deliver data in a timely manner.D. Jitter: Variation in the packet arrival time. it’s the uneven delay in the delivery of audio

or video packets.- Components: 1. Message: information to be communicated.2. Sender: the device that sends the data message.3. Receiver: the device that receives the message.4. Transmission medium: the physical path by which a message

travels.5. Protocol: set of rules that govern data communications.• Direction of data flow:

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Business Applications:

Goals: sharing physical resources such as printers or sharing information. example: one or more database with company information.- a computer network can provide powerful communication

medium among employees: Email.- telephone calls between employees can be carried by

computer network (IP telephony or voice over IP -VoIP-)- doing business electronically: E-commerce.

Home Applications: - users can access info and communicate with other using e-commerce.

- much of this info is accessed using client-server model.- different model for accessing information is peer-to-peer

communication.

Mobile Users: - combinations of wireless networks and mobile computing.

example in PDA >

Simplex: Half-Duplex Full-Duplex

communication is unidirectional (only one transmit; the other receives), example: keyboard(input) and monitors(output).

each station can both transmit and receive but one at a time. (walkie talkies)

both stations can transmit and receive at the same time. (telephone)

24/05/1439 AH2. Networks: set of devices(often referred to as nodes) connected by

communication links. (node = printer/computer…) Network criteria: - performance: depends on; a. number of usersb. type of transmission mediac. capabilities of connected HW and the efficiency os SW- reliability: measured by frequency of failure, the time it takes to recover from

failure, a network’s robustness in catastrophe.- security: protecting data from unauthorized access.• Physical Structures: A. Types of connections:

B. topology: the way in which network is laid out physically.

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Point-to-point Multipoint

between two devices(cable connected between two ends or microwave satellite are possible).

AKA: multi-drop. connection is one in which more that two specific devices share a single link.- capacity of a the channel is shared as:a. spatially: if several devices can use link simultaneouslyb. temporally: time shared connection

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Mesh Star Tree Bus Ring

- every device has a dedicated point-to-point link to every other devices

- connections = n(n-1)/2 - every device must have

n-1 (I/O) ports.- Advantages: 1. privacy/security2. Eliminating traffic

problems3. it’s robust4. fault identification and

isolation- Disadvantages: 1. installation and

reconnection difficulty2. sheer bulk can be

greater that the available space

3. HW required to connect each link(ports and cables); expensive.

- each device has link only to a central controller (hub)

- doesn’t allow direct traffic between devices.

- sending data between devices:

device > hub > device - Advantages: 1. easy to install,

cheaper2. robustness3. Identification and

fault isolation- Disadvantages: 1. dependency of whole

topology on hub; if it goes down the whole system is dead.

- not every plugs directly into the central hub.

- Advantages and disadvantages the exact same as “star topology”

- nodes are connected to the bus cable by drop lines and taps.

- a drop line is a connection running between the device and the main cable.

- Advantages: 1. ease of installation, use

less calming than mesh or star.

- Disadvantages: 1. a fault in bus

cable(break) stops all transmission even between devices on the same side of the problem.

2. Reconnection, it can be difficult to add new devices.

- each device has a medicated point-to-point connection only with the two devices on either side of it.

- a signal is passed along the ring in one direction from device until it reaches its destination.

- each device in the ring incorporate as repeater.

- Repeater: regenerates the signal. it receives a weakened signal, creates a copy, bit for bit, at the original strength.

- Advantages: 1. easy to install and

reconfigure.2. fault isolation is simplified.- Disadvantages: 1. unidirectional traffic- another kind: Dual-ring

topology

24/05/1439 AHHybrid topology:

Network categories -two primary categories-: - local-area networks- wide-area networkscategories: 1. PAN(Personal Area Network): bluetooth is used to connect wirelessly,

it can be built with other technologies that communicate over short ranges, such as RFID.

2. LAN(Local Area Network): privately owned network in buildings like home or office.

- every computer has a radio modem and an antenna that to uses to communicate with other computers.

- each computer talks to a device in the ceiling called AP(Access Point), wireless router/ base station.

- IEEE 802.11 = WiFi, runs at speeds 11-100s Mbps.- most wired LANs uses copper wires but some uses optical fiber.- wired LANs = speeds of 100 Mbps - 1 Gbps, have low delay and make very few

errors.- signals easier to send over wires/optical fibers than air.- built from point-to-point links.- IEEE 802.3 = Ethernet << most common wired LAN.3. MAN 4. WAN(Wide Area Network): - large geographical area(country/continent).- consists of hosts and subnets owned and operated by different

people.- subnets components: transmission lines and switching elements.- when data arrive on an incoming line, the switching element must

choose an outgoing line on which to forward them.- routers connect different kinds of networking technology.Heterogeneous Networks The internet:

- people connected to one network often want to communicate with people attached to different one.- collection of interconnected networks is called an internetwork/internet.

- ISP(Internet Service Providers).- NAP(Network Access Points): complex switching stations(normally run by a third

party) which connects different national ISPs.- Peering Points: private switching stations which connects ISPs one to another.

Lecture 2: Network models: 1. OSI model: International Standards Organization(ISO) is a

multinational body dedicated to worldwide agreement on international standards and it covers all aspects of network communications is the Open System Interconnection(OSI).

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24/05/1439 AH- purpose of OSI model is to show how to facilitate communication between

two different systems without requiring change to the logic of the underling HW and SW.

- it’s flexible, robust and interoperable. - layer calls upon the services of the layer below it. eg:(3 uses 2).- between machines, layer x in machine 1 communicates with

layer x in machine 2, governed by an agreed-upon series of rules(protocols).

- each interface defines the information and services a layer must provide for the layer above it.

• Encapsulation: - at the sender device, in each layer, a header or a trailer can be

added to the data unit(payload). the trailer is added only at layer 2.- the data portion of a packet at level N-1 carries the whole packet(data and

header and maybe trailer) from level N. << this called encapsulation.- at the receiver device, each lower layer removes the header added by the

corresponding layer and sends the payload to the upper layer in a process called decapsulation.

• Layers in OSI mode: - 1,2,3 >> network support layers. - 5,6,7 >> user support layers. - 4 >> links two groups and ensure that what the lower layers have

transmitted is in a form that the upper layer can use. - the upper layers almost implemented in SW, lower layers are SW + HW,

except for the physical layer almost HW.

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1. Physical layer responsible for movement of individual bits from hop(node) to hop.

2. Data link layer

- responsible for moving frames from hop(node) to hop.- transforms physical layer to reliable link & makes it appear

error-free- other responsibilities: framing, physical addressing, flow

control, error control, access control.

3. Network layer

responsible for the delivery of individual packets from the source host to destination host(node with all layers). - if two systems are connected to the same link then no

need for network layer.- other responsibilities: logical addressing(IP address) and

Routing.

4. Transport layer

responsible for delivery of end users(source & destination).- other responsibilities: Service-point addressing,

segmentation and reassembly, connection control, flow control and error control.

5. Session layerresponsible for dialog control and synchronization.- it establishes, maintains and synchronizes the interaction

among communication.

6. Presentation layer responsible for translation, compression and encryption.

7. Application layerresponsible for providing services to the user. - specific responsibilities: Network virtual terminal, file

transfer, mail services, and directory services.

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2. TCP / IP protocol suite: - layers in the TCP/IP don’t exactly match those in the OSI model.- was defined as having four layers: host-to-network,

internet, transport and application. - However, when TCP/IP is compared to OSI, we can say

that the TCP/IP made of five layers: physical, data link, network, transport and application.

- it’s a Hierarchical protocol(it means each upper-level protocol is supported by one or more lower-level protocols.

- contains relatively independent protocols that can be mixed and matched depending on the needs of the system.

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1. Physical layerresponsible for carrying individual bits in a frame across the link in the form of either electrical or optical signals.- logical unit between two physical layers in two devices is a bit. - there are several protocols that transforms a bit to a signal.

2. Data link layer- the main functions are almost the same as of that for OSI model.- the logical unit between two data link layers in two devices is a frame.

3. Network layer

- the same main function of that in OSI model.- the logical unit between two network layers in two devices is a packet.- at the network layer supports the Inter-networking protocol(IP): • the transmission mechanism used by the TCP/IP protocols.• it’s an unreliable and connectionless protocol -a best- effort delivery service.• the term best effort means that IP provides no error checking or tracking.- IP uses four supporting protocols:• ARP(Address Resolution Protocol): is used to associate a logical address with a physical address.• RARP(Reverse Address Resolution Protocol): allows a host to discover its internet address when it knows only its

address.• ICMP(Internet Control Message Protocol): mechanism used by hosts and gateways to send notification of

datagram problems back to the sender + sends query and error reporting messages.• IGMP(Internet Group Message Protocol): is used to facilitate the simultaneous transmission of a message to a

group of recipients.

4. Transport layer

- represented int TCP/IP by: TCP and UDP.- IP is a host-to-host protocol, meaning that it can deliver a packet from one physical device to another.- the logical unit between two transport layers in two devices is a segment/datagram. - UDP/TCP are transport level protocols responsible for delivery of a datagram or segment from end-to-end(from the

source node to the destination node).1. Transmission Control Protocol(TCP): Advantages: - connection oriented; that first establishes a logical connection between transport layers at two hosts before

transferring data.- provides flow control, error, control, and congestion control.Disadvantages: - connection overhead- connection delay2. User Datagram Protocol(UDP): Advantages: - less overhead in the connection established.- lower delay in the connection established.Disadvantages: - it’s a connectionless protocol that transmits user datagrams without first creating a logical connection.3. Stream Control Transmission Protocol(SCTP): newly designed for applications emerging in the multimedia.

5. Application layer

- the logical unit between two application layers in two devices is a message.- Process(running program)-to-process communication is the duty of application layer.- many protocols are defined at this layer such as: HTTP, FTP, SMTP, DNS, TELNET.