Computer Network Fundamentals (IT 221)

Computer Network Fundamentals (IT 221) أساسيات شبكات الحاسب Business Applications: Lecture 1: Computer network: collection of autonomous computers interconnected by a single technology. 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: Mobile Users: - 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. - combinations of wireless networks and mobile computing. example in PDA > 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: 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) 1

2. Networks: set of devices(often referred to as nodes) connected by communication links. (node = printer/computer ) Network criteria: - performance: depends on; a. number of users b. type of transmission media c. 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: 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 simultaneously b. temporally: time shared connection B. topology: the way in which network is laid out physically. 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. 1. privacy/security 2. Eliminating traffic problems 3. it s robust 4. fault identification and isolation 1. installation and reconnection difficulty 2. 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 1. easy to install, cheaper 2. robustness 3. Identification and fault isolation 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. 1. ease of installation, use less calming than mesh or star. 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. 1. easy to install and reconfigure. 2. fault isolation is simplified. 1. unidirectional traffic - another kind: Dual-ring topology 2

Hybrid topology: Network categories -two primary categories-: - local-area networks - wide-area networks categories: 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). 3

- 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. 1. Physical layer responsible for movement of individual bits from hop(node) to hop. - responsible for moving frames from hop(node) to hop. - transforms physical layer to reliable link & makes it appear 2. Data link layer 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 layer responsible for dialog control and synchronization. - it establishes, maintains and synchronizes the interaction among communication. 6. Presentation layer responsible for translation, compression and encryption. 7. Application4layer responsible for providing services to the user. - specific responsibilities: Network virtual terminal, file transfer, mail services, and directory services.

1. Physical layer 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. responsible 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 3. Network layer 4. Transport layer 5. Application 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. - 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. - 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 delay 2. 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. - 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. 5