Comp247-notes - Summary Data Communications PDF

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This is the summary notes of COMP247, Semester I, 2018. ...

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COMP 247 – DATA COMMUNICATIONS WEEK 1: INTRO TO DAT A COM MUNIC AT ION S

WHY N ET WOR KS AR E IM PORT ANT

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It is a necessity to have a network for every business, organisation. Modern organisations rely on the efficient transmission of data Enables distributed systems, real-time communication, e-commerce, and the Web

Data communications is the movement of computer information’s from one point to another by means of electrical or optical transmission systems •

facilitate more efficient use of computers and improve the day-to-day control of a business by providing faster information flow.

This contrasts with the broader term Telecommunications, which includes the transmission of voice and video (images and graphics) as well as data and usually implies longer distances. BASIC N ETWOR K COMP ONENTS

There are three basic hardware components for a data communications network: • Client is a user device to access network and receive data from server – e.g., desktops, laptops, tablets, cell phones, etc. • Server is a device that stores and transmits data to a client – e.g., Web server, mail server, file server • Circuit is a pathway or connection between client and server – e.g., copper wire, fibre optic cable, wireless Both the server and client also need special-purpose network software that enables them to communicate. • •

A switch, like a hub, is a device that connects a number of computers together to make a LAN. Most switches operate at the data link layer. The router is a special device that connects two or more networks. The router enables computers on this network to communicate with computers. Routers operate at the network layer. A Router is concerned with forwarding a message to its final destination – it is thus a long-distance device

Such networks are called peer-to-peer networks because the computers function as equals, rather than relying on a central server or host computer to store the needed data and software. 1|P age

TYPES OF NETWO RKS

• Local Area Networks (LANs) – covers a clearly defined small area, such as one floor or work area, a single building, or a group of buildings. LANs often use shared circuits, where all computers must take turns using the same circuit. • Backbone Networks (BNs) – most LANs are connected to a BN, a larger, central network connecting to several other networks and may span hundreds of feet to several miles • Wide Area Networks (WANs) – is a computer network that covers relatively larger geographical area such as a state, province or country. It provides a solution to companies or organizations operating from distant geographical locations who want to communicate with each other for sharing and managing central data or for general communication. Most organizations lease circuits and use those to transmit their data. Two other common terms are intranets and extranets. • Intranet An intranet is a LAN that uses the same technologies as the Internet (e.g., Web servers, Java, HTML) but is open to only those inside the organization. – Open only those inside the organization – e.g., employees accessing budgets, calendars • Extranet An organization may use an extranet to provide secure and safe access to individuals who work for different organizations but require company data. – e.g., A company providing access to outside suppliers/contractors NETWOR K MODE LS

Network Models divide communication functions into layers. The two most important network models are the Open Systems Interconnection Reference (OSI) model and the Internet model. • Open Systems Interconnection Reference Model (OSI model) -

Created by International Standards Organization (ISO) as a framework for computer network standards in 1984 Based on 7 layers

• Internet Model -

the Internet model collapses the top three OSI layers into one layer. Based on 5 layers Based on Transmission Control Protocol/ Internet Protocol (TCP/IP) suite 2|P age

MESSAG E TR AN SMISSI ON US ING LAYER S

Each computer in the network has software that operates at each of the layers and performs the functions required by those layers. Each layer in the network uses a formal language, or protocol, that is simply a set of rules that define what the layer will do and that provides a clearly defined set of messages that software at the layer needs to understand. -

For example, the protocol used for Web applications is HTTP.

• Protocol Data Unit (PDU) contains information that is needed to transmit the message through the network. – Each layer adds a PDU – PDUs act like nested envelopes – Encapsulation occurs when a higher level PDU is placed inside of a lower level PDU

The figure shows how a message requesting a Web page would be sent on the Internet.

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Advantages of Layers -

Networking functionality is modular and the software/hardware at any layer can be more easily substituted Easier to troubleshoot or make changes to one layer at a time Application developers only need to worry about the application layer in their programs

Disadvantages of Layers -

Inefficient because the encapsulation/de-encapsulation at each layer requires processing Inefficient because encapsulation in a PDU increases overhead at each layer

NETWOR K ST AN DARDS

Standards ensure that hardware and software from different vendors work together and “speak the same language”. The use of standards makes it much easier to develop software and hardware that link different networks because software and hardware can be developed one layer at a time. There are two types of standards: formal and de facto. De jure standards The formal standardization process has three stages: specification, identification of choices, and acceptance. – Formalized by an industry or government body De facto standards – In the communications industry, de facto standards often become formal standards once they have been widely accepted. •

For example, Microsoft Windows is a product of one company and has not been formally recognized by any standards organization, yet it is a de facto standard.

There are many different standards used in networking today. The figure outlines some of the most commonly used data communication standards. For a network to operate, many different standards must be used simultaneously. The sender of a message must use one standard at the application layer, transport layer, network layer, data link layer, and the physical layer. Each layer and each standard is different, but all must work together to send and receive messages.

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The application layer is the application software used by the network user. The transport layer takes the message generated by the application layer and, if necessary, breaks it into several smaller messages. The (PDU) at the transport layer is called a segment. The network layer addresses the message and determines its route through the network. The data link layer formats the message to indicate where it starts and ends, decides when to transmit it over the physical media, and detects and corrects any errors that occur in transmission. The physical layer is the physical connection between the sender and receiver, including the hardware devices (e.g., computers, terminals, and modems) and physical media (e.g., cables and satellites). Each layer, except the physical layer, adds a protocol data unit (PDU) to the message.

TREN DS

Bring your own device (BYOD) – Huge demand for employees to connect their personal smartphones, tablets, and other devices to organizational networks

The Internet of Things – Everything connects to the network – Networks need to support the increased demands of these devices

IMPLIC ATI ON S FOR M AN AG EM EN T

• Networks and the Internet change (almost) everything • Today’s networking is driven by standards • As network demand increases, so will storage demands

WHAT I S A N ETWOR K AN D WH Y WE HA VE THEM ?

There are many ways to describe and analyse data communications networks. All networks provide the same basic functions to transfer a message from sender to receiver, but each network can use different network hardware and software to provide these functions. A network consists of two or more computers that are linked to share resources, exchange files, or allow electronic communications. The computers on a network may be linked through cables, telephone lines, radio waves, satellites, or infrared light beams. Computer system is a memory hierarchy

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Basic problem is to get data to where it will be used Efficiency based on data being near processor

So, we have distribution mechanisms – memory sticks, portable disk drives,

Memory hierarchy is an implementation issue Applications should not worry about whether data comes from registers, cache, RAM, or virtual memory nor whether data comes from tape or over network •

Computational models abstract these implementation details away

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Programmer is only concerned with variables abstract away hierarchy

We want the fact that different network technologies are used to be transparent to the user and application. Advances in technology are enabling us to consolidate these different kinds of networks onto one platform, referred to as the converged network. Unlike dedicated networks, converged networks can deliver voice, video streams, text, and graphics between many different types of devices over the same communications channel and network structure. -

provides access to a wide range of alternative and new communication methods that enable people to interact directly with each other almost instantaneously. This network infrastructure uses the same set of rules, agreements, and implementation standards.

Traditional Networks (Left) and Converged Network (Right)

Circuit-switched networks are connection oriented. This means that before communication can take place a circuit needs to be established. For example, a landline telephone call establishes the circuit when you dial the number and when the receiver answers. That number is the destination of the call and is only needed in call establishment. This circuit is then exclusively used for that call.

With a packet switch network, no connections need to be made. Each packet is placed in the network – the important point is that each packet must carry the destination address with it. Packet-switched networks make more efficient use of resources. Packet switching is more flexible 6|P age

and uses modern technologies. Telephone calls can be sent over a packet-switched network. Thus, we have convergence of these technologies, which is everything becoming packet switched.

PROT OCO L ST AC K

Because the protocols are used at different layers and are stacked on top of one another, the set of software used to understand the different protocols is often called a protocol stack. -

Each layer specifies a different protocol for handling a function or subsystem of the communication process.

3 important points -

There are many protocols in networks. Protocols are arranged in a layered fashion. Several protocols may work together in a protocol stack.

Transport and Network Layer Protocols – TCP/IP are the standard transport and network protocols used today. They perform addressing (finding destination addresses), routing (finding the “best” route through the network), and segmenting (breaking large messages into smaller packets for transmission and reassembling them at the destination).

Addressing – Before you can send a message, you must know the destination address. It is extremely important to understand that each computer has several addresses, each used by a different layer. One address is used by the data link layer, another by the network layer, and still another by the application layer.

Types of addresses

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ADDRE SSES IN TH E INT ER NET

In the internet, this is a called an IP address – IP stands for Internet Protocol •

IP is the network layer protocol for the internet, responsible for addressing and delivery of messages

IPV4 A DDRESS ES

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32 bit (4 byte) addresses In the computer stored in binary, but for human convenience written in decimal A byte can hold values in the range 0-255 Each byte in an IP address written separately, so an example IP address is  127.97.201.4

SECURI TY

Securing a network infrastructure includes physically securing devices that provide network connectivity, and preventing unauthorized access to the management software that resides on those devices. In order to achieve the goals of network security, there are three primary requirements: •

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Ensuring confidentiality: only the intended and authorized recipients can access and read data. This is accomplished by having a strong system for user authentication, enforcing passwords that are difficult to guess, and requiring users to change their passwords frequently. Maintaining communication integrity: ensure information has not been altered in transmission, from origin to destination. Data integrity is made possible by requiring validation of the sender and by using mechanisms to validate that the packet has not changed during transmission. Ensuring availability: means having the assurance of timely and reliable access to data services for authorized users. Network firewall devices, along with antivirus software, can ensure system reliability and the robustness to detect, repel, and cope with such attacks.

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WEEK 2: THE IN TERNET

THE INT ERNET

The Internet is the most used network in the world, the Internet is a network of networks—a set of separate and distinct networks operated by various national and state government agencies, nonprofit organizations, and for-profit corporations. The Internet exists only to the extent that these thousands of separate networks agree to use Internet protocols and to exchange data packets among one another. All networks that connect to the Internet must rigidly conform to an unyielding set of standards for the transport and network layers; without these standards, data communication would not be possible.

THE W ORL D-WI DE WEB (WWW)

The WWW provides a graphical users interface and enables the display of rich graphical images, pictures, video, and sound clips over the Internet. The internet is the transport mechanism that enables the WWW and the other services. In order for the requests from the Web browser to be understood by the Web server, they must use the same standard protocol. The standard protocol for communication between a Web browser and a Web server is Hypertext Transfer Protocol (HTTP).

INTERN ET ARCHIT EC TU RE

Basic Internet architecture. ISP = Internet service provider; MAE = metropolitan area exchange; NAP = network access point

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Internet service providers (ISPs) connect the networks of their customers to the Internet. NAPs were originally designed to connect only national ISPs. These National ISPs (Tier 1 ISPs) provide services for their customers and to Regional ISPs (Tier 2 ISPs). These regional ISPs rely on the national ISPs to transmit their messages to national ISPs in other countries. Regional ISPs, in turn, provide services to their customers and to Local ISPs (Tier 3 ISPs) who sell Internet access to individuals. –

ISPs are autonomous systems and share routing info using BGP

ISP backbone Larger backbone connections operating at 10 gbps and experimenting with 40 Gbps and 160 Gbps – –

Requires faster backbone switches and routers Internet peak traffic estimated to reach 1 Pbps by 2018

CONN ECT IN G TO AN ISP

A point of presence (POP) is the location where an ISP provides service to its customers. The pop connects to the rest of the ISP’s network. This connection may be via a dial-up modem over a telephone line or via a higher-speed circuit such as DSL, cable modem, or a T1. –

Authentication is performed at the POP.

INTERN ET ACCES S T EC HNOL OG IE S

Internet access technologies cannot be used for general-purpose networking from any point to any point. •

Some organisations use WAN technologies to connect to their ISP

Common broadband technologies to connect to ISPs include: – – – –

Digital subscriber line (DSL) Cable Fibre to the home Wireless

Digital Subscriber Line (DSL) is a family of point-to-point technologies designed to provide highspeed data transmission over traditional telephone lines. The reason for the limited capacity on traditional telephone circuits lies with the telephone and the switching equipment at the end offices. – –

The actual cable in the local loop from a home or office to the telephone company end office can provide much higher data transmission rates. Data Link: PPP (Point to Point Protocol)

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Many DSL technologies exist, but the most commonly implemented include: •

Asymmetrical DSL o 3 channels of different width o Faster download than upload o Bandwidth dependent on distance from equipment

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Very-high-data-rate DSL o Like Adsl, but with higher data rates and shorter range o Often paired with fibre circuits to the node o May be used for HD TV in addition to data and voice transmission

Cable Typically offered by cable television service providers, the Internet data signal is carried on the same coaxial cable that delivers cable television. It provides a high-bandwidth, always-on connection to the Internet. With cable modems, each user must compete with other users for the available capacity. -

Data Over Cable Service Interface Specifications (DOCSIS)

Fibre to the Home -

A dedicated point-to-point fibre optic service Architecturally similar to DSL and cable Expensive to deploy and these networks are emerging slowly

Wireless The most popular types of fixed wireless are WiMax and wireless DSL, which requires a line of sight between the communicating transmitters. WiMax is sometimes called the new 4G mobile Internet. It works similarly to Wi-Fi, in that it enables mobile users to connect into the Internet at speeds of 40 to 70Mbps. LTE (Long Term Evolution) is what is being deployed as “4G”

Satellite dishes require a clear line of sight to the satellite, so satellite service might not be an option in heavily wooded areas or places with other overhead obstructions. The availability of satellite Internet access is a real benefit in those areas that would otherwise have no Internet connectivity at all.

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INTERN ET GOVER NAN CE

Because the Internet is a network of networks, no one organization operates the Internet. Internet Society (ISOC) is a non-profit organization founded in 1992 to provide leadership in Internet-related standards, education, access, and policy. Standards are developed through four related organizations governed by ISOC.

BUILDIN G THE FUT UR E IN TE RN ET

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The process of creating the backbone to allows higher connection speed to happen NBN will enables us to have a better connection than better before Besides providing very high-speed Internet connections, these networks are intended to experiment with new protocols that one day may end up on the future Internet.

BIG CH AL LENGES F OR M A NAG EM EN T

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The intern...