Data Communication

DATA COMMUNICATION

Definition • Two or more computers connected via a communications medium OR • The hardware and software that enable the user to access a remote device

Why Have Data Communication? Communication between computers allows: • Sharing of resources • Distribution of processing functions • Compatibility of dissimilar equipment and software • Increased performance with reduced cost.

Signal Types Message may take many forms in data communications, • • • • •

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e.g. a user file containing graphics, text, or sound data, machine generated request, or status message. Senders and receivers may be humans using a computer or a variety of programmable devices Medium used can be space, telephone wire, optic fiber etc. Encoders translate message into signals suitable for medium and back again for receiver (e.g. modem).

Data is usually transmitted using one of three types of signals: • • • •

An electrical voltage (co-axial cable, wire); An electromagnetic wave (satellite, wireless LAN) Light pulses (optic fiber). E.g. phone line carries analog electrical voltage signals to represent sound.

Protocols/Standards

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HTTP (Hypertext Transfer Protocol) HTTP is a request/response standard between a client and a server. A client is the end-user, the server is the web site. The client making an HTTP request - using a web browser, spider, or other end-user tool - is referred to as the user agent The responding server - which stores or creates resources such as HTML files and images - is called the origin server. In between the user agent and origin server may be several intermediaries, such as proxies, gateways, and tunnels. HTTP is not constrained to using TCP/IP and its supporting layers, although this is its most popular application on the Internet. Indeed HTTP can be "implemented on top of any other protocol on the Internet, or on other networks. HTTP only presumes a reliable transport; any protocol that provides such guarantees can be used.“

• Typically, an HTTP client initiates a request. It establishes a Transmission Control Protocol (TCP) connection to a particular port on a host (port 80 by default; see List of TCP and UDP port numbers). An HTTP server listening on that port waits for the client to send a request message. Upon receiving the request, the server sends back a status line, such as "HTTP/1.1 200 OK", and a message of its own, the body of which is perhaps the requested file, an error message, or some other information.

• FTP (File Transfer Protocol) • FTP is a file transfer protocol for exchanging and manipulating files over a TCP computer network. • FTP runs exclusively over TCP. It defaults to listen on port 21 for incoming connections from FTP clients. A connection to this port from the FTP Client forms the control stream on which commands are passed to the FTP server from the FTP client and on occasion from the FTP server to the FTP client. FTP uses out-of-band control, which means it uses a separate connection for control and data. Thus, for the actual file transfer to take place, a different connection is required which is called the data stream. Depending on the transfer mode, the process of setting up the data stream is different.

Security Problem -FTP •

The original FTP specification is an inherently insecure method of transferring files because there is no method specified for transferring data in an encrypted fashion. This means that under most network configurations, user names, passwords, FTP commands and transferred files can be "sniffed" or viewed by anyone on the same network using a packet sniffer. This is a problem common to many Internet protocol specifications written prior to the creation of SSL such as HTTP, SMTP and Telnet. The common solution to this problem is to use either SFTP (SSH File Transfer Protocol), or FTPS (FTP over SSL), which adds SSL or TLS encryption to FTP as specified in RFC 4217

TCP/IP •

The Internet Protocol Suite (commonly TCP/IP) is the set of communications protocols used for the Internet and other similar networks. It is named from two of the most important protocols in it: the Transmission Control Protocol (TCP) and the Internet Protocol (IP), which were the first two networking protocols defined in this standard. Today's IP networking represents a synthesis of several developments that began to evolve in the 1960s and 1970s, namely the Internet and LANs (Local Area Networks), which, together with the invention of the World Wide Web by Tim Berners-Lee in 1989, have revolutionized computing.

Layers In TCP/IP • The TCP/IP suite uses encapsulation to provide abstraction of protocols and services. Such encapsulation usually is aligned with the division of the protocol suite into layers of general functionality. Generally an application (the highest level of the model) uses a set of protocols to send its data down the layers, being further encapsulated at each level.

Implementation of TCP •

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Today, most operating systems include and install a TCP/IP stack by default. For most users, there is no need to look for implementations. TCP/IP is included in all commercial Unix systems, Mac OS X, and all free-software Unix-like systems such as Linux distributions and BSD systems, as well as all Microsoft Windows operating systems. Unique implementations include Lightweight TCP/IP, an open source stack designed for embedded systems and KA9Q NOS, a stack and associated protocols for amateur packet radio systems and personal computers connected via serial lines.

SMTP •

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SMTP is a relatively simple, text-based protocol, in which one or more recipients of a message are specified (and in most cases verified to exist) along with the message text and possibly other encoded objects. The message is then transferred to a remote server using a procedure of queries and responses between the client and server. SMTP is a "push" protocol that cannot "pull" messages from a remote server on demand. To retrieve messages only on demand, which is the most common requirement on a single-user computer, a mail client must use POP3 or IMAP. Another SMTP server can trigger a delivery in SMTP using ETRN. It is possible to receive mail by running an SMTP server. POP3 became popular when single-user computers connected to the Internet only intermittently; SMTP is more suitable for a machine permanently connected to the Internet.

Limitations SMTP •

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One of the limitations of the original SMTP is that it has no facility for authentication of senders. Therefore the SMTP-AUTH extension was defined. However, the impracticalities of widespread SMTP-AUTH implementation and management means that E-mail spamming is not and cannot be addressed by it. Modifying SMTP extensively, or replacing it completely, is not believed to be practical, due to the network effects of the huge installed base of SMTP. Internet Mail 2000 was one such proposal for replacement.

Bits, Bytes and Packets Another important role of protocols is to determine format of data being sent, so sender & receiver agree on meaning of stream of bits. Message may be sent as single bytes, complete message, or (most commonly) message broken up into pieces called packets, with exact format determined by protocol. Each packet contains header information such as source & destination address, amount of data, error-checking code.

Bandwidth Bandwidth determines amount of data that can be transmitted per unit time. Greater bandwidth = greater possible data transmission rate. Available bandwidth varies between different media, e.g. coaxial cable, optic fiber, satellite etc all have different bandwidth characteristics. Each media can only transmit signals with certain ranges of frequencies. Higher the central frequency, greater the possible frequency range, & greater potential bandwidth of the media.

Transmission Impairments • Final signal received differs from the one sent because of impairments to transmission, possibly resulting in errors. Impairments are: 1 Noise - caused by various factors: thermal, crosstalk, and impulse. 2 Attenuation - signal strength decreases over distance. Corrected by use of repeaters (digital) and amplifiers (analog).

Communication Modes • Elementary types of data flow over a transmission path. a. Simplex - only allows information flow in one direction. Devices connected to such a circuit are either a send only or receive only device. For example, a data collection terminal on a factory floor or a line printer. - e.g. host to dumb terminal, radio & TV broadcast

Half Duplex - allows information flow in both directions, but only one at a time. It requires two wires this is the most common type of transmission for voice communication because only one person is supposed to speak at a time. It is also used to connect a terminal with a computer. The terminal might transmit data and then the computer responds with an acknowledgement. E.g. press-to-talk radio systems, older modems. Sender------------------------------------------------------------------> Receiver OR <------------------------------------------------------------------

Full Duplex - allows simultaneous information flow in both directions. This involves a special switching circuit requires a small amount of time is approximately 150 milliseconds. With high-speed capabilities of computer, this turnaround time is unacceptable in many instances. Also some applications require simultaneous transmission in both the directions. In such cases, a full duplex system is used that allows information to flow simultaneously in both directions on the transmission path. It requires four wires. E.g. (common in computer communications), telephone. Sender-------------------------------------------------------------------> Receiver AND <------------------------------------------------------------------

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