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Data and Computer Communications. Chapter 1 – Data Communications, Data Networks, and the Internet. Ninth Edition by William Stallings. Data Communications, Data Networks, and the Internet.
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Data and Computer Communications Chapter 1 – Data Communications, Data Networks, and the Internet Ninth Edition by William Stallings
Data Communications, Data Networks, and the Internet “The fundamental problem of communication is that of reproducing at one point either exactly or approximately a message selected at another point” - The Mathematical Theory of Communication, Claude Shannon
1-1 DATA COMMUNICATIONS The term telecommunication means communication at a distance. The word data refers to information presented in whatever form is agreed upon by the parties creating and using the data. Data communications are the exchange of data between two devices via some form of transmission medium such as a wire cable.
Figure 1.2 Data flow (simplex, half-duplex, and full-duplex)
1-2 NETWORKS A network is a set of devices (often referred to as nodes) connected by communication links. A node can be a computer, printer, or any other device capable of sending and/or receiving data generated by other nodes on the network.
Figure 1.3 Types of connections: point-to-point and multipoint
Changes in Networking Technology • * Corporate WAN needs • * Digital electronics * Emergence of high-speed LANs
Convergence • The merger of previously distinct telephony and information technologies and markets • Layers: • applications • these are seen by the end users • enterprise services • services the information network supplies to support applications • infrastructure • communication links available to the enterprise
Benefits Convergence benefits include:
Capacity Transmission Lines Reliability The basic building block of any communications facility is the transmission line. Cost The business manager is concerned with a facility providing the required capacity, with acceptable reliability, at minimum cost. Transmission Line
Transmission Mediums Two mediums currently driving the evolution of data communications transmission are: Fiber optic transmissions and Wireless transmissions
Networking Advances in technology have led to greatly increased capacity and the concept of integration, allowing equipment and networks to work simultaneously.
Network Hardware Classifying networks based on their scale: • Local Area Networks • Metropolitan Area Networks • Wide Area Networks • Wireless Networks • Home Networks • Internetworks
LANs and WANs There are two broad categories of networks:
Wide Area Networks (WANs) • Span a large geographical area • Require the crossing of public right-of-ways • Rely in part on common carrier circuits • Typically consist of a number of interconnected switching nodes
Wide Area Networks Alternative technologies used include: • Circuit switching • Packet switching • Frame relay • Asynchronous Transfer Mode (ATM)
Circuit Switching • Uses a dedicated communications path • Connected sequence of physical links between nodes • Logical channel dedicated on each link • Rapid transmission • The most common example of circuit switching is the telephone network
Packet Switching • Data are sent out in a sequence of small chunks called packets • Packets are passed from node to node along a path leading from source to destination • Packet-switching networks are commonly used for terminal-to-terminal computer and computer-to-computer communications
Asynchronous Transfer Mode (ATM) • Referred to as cell relay • Culmination of circuit switching and packet switching • Uses fixed-length packets called cells • Works in range of 10’s and 100’s of Mbps and in the Gbps range • Data rate on each channel dynamically set on demand
Network Hardware Wireless Networks Categories of wireless networks: • System interconnection • Wireless LANs • Wireless WANs
Network Hardware Wireless Networks • System interconnection • Bluetooth a short-range wireless network. Allows system components together, digital cameras, headsets, scanners, and other devices to connect to a computer by merely being brought within range. • Wireless LANs • Every computer has a radio modem and antenna with which it can communicate with other systems. • Standard for wireless LANs: IEEE 802.11, which most systems implement and which is becoming very widespread.
Network Hardware Wireless Networks • Wireless WANs (cont.) • Wireless LANs an operate at rates up to about 50 Mbps over distances of tens of meters. While cellular systems (Wireless WANs) operate below 1 Mbps, but the distance between the base station and the computer or telephone is measured in kilometers rather than in meters. • High-bandwidth wide area wireless networks are also being developed (IEEE 802.16).
The Internet • Internet evolved from ARPANET • Developed to solve the dilemma of communicating across arbitrary, multiple, packet-switched network • TCP/IP provides the foundation
Figure 1.12 A heterogeneous network made of four WANs and two LANs
The Channel Allocation Problem To allocate a single broadcast channel among competing users, we can use: • Static Channel Allocation in LANs and MANs • Dynamic Channel Allocation in LANs and MANs
Static Channel Allocation in LANs and MANs Frequency Division Multiplexing (FDM) is an example of static channel allocation where the bandwidth is divided among a number of N users. When there is only a small and constant number of users, each of which has a heavy (buffered) load of traffic (e.g., carriers' switching offices), FDM is a simple and efficient allocation mechanism. However, when the number of senders is large and continuously varying or the traffic is bursty, FDM presents some problems. 1) when fewer than N users are currently interested in communicating, a large piece of valuable spectrum will be wasted. 2) when more users wants to communicate, those who have not been assigned a frequency will be denied permission. 3) even assuming that the number of users could somehow be held constant at N, each user traffic usually changes dynamically over time.
Multiple Access Protocols • ALOHA • Carrier Sense Multiple Access Protocols • Collision-Free Protocols • Limited-Contention Protocols • Wavelength Division Multiple Access Protocols • Wireless LAN Protocols
Pure ALOHA The basic idea of an ALOHA system is simple: let users transmit whenever they have data to be sent. There will be collisions, of course, and the colliding frames will be damaged. If the frame was destroyed, the sender just waits a random amount of time and sends it again. How the channel know that there is a collision: • Due to the feedback property of broadcasting, a sender can always find out whether its frame was destroyed by listening to the channel, the same way other users do. With a LAN, the feedback is immediate; with a satellite, there is a delay of 270 msec before the sender knows if the transmission was successful. • If listening while transmitting is not possible for some reason, acknowledgements are needed.
Pure ALOHA (2) In pure ALOHA, frames are transmitted at completely arbitrary times. The throughput of ALOHA systems is maximized by having a uniform frame size rather than by allowing variable length frames.
Other protocols Slotted ALOHA: It assumed the time is divided into discrete intervals. The station can send at the beginning of the next time interval whenever it have data ready after the start of the current time interval. 1- Persistent CSMA: When a station has data to send, it first listens to the channel to see if anyone else is transmitting at that moment. if the channel is idle, it start transmission. If the channel is busy, the station waits until it becomes idle. When the station detects an idle channel, it transmits a frame. If a collision occurs, the station waits a random amount of time and starts all over again. Nonpersistent CSMA: same as 1-persistent except that the station does not continually sense the channel when it finds it busy, rather it waits a random period of time and then sense the channel again. When the channel becomes idle it transmit. p-Persistent CSMA: same as Nonpersistent CSMA but the station transmit with probability p when the channel is idle.
Wireless LAN Protocols A system of notebook computers that communicate by radio can be regarded as a wireless LAN A common configuration for a wireless LAN is an office building with base stations (also called access points) strategically placed around the building. All the base stations are wired together using copper or fiber. A simplifying assumption that all radio transmitters have some fixed range will be used follow. When a receiver is within range of two active transmitters, the resulting signal will generally be garbled and useless.
