1.1 Data Communication Components Data Representation Direction of Data Flow
Figure 1.1 Five components of data communication 1.Message – information (data) to be communicated. i.e. text, numbers, pictures, sound, or video (or combination of these) 2.Sender – device that sends the data message. i.e. computer, workstation, phone, video camera and so on. 3.Receiver - device that receives the message. i.e. computer, workstation, phone, video camera and so on. 4.Medium – the physical path by which a message travels from sender to receiver. i.e twisted pair, coaxial, fiber, radio waves and so on 5.Protocol – a set of rules (agreement between the communicating device) that governs data communications.
Text Represented as a bit pattern, a sequence of bits (0s or 1s). i.e. (A,B,…..,Z) uppercase letter, (0,1,2,…,9) numeric characters and so on. ASCII – 7 bits for each symbol Extended ASCII Unicode ISO Numbers Images Audio Video
Figure 1.2 Simplex unidirectional (as on a one-way street). Only one of the two devices can transmit, the other can only receive. i.e. Keyboard and traditional monitors (simplex devices) Half-Duplex Direction of Data Flow 1.Simplex 2.Half-Duplex 3.Full-Duplex
Figure 1.3 Half-duplex -Each station can transmit and receive, but not at the same time -Analogy: one-lane road with two-directional traffic -i.e. walkie-talkies
Figure 1.4 Full-duplex -Also called duplex -Both station can transmit and receive simultaneously -Analogy: two-way street with traffic flowing in both directions at the same time -Sharing occur in two ways: 1. the link contain 2 separate transmission path, 2. capacity of the channel divided between signals traveling in both direction
1.2 Networks Issues to be discussed: Distributed Processing task divided among multiple computers Network Criteria Performance, reliability and security Physical Structures Refer to type of connection and physical topology Categories of Networks LAN, MAN, and WAN A set of devices (nodes) connected by communication links.
Figure 1.5 Point-to-point connection- provide dedicated link between 2 devices. The entire capacity of the link is reserved for transmission between those 2 devices. Type of Connection In general there are 2 types of connections: 1.Point-to-Point 2.Multipoint Q: List an example of point-to-point connection
Figure 1.6 Multipoint connection -Also called ‘multidrop’ connection ->2 devices share a single link -The capacity of the channel is shared (spatially or temporally) -If several devices used the link simultaneously, it is a spatially connection. -If users must take turns, it is a timeshare connection.
defines the physical or logical arrangement of links in a network Topology of network - geometric representation of the relationship of all links and nodes Five topology describes how devices in a network are interconnected rather then their physical arrangement
each node has a dedicated point-to-point link to every other node For n devices, physical channels = n(n-1)/2 and devices I/O ports = (n-1) input/output ports
Advantages Provides dedicated links – where each connection can carry its own data load – eliminating traffic problems Robust - if one link becomes unusable, it does not incapacitate the entire system Privacy and security- a dedicated line guarantees security Point-to-point links - easy for fault identification and fault isolation Disadvantages the amount of cabling and number of I/O ports required expensive
Each device has a dedicated point-to-point link only to a central controller (or hub). Does not allow direct traffic between devices as in Mesh Controller acts as an exchange to relay data
Advantages only one link and one I/O port required per device less expensive than mesh topology Robustness - if one link fails, only that link is affected, also lends to easy fault identification and fault isolation easy fault identification and fault isolation (as long as the hub is working) Disadvantages if the hub fails, the network is down requires more cabling than tree, ring and bus topologies
Uses multipoint configuration One long cable (the backbone) - links all the devices in the network Drop line – connection between the device and the main cable Tap – a connecter that creates contact with the metallic core
Advantages easy installation –backbone cable can be connected to the nodes by drop lines of various lengths. Therefore using less cabling than mesh, star or tree Disadvantages difficult reconfiguration – difficult to add new devices as bus topology is usually designed optimally efficient fault isolation - a fault or break in the bus cable stops all transmission
Dedicated point-to-point configuration between two devices on either sides Signal will pass along devices in the ring in one direction until it reaches the destination
Advantages each device incorporates a repeater easy to install and reconfigure – to add or delete requires moving only two connections – constraints are media and traffic considerations (max ring length and number of devices) Fault isolation is simplified – one device does not receive signal in specified period, it can issue an alarm – which contains the problem and location Disadvantages in unidirectional traffic, a break in the ring can disable the entire network (some implementations use a dual ring or a switch to solve this problem)
combination of several topologies as subnetworks linked together in a larger topology. Figure 1.9 A hybrid topology: a star backbone with three bus networks
Determined by its size, ownership, distance covered and physical architecture
Usually privately owned and links the devices in a single office, building or campus LAN are design to allow resource to be shared between devices Etc h/w (printer), s/w (application program) or data Figure 1.10 An isolated LAN connecting 12 computers to a hub in a closet
LAN usually uses one type of transmission medium Size limited to a few kilometers Common topologies are bus, ring and star High data rate – 4/10/16 Mbps to 100 Mbps or gigabits
long-distance transmission over large geographical areas (continent/world) utilize public, leased or private communication in combinations An enterprise network is a WAN owned by one company
1. 27 Figure 1.11 WANs: a switched WAN and a point-to-point WAN
Extend over entire city, may be wholly owned and operated by a private company, popular service SMDS (Switch Multi- Megabit Data Services)
internet – network of networks Individual networks are joined into internetworks using internetworking devices
1. 30 Figure 1.12 A heterogeneous network made of four WANs and two LANs
1.3 The Internet The most notable internet is called the Internet (uppercase letter I), a collaboration of more than hundreds of thousands of interconnected networks. The Internet has revolutionized many aspects of our daily lives. It has affected the way we do business as well as the way we spend our leisure time. The Internet is a communication system that has brought a wealth of information to our fingertips and organized it for our use.
1958 – After USSR launches Sputnik, first artificial earth satellite, US forms the Advanced Research Projects Agency (ARPA), the following year, within the Department of Defense (DoD) to establish US lead in science and technology applicable to the militaryARPA 1967 – ARPANET, a small network of connected computers. 1969 – ARPANET was born. 4 nodes were inter- connected: UCLA,SRI,UCSB, U. of Utah. Charley Kline from UCLA sent 1 st packet. First RFC by Steve Crocker - 32
1972-1974 – Robert Kahn and Vint Cerf develop protocols to connect networks without any knowledge of the topology or specific characteristics of the underlying nets 1972 – Robert Kahn gives first public demonstration of ARPAnet (now 15 nodes) at International Conference on Computer Communication - 33
1974 – First full draft of TCP produced November 1977 - First three-network TCP/IP based interconnection demonstrated linking SATNET, PRNET and ARPANET in a path leading from Menlo Park, CA to Univ. College London and back to USC/ISI (Marina del Ray, CA) 1978 – TCP split into TCP and IP - 34
Today most end users who want Internet connection use the services of Internet service providers (ISPs). International ISP National ISP Regional ISP Local ISP
1. 36 Figure 1.13 Hierarchical organization of the Internet
1.4 Protocols and Standards Protocol defines what, how and when it is communicated. Key elements are: Syntax: structure of format of data Syntax: structure of format of data Semantics: meaning of each section of bits Semantics: meaning of each section of bits Timing: two characteristics – when and how fast data can be sent. Timing: two characteristics – when and how fast data can be sent.
Standards Creation Committees: International Organization of Standardization (ISO) International Telecommunication Union- Telecommunication Standards Sector (ITU-T) American National Standards Institute (ANSI) Institute of Electrical and Electronics Engineers (IEEE) Electronic Industries Association (EIA) Forums Regulatory Agencies
An Internet standard is a thoroughly tested specification that is useful to and adhered to by those who work with the Internet. An Internet draft is a working document (a work in progress) with no official status and a 6- month lifetime. Upon recommendation from the Internet authorities, a draft may be published as a Request for Comment (RFC)
3 March, 200442 P2P configuration can make systems vulnerable Bandwidth problems
3 March, 200443 Dependent on a (reasonably) reliable network connection Single point of failure for network and voice communications Both parties must have VoIP capability to talk Integration difficulties