1_1_network

1_1_network

1. is set of devices (often referred to as nodes)

   • connected by communication links
     • is the physical path by which message travels from sender to receiver.
       • example
         • fiber-optic cable
         • copper wire
         • air carrying radio waves
   • A node can be a computer, telephone, or any other device capable of sending and receiving messages.

2. Introduction

   • The Networking Problem
     • “Noise” damages (corrupts) the messages
     • Establishing and maintaining physical communication
     • The visible network variables
       • Delivery
         • The network must deliver data to the correct destination. Data must be received only by the intended recipients and not by others.
       • Correctness:
         • Data must be delivered accurately
       • Timeliness
         • Data must be delivered before they need to be put to use; else, they would be useless.
     • Different network topologies
       • Connection topology
         • distributed-topology network
           • resembles a fisherman’s net, has the greatest resilience to element (node or link) failures.
         • actual topology
           • the Internet topology evolved in a “self-organizing” manner.
         • decentralized-topology network
         • distributed topology
     • the connections between the major Internet Service Providers
       • Component characteristics
         • reliability
         • performance of individual hardware components (nodes and links)
       • Performance metrics
         • success rate of transmitted messages or average delay of message delivery
       • Different applications (data/voice/multimedia) have different requirements for network service in order to be useful to human users:
         • some applications are impaired by message loss and others are impaired by delay or jitter
     • Digital signal distortion
       • Heterogeneity
         • Diverse software and hardware of network components need to coexist and interoperate
       • Autonomy
         • Different parts of the Internet are controlled by independent organizations
       • Scalability
         • a global network like the Internet consists of many autonomous domains, there is a need for standards that prevent the network from becoming fragmented into many noninteroperable pieces (“islands”).
   • Communication Links
     • The input pulses must be well separated because too short pulses will be “smeared” together. This can be observed for the short duration pulses at the right-hand side of the pulse train. Obviously, the receiver of the signal in the bottom row will have great difficulty figuring out whether or not there were pulses in the transmitted signal. You can also see that longer pulses are better separated and easier to recognize in the distorted signal. The minimum tolerable separation depends on the physical characteristics of a transmission line . If each pulse corresponds to a single bit of information, then the minimum tolerable separation of pulses determines the maximum number of bits that can be transmitted over a particular transmission line. This number limits the data rate of a communication link (or transmission rate, or transmission capacity), which is the rate at which the network interface accepts data bits. The data rate of a link is measured in the units of bits per second (bps)
       • example
         • copper vs. optical fiber
       • An important attribute of a communication link is how many bitstreams can be transmitted on it at the same time. If a link allows transmitting only a single bitstream at a time, then the nodes connected to the link must coordinate their transmissions to avoid different bitstreams corrupting each other (known as data collision).
         • Point-to-point links often support data transmissions in both directions simultaneously. This kind of a link is said to be full duplex.
           • A point-to-point link that supports data flowing in only one direction at a time is called half duplex.
             • the nodes on each end of this kind of a link can both transmit and receive, but not at the same time—they only can do it by taking turns. It is like a one-lane road with bidirectional traffic.
               • In TDM or FDM, the communication link transmission capacity is subdivided into m portions and each portions is assigned to a different logical stream of bits. In FDM, the channel bandwidth W is subdivided into m channels, each with bandwidth W/m (actually slightly less, to allow for guard bands between channels)
           • A full-duplex link can be implemented in two ways: either the link must contain two physically separate transmission paths, one for sending and one for receiving, or the capacity of the communication channel is divided between signals traveling in opposite directions.
     • Wireless Link
       • The received signal strength decreases exponentially with the senderreceiver distance. As any other communication medium, the wireless channel is subject to thermal noise, which distorts the signal randomly
         • that the interference of simultaneously transmitting sources never disappears—it only is reduced exponentially with an increasing mutual distance
   • Packets and Statistical Multiplexing
     • In computer communication networks, messages are represented as strings of binary symbols (0or 1), known as bits. Generally, messages are of variable length and some of them may still be considered too long for practical network transmission. There are several reasons for imposing a limit on message length. One is that longer messages stand a higher chance of being corrupted by an error
       • Another reason is to avoid the situation where a sending application seizes the link for itself by sending very long messages while other applications must wait for a long time.
   • Communication Protocols
     • is a set of rules agreed-upon by interacting entities protocols serve as mechanisms by which organizations or individuals manage the complexity of articulating their work
       • An important characteristic of protocols is that the units of communication are data packets. Each data packet consists of a header that contains the packet guidance information to help guide the packet from its source to its destination, and the payload, which is the user information to be delivered to the destination address.