Computer Network

24 flashcards · Computer Science · by Howard Su

Purpose of Computer Network?
_ enable communicate among the participants _ resource sharing: different tasks to different computers; avoid bottlenecks _ resource pooling: combine resources and functionalities of multiple machines _ resource balancing: increase availability of the services by redundancy
Required Components for Computer Network?
_ > 1 computers with network services running => communicate or access shared resources; computers = hosts _ transmission medium to send/receive data: wire, air (wireless data transmission) _ network protocols: rules specifying how computers can communicate
Network Services?
_ provide resources _ server: provides network service _ client: uses network service _ being both server and client = peer ** terms apply only to network services, not hardware
Transmission Media?
_ guided transmission media: copper cable (electrical impulses); fiber-optic cable (light impulses) _ wireless transmission: + directed: radio technology (electromagnetic waves in radio frequency spectrum); infrared (electromagnetic waves in spectral range); laser (light impulses) + undirected: based on radio technology
Protocol?
_ set of previously made agreements between communication partners _ including: rules for connection establishment and termination; method of synchronization between sender and receiver; measures for detection and treatment of transmission errors; valid messages (vocabulary); format and encoding _ syntax = format of valid messages _ semantics = vocabulary and meaning of valid messages
Types of Computer Networks?
_ PAN (Personal Area Network): network of small mobile devices; USB, FireWire, WLAN, Bluetooth, IrDA; few meters range _ LAN (Local Area Network): local network (inside a building); Ethernet, WLAN, Token Ring; 500 - 1000m (depends on transmission medium, time using, environment and transmission power) _ MAN (Metropolitan Area Network): connects LAN; Fiber-optic cables (less signal weakening and higher data transmission rates), WiMAX (IEEE 802.16); 100km, covers a city or agglomeration area) _ W…
Communication Modes?
_ Synchronous ("Rendez-Vouz"): + sender and receiver need to be present at the same time + require to wait for the other side to be ready + phone calls, video conference _ Asynchronous + sender and receiver act independently from each other + require buffering + instant messaging, email
Unicast and Broadcast?
_ unicast: one-to-one = one host sends information to exactly one other host _ broadcast: one-to-all = one host sends information to all other hosts in the network
Multicast and Anycast?
_ group communication _ multicast: one host sends information to all hosts in a given group _ anycast: one-to-any = one host sends information to one (any) host in a given group
Connection-oriented and Connectionless?
_ connection-oriented: stateful + connection establishment, data transfer, connection termination + establish virtual path between involved hosts + sequent data exchanged between same hosts + used for reliable services _ connectionless: stateless + no path between involved hosts + low latency services
Communication Channels?
_ means/ways of data transmission _ Ex: telephone calls, presentation, email, etc.
Directional Dependence (Anisotropy) of Data Transmission?
_ give a communication channel with >2 endpoints + simplex: only one side of channel can send data (one direction); radio, TV, pager + duplex (full-duplex): both sides can send data simultaneously; phone, networks with twisted pair cables + half-duplex: both sides can send data but not simultaneously (one direction at a time); networks with fiber-optic cables or coaxial cables, wireless networks with a single channel
Bandwidth, Throughput, Goodput and Latency?
_ factors influencing the performance of a computer network: + bandwidth: number of bits can be transmitted within a period of time via the network (ideal case) + throughput: bandwidth but it actual case + goodput: actual rate of data the user benefits from + latency (delay): time for a message to travel from one end to the most distant end in the network + latency = propagation delay + transmission delay + waiting time + propagation delay = distance/(speed of light * velocity factor ) + transm…
Bandwidth-Delay Product?
_ volume of a network connection: product of bandwidth and latency = maximum number of bits that can be inside the line between 2 hosts
Reference Model?
_ describe computer networks by the technologies used _ several layers; each layer addresses a particular aspect of communication and offers interfaces to the neighboring layer _ each layer defines their own protocol that define syntax and semantics of parts of the transmitted message (header and trailer) => encapsulated _ single protocols in a layer can be modified or replaced without affecting other aspects of communication ** no concrete implementation for a layer => multiple protocols (not…
TCP/IP Reference Model
_ 4 layers: application <=> transport <=> internet <=> link _ each layer adds additional information as header to the message (sometimes also trailer at the end of the message like Ethernet in link layer) _ the receiver analyzes the header and trailer on the same layer
Hybrid Reference Model?
_ extension of TCP/IP reference model _ 5 layer model: dividing Link Layer into 2 layers: Data Link Layer and Physical Layer
OSI Reference Model?
_ 2 additional layers (Presentation and Session) are placed below Application Layer and above Transport Layer _ central concepts: + services: define what layer does (semantics) + interfaces: define how to access + protocols: describe how the layer is implemented
Physical Layer?
_ transmit bitstream _ physical connection to the network _ convert data into signals _ sender site: signals are modulated onto the medium _ receiver site: signals are demodulated from the medium _ devices: repeater, hub (multiport repeater)
Data Link Layer?
_ ensures error-free data exchange of frames between devices in physical networks + handles transmission errors with checksums (represent the number of bits in a transmission message) + controls the access to the transmission medium _ specify physical network addresses (MAC addresses) _ sender site: packs the Network Layer packets into frames and transmits them via a physical network _ receiver site: identifies frames in bitstream from Physical Layer _ devices: bridges, layer-2-switches (multip…
Network Layer?
_ forward packets between logical networks (over physical networks): defines logical addresses (IP addresses) => each IP packet is routed independently to its destination (connectionless) _ sender site: packs the segments of Transport Layer into packets _ receiver site: unpacks the packets in frames from Data Link Layer _ devices: routers, layer-3-switches
Transport Layer?
_ transport segments between processes on different devices (end-to-end protocols) _ transport protocols implement: + connectionless communication: UDP (User Datagram Protocol) in TCP/IP networks + connection-oriented communication: TCP (Transport Control Protocol) in TCP/IP networks _ sender site: packs the data of Application Layer into segments _ receiver site: unpacks the segments inside the packets from the network layer _ addresses processes with port numbers
Application Layer?
_ contains all protocols that interact with the application programs (browser or mail) _ some Application Layer protocols: HTTP, FTP, SMTP, POP3, DNS, SSH, Telnet
Session Layer and Presentation Layer?
_ OSI Reference Model only _ session: controls the dialogues (connections) between processes + services: checkpointing (and recovery), authentication, authorization + protocols: H.245, L2TP, PAP, SOCKS + commonly used for RPCs _ presentation: contains rules for format of messages + notify receiver about the format of the message + data records, types and length can be defined and specified + compression and encryption can be implemented _ functionalities of Session and Presentation Layer are us…