Computer Network and the Internet Flashcards
Computer network definition
group of devices that uses common communication protocols over digital interconnections to share resources
Computer network main elements
Node, interconnections
Computer network: problems that brought to their development
Low flexibility of devices
Low exploitation of available computer resources
Computer networks have grown exponentially because
Standardization of protocols
Development of network design methodologies
Tech advancements
Computer network foundation principle
= client-server architecture
client = device that accesses shared resources
server = device that provides services
Computer networks are beneficial because
Allows resource (data + peripheral devices) sharing
Good ratio price-performance
High reliability
Expand incrementally
Computer network requirements
Connectivity
Resource sharing
Reliability
Performance
What does connectivity mean for Computer networks?
Nodes are interconnected through a communication subnetwork
Connectivity: types of nodes
End system:
- Master: manages communication
- Slave: forwards request/receives mex
Intermediate system: connects transmission channels
Connectivity: types of connections and topologies
Point-to-point:
- star
- tree
- ring
- complete mesh
- uncomplete mesh
Multipoint: slave can use channel if authorized by the master
Broadcast:
- Wireless: data travels through air, received by antenna, the broadcast channel has arbitration policies
- Bus
- Ring
Connectivity: a communication subnetwork is composed by
Intermediate Systems (that connect)
Transmission Channels
If the IS connects different subnetworks we talk about routers, if they are in the same subnetworks we talk about switches
Connectivity: the presence of IS forms switched networks: types
Circuit-switched network
- A private connection is opened between the nodes that want to communicate through a virtual path
- Time-based pricing
- Pros: delays only for electromagnetic propagation, no congestion, IS are low cost because don’t have intelligence
- Cons: wait for path search, low use of communication channel
Packet-switched network
- Mex are sent on the channel in limited-size packets. Each IS decides best route to sent the packet for it to reach its destination node
- Volume-based pricing
- Pros: efficient use of transmission channels, error detection and recovery done by IS
- Cons: possible packet lost, IS ar eintelligent so they cost more
Resource sharing operates on the principle of
Multiplexing (multiplexer = circuit that decides which node accesses the channel) and demultiplexing (circuit that sends data received through the channel to its destination)
Method of multiplexing-demultiplexing
Time-Division multiplexing:
- time divided into equal quanta of time, each stream can transmit for a quantum of time
Frequency division multiplexing
- all streams transmit packets on the channel with != frequencies, multiplexer decides priority of packages
Problems with de-multiplexing methods
Inefficient: time/frequency busy even if no packets, hard to add streams
new -> statistical multiplexing: quantum of time given only if the stream has something to transmit
What does reliability mean?
A network must ensure that the correct message arrives
Reliability: categories of fault
Altered bits -> parity bit
Packet loss (node overload, bit-level error, management sw error)
Physical connection/node down (operator’s fault, power down, sw not working)
Performance: influenced by
bandwidth -> P=n_of_bits*time_interval
latency -> time taken for a bit to propagate from one end of the channel to the other, Tprop + Ttrans + Tacc
Tprop
Propagation delay = dist/Vlight
T acc
queuing delay
Ttrans
Time to transmit a datum
Dim/P
size_of_data/bandwidth
L*P
= volume of the channel
= number of bits to send before the first bits reaches its destination
Bidirectional latency =
Round-Trip time
Networking devices
NIC card
Switch
Hub
Repeater
Bridge
Router
gateway
Firewall
NIC Card
Network Interface Card: circuit board that enables devices to connect to computer networks, each has a unique Media Access Control address
Switch
- determines if data should remain on LAN
- transfers data to the right connection
Hub
- connects devices to make them function as a single network segment
- receives data broadcasting it
- regenerates incoming data signals
Repeater
regenerates signals
Bridge
- provides connection between LANs
- converts data formats
- manages data transmission
Router
- regenerates signals
- concentrates connections
- converts data transmission formats
- manages data transfers
- connects to WANs
Gateway
translator among heterogeneous systems
Firewall
for network security and access rules: rejects access from unrecognized sources
Network media: function
Carry information through a LAN
Network media: info can be carried trhough
Wires
-> Ethernet = tech for connecting devices in a wires LANs/WANs, it uses protocols that define how data is formatted and transmitted over physical media
- copper cables: common easy cheap, but the further teh signal is transmitted the weaker it becomes
- - Shielded Twisted Pair: popular
- - Coaxial cable: for longer distances: longest distances
- - Unshielded Twisted Pair: chapest, greatest speed, 4 pair of insulated cables
Wireless tech
- Wifi: access point is wireless LAN base station
- Bluetooth: short-range, radio waves
Network Architecture: function
to guide design and construction choices of a network
Network Architecture: connection in a computer network must be
general
economically convenient
fair
robust
high performance
Network Architecture: construction details are hided through
stratification
Network Architecture: stratification rules
- the purpose of each level is providing the higher one with services masking services’ implementation
- levels added subsequently on hw basis
- hw on the basis offers services
Network Architecture: why is stratification important
- it breaks down network building
- makes network architecture modular
Network Architecture: stratification schema
Application programs
Process to process channels
Computer to computer communication
Hardware
Network Architecture: layers obtained with stratification are
levels of abstraction
Network Architecture: protocol defintion
objects that form the layers of a network architecture
Network Architecture: protocols functions
- define a local communication service
- rule for exchanging information between a protocol and its peers
Network Architecture: protocols 2 interfaces
- peer interface: defines form and meaning of messages exchanged between same level protocols
- service interface: allows objects on the same device to use protocol’s communication services
Network Architecture: encapsulation method
Each layer adds to the message header information deleted and interpreted by the receiver’s protocol on the same level
Network Architecture: ISO-OSI definition
standardized network architecture
ISO-OSI protocol layers
7 APPLICATION: used by user
6 PRESENTATION: syntax and semantics check
5 SESSION: allows to establish a work session between != computers
4 TRANSPORT: guarantees quality layer 3
3 NETWORK: controls subnet operations
2 DATA LINK: correctness and reliability of connection
1 PHYSICAL: encoding of mex
