Network Technologies &

Principles

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Network Technologies & Principles

Communication Subsystem.

Types of Network.

Principles of Network.

Distributed Protocols

Communication Subsystem

The hardware and software within a distributed system which provides the communication facilities is known as the communication subsystem.

Consists of:

– Transmission media: providing the physical connectivity, e.g. wire, cable, fiber and wireless channels;

– Hardware devices: providing the linkage, e.g. routers, bridges, hubs, repeaters, network interfaces and gateways;

– Software components: managing the communication, e.g. protocol stacks, communication handlers and drivers.

Network Types

Local Area Networks (LANs)

– High-speed communication on proprietary grounds (on-campus).

– Based on twisted copper wire, coaxial cable or optical fibre.

– Total system bandwidth is high and latency is low.

– Most typical solution: Ethernet with 100 Mbps

Metropolitan Area Networks (MANs)

– High-speed communication for nodes distributed over medium-range distances, usually belonging to one organization.

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– Based on high bandwidth copper and optical fibre.

– Providing "back-bone" to interconnect LAN's.

– Technology often based on ATM, FDDI or DSL.

Wide Area Networks– Communication over long distances (cities,

countries, or continents).– Covers computers of different

organizations.– High degree of heterogeneity of underlying

computing infrastructure.– Involves routers to manage network and

route messages to their destinations.

– Speeds up to a few Mbps possible, but around 50-100 Kbps more typical.

– Most prominent example: the Internet.

Wireless Networks– End user equipment accesses network

through short or mid range radio or infrared signal transmission

– Wireless WANs:• GSM (up to about 20 Kbps), UMTS (up to

Mbps), PCS.– Wireless LANs/MANs:

• WaveLAN (2-11 Mbps, radio up to 150 meters).

– Wireless Personal Area Networks:• Bluetooth (up to 2 Mbps on low power

radio signal, < 10 m distance).

Network PrinciplesPacket Transmission

A packet is a sequence of binary data with addressing information to identify the source and destination computers.

A network message with arbitrary length is divided before transmission into packets of restricted length.

Restricted length packets are used:– To allow each computer in the network to

allocate sufficient buffer storage to hold largest possible incoming packet.

– To avoid long waiting for communication channels to be free if long messages ware transmitted without subdivision.

Network PrinciplesSwitching Schemes

A switching system is required to transmit information between two arbitrary nodes in the network using shared communications link.

Four types of switching are used in computer network:

– Broadcast:

• Requires no switches.

• All messages are sent to all connected computers.

• Each computer is responsible extracting messages addressed to itself.

• Used approach in Ethernet and wireless networks.

– Circuit switching:• Approach taken in the telephone system.• A physical link is established between the

sender and the receiver. – Packet switching:

• Otherwise known as store-and-forward (postal system).

• At each switching node (connection point) a computer manages the packets by reading each one into memory, examining its destination, and choosing an outgoing circuit appropriately.

– Frame relay:• Reading in and storing the whole of each

packet introduces a performance overhead which can become significant.

• In ATM networks a frame of fixed size is used in place of a packet and only its header needs to be examined.

• The remainder of the frame is simply relayed as a stream of bits.

Networking Performance Parameters Latency - time to transfer “empty” message Bandwidth or data transfer rate - how many

bits/sec can be transferred (how thick the “pipe” is)

message_transfer_time = latency + msg_length / data_transfer_rate

Consider: a modem connection vs. a van of magnetic tapes traveling an interstate highway

QoS: Quality of Service (bandwidth/latency guarantees for particular connections)

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OSI Protocol Stack

OSI - Open Systems Interconnect Application - application interfaces (httpd, ftp) Presentation - network representation for

data Session - connections, encryption Transport - message à packets Network - network-specific packets, routing Data Link - transmission of packets between

“directly” connected machines + error issues Physical - hardware (“I can touch it”)

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Communication Through Layers

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Presentation

Session

Transport

Network

Data Link

Physical

Application

Presentation

Session

Transport

Network

Data Link

Physical

Application

TCP/IP Protocol Stack

ISO stack is good as a model for understanding networks Layers in “real” network stacks aren’t so differentiated TCP/IP stack has won primarily because of the free

implementation shipped in early versions of BSD Unix Addresses above IP are (port, address) combinations

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Physical

IP

TCPUDP

ApplicationApplication

Transport

Network

Transport Protocols

UDP (User Datagram Protocol)– Connectionless– Fast setup– Easy one-to-many communication– Datagram-oriented (fixed size chunks of data)– Packet reordering– Packet loss (no flow control, bad packets dropped)– Packet duplication– (Absolute) maximum datagram length: 64K– Usable maximum is more complicated– 8K is generally safe for modern systems

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Transport Protocols, Cont.

TCP (Transmission Control Protocol)– Connection-oriented– Byte stream-oriented– Slower setup– Consumes file handles: one per connection– Flow control, automatic retransmission

• No packet reordering (delivery is FIFO)• No packet loss• No duplication

– Theoretically “no” limit on size of objects that can be dumped into a TCP stream

– In practice, limits exist

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