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Networks – basics

A network consists of: Transmission media (wire, cable, …). Hardware devices (routers, switches, …). Software components (protocol stacks, drivers, …).

Terminology: Host: Computers and other devices that use a network. Node: Any computer or switching device attached to a network. Subnet: Set of interconnected nodes.

Design issues: Performance, scalability, reliability, security, mobility, quality of service, multicasting.

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Networks – types

Range Bandwidth (Mbps) Latency (ms)

LAN 1-2 kms 10 – 1000 1 – 10WAN worldwide 0.010 – 10000 [1] 100 – 500MAN 2-50 kms 1 – 150 10Wireless LAN 0.15-1.5 km 2 – 54 [2] 5 – 20Wireless WAN worldwide 0.010 – 2 100 – 500Internet worldwide 0.010 – 2 100 – 500

[1]: OC-192 over ATM: http://newsroom.cisco.com/dlls/innovators/switching/eugene_wang_profile.html

[2]: IEEE 803.11a:http://www.wi-fiplanet.com/tutorials/article.php/2109881

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Networks – packet transmission

Message: Sequence of data items of arbitrary length.

Messages subdivided into packets.Switching schemes:

Broadcast. Circuit switching. Packet switching. ATM / Frame relay.

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Protocols – basics

Protocol: Set of rules and formats to be used for communication between processes in order to perform a given task.

Should include specification of: Sequence of messages that must be exchanged. Format of the data in the messages.

Implemented by a pair of software modules in the sending and receiving computers.

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Protocols – layers

Layer n

Layer 2

Layer 1

Message sent Message received

Communicationmedium

Sender Recipient

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Protocols – encapsulation and headers

Presentation header

Application-layer message

Session header

Transport header

Network header

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Protocols – the ISO Open Systems Interconnection (OSI) model

Application

Presentation

Session

Transport

Network

Data link

Physical

Message sent Message received

Sender Recipient

Layers

Communicationmedium

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Protocols – internetwork layers

Underlying network

Application

Network interface

Transport

Internetwork

Internetwork packets

Network-specific packets

MessageLayers

Internetworkprotocols

Underlyingnetworkprotocols

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Protocols – network layer routing (WAN)

Hosts Linksor local networks

A

D E

B

C

1

2

5

43

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Routers

Routing algorithms, adaptive routing (congestion control)

Packet delivery: Datagram or virtual circuit?

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Internet protocols – Internetworking

Internetwork: Network which integrates a number of different subnets.

Needs: Unified internetwork addressing scheme (Internet: IP

addresses) Protocol defining format of internetwork packets and

specifying rules for handling (Internet: IP protocol). Interconnecting components that route packets to

their destinations (Internet: Internet routers).

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Internet protocols – the TCP/IP protocol suite

Messages (UDP) or Streams (TCP)

Application

Transport

Internet

UDP or TCP packets

IP datagrams

Network-specific frames

MessageLayers

Underlying network

Network interface

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Internet protocols – encapsulation and headers

Application message

TCP header

IP header

Ethernet header

Ethernet frame

port

TCP

IP

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Internet protocols – IP

Internet Protocol.Transmits datagrams from one host to another,

if necessary via intermediate routers.Unreliable, best-effort delivery semantics.Address resolution: Conversion of Internet

addresses to network addresses (for a given network).

Routing: Each router in the Internet implements IP-layer software to provide a routing algorithm.

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Internet protocols – IP packet layout and addressing

7 24

Class A: 0 Network ID Host ID

14 16

Class B: 1 0 Network ID Host ID

21 8

Class C: 1 1 0 Network ID Host ID

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Class D (multicast): 1 1 1 0 Multicast address

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Class E (reserved): 1 1 1 1 unused0

dataIP address of destinationIP address of source

header

up to 64 kilobytes

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Internet protocols – TCP and UDP

UDP features: Transport-level replica of IP. No guarantee of delivery. No setup cost, no acknowledgement messages. Message size up to 64 kbytes.

TCP features: Reliable delivery. Arbitrarily long sequences of bytes. Connection-oriented. Mechanisms: Sequencing, flow control,

retransmission, buffering, checksum.

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Ethernet (IEEE 802.3) – Basics

Carriers Sense Multiple Access with Collision Detection (CSMA/CD)

Xerox – EthernetRandom Access

Stations access medium randomly

Contention Stations contend for time on medium

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Ethernet – ALOHA

Sender Go ahead and send! Retransmit if no ACK

Problems Collisions Low utilization (18%)

Slotted ALOHA is an improvement (max utilization 37%)

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Ethernet – CSMA

Carrier Sense Multiple Access Observations

Propagation time is much less than transmission time All stations know that a transmission has started almost

immediately Sender

First listen for clear medium (carrier sense) If medium idle, transmit If two stations start at the same instant, collision Wait reasonable time Retransmit if no ACK

Max utilization depends on propagation time (medium length) and frame length

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Ethernet – CSMA/CD

Carrier Sense Multiple Access – Collision Detection Observation: With CSMA, collision occupies medium for

duration of transmission Sender

If medium idle, transmit If busy, listen for idle, then transmit Station listens whilst transmitting If collision detected, transmit jam signal, then cease

transmission

After jam, wait random time then start again Binary exponential back off

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Ethernet –CSMA/CDOperation

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Ethernet – Collision Detection

Bus Collision produces much higher signal voltage than

signal Collision detected if cable signal greater than single

station signal

Star Activity on more than one input is collision Special collision presence signal

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Summary

Networks.Protocols. Internet protocols (TCP/IP).Ethernet