COM211 Communications and NetworksCDA CollegePelekanou OlgaEmail: fe_olga@mail.ruwww.cdacollege.ac.cy/site/info-com-technology-ll/

• Lecture 1: Introduction to Computer Networks, OSI Model

• Lecture 2: Hardware building blocks and encoding

• Lecture 3: Physical Media and Cabling

• Lecture 4: Protocols

• Lecture 5: LAN and WAN Technologies

• Lecture 6: Data Link Layer and Ethernet

• Lecture 7: Midterm

• Lecture 8: Network Layer of OSI

• Lecture 9: Transport Layer of OSI

• Lecture 10: Application Services• Lecture 11: Security in Computer Networks• Lecture 12: Wireless Networks • Lecture 13: Revision for the Final examinations

Andrew S. Tanenbaum, Computer Network, Prentice-Hall

Doughlas E. Comer, Computer Networks and Internet

Larry L. Peterson and Bruce S. Davie, Computer Networks: A Systems Approach

A collection of autonomous computers interconnected by a single technology.

Two computers are said to be interconnected if they are able to exchange information.

Interconnect machines. Maintain data confidentiality, data

integrity, and system accessibility. Support growth by allowing more and

more computers, or nodes, to join in (scalability).

Support increases in geographical coverage.

Is not a single network but a network of networks

(a)

(b)

point-to-point

multiple-access

Geographical coverage and scalability are limited.

Each node needs one interface for each link.

Transmission technology

◦ Broadcast links

◦ Point – to – point links

A single communication channel that is shared by all the machines on the networks

Packets (short messages) sent by any machine are received by all the others

The machines ignored or processes the packet from the address field

Broadcasting: addressing a packet to all destinations

Multicasting: transmission to a subset of the machines

Unicasting: one sender and one receiver

Consist of many connections between individual pairs of machines

To go from the source to the destination, a packet may have first visit one or more intermediate machines

Often multiple routes, of different lengths, are possible◦ Find the good ones

As a general rule

◦ Smaller geographically localized networks tend to use broadcasting

◦ Large networks usually are point – to -point

Defines the way in which computers, printers, and other devices are connected

Describes the layout of the wire and devices as well as the paths used by data transmissions.

This topology connects all devices to each other for redundancy and fault tolerance

It is used in WANs to interconnect LANs and for mission critical networks (banks, financial institutions, etc.)

Implementing the mesh topology is expensive and difficult

All the devices on a bus topology are connected by one single cable.

The star topology is the most commonly used architecture in Ethernet LANs.

Larger networks use the extended star topology also called tree topology. When used with network devices that filter frames or packets, like bridges, switches, and routers, this topology significantly reduces the traffic on the wires by sending packets only to the wires of the destination host.

A frame travels around the ring, stopping at each node. If a node wants to transmit data, it adds the data as well as the destination address to the frame.

The frame then continues around the ring until it finds the destination node, which takes the data out of the frame.

Types: Single ring – All the devices share a single cable Dual ring – Allows data to be sent in both directions

and provides redundancy

Physical Media Network Devices Computers Protocols Services

Twisted Pair Coaxial Fiber Optics Wireless Transmissions

Hub Switches ( Level 2 and 3 ) Routers Wireless Access Points Modems NIC’s

End Devices: Acts as a source/destination. For message transmitting or receiving.

Server: In a client/server network environment, network services are located in a dedicated computer whose only function is to respond to the requests of clients. The server contains file sharing, http and other services that are continuously available to respond to client requests.

Client: Our computers. We request a service from a server (ex. We log into gmail to check our email )

A protocol, in contrast, is a set of rules governing the format and meaning of the packets, or messages that are exchanged by the peer entities within a layer. Entities use protocols to implement their service definitions. They are free to change their protocols at will, provided they do not change the service visible to their users.

TCP, UDP, IP, X.25, ICMP, IPSec

DHCP - Dynamic Host Configuration Protocol

DNS – Domain Name System HTTP - Hypertext Transfer Protocol SSH – Secure Shell Telnet SNMP - Simple Network Management

Protocol SMTP - Simple Mail Transfer Protocol FTP – File Transfer Protocol IRC – Internet Relay Chat POP – Post Office Protocol

To interconnect two or more networks, one needs a gateway or router. Host-to-host connectivity is only possible if there’s a uniform addressing scheme and a routing mechanism.

Messages can be sent to a single destination (unicast), to multiple destinations (multicast), or to all possible destinations (broadcast).

Divide time into equal-sized quanta and assign each them to flows on the physical link in round-robin fashion.

Flows are transmitted simultaneously on the link, but each one uses a different frequency.

Flows are transmitted simultaneously on the link, but each one uses a different coding scheme.

For a chosen group of nodes, a unique coding scheme can be used. Each bit is encoded in multiple pulses. Multiple senders may use the same time slots with different coding.

Flow is broken into packets and sent to a switch, which can deal with the arriving packets according to the switch policy (FIFO, round-robin, etc).

switch

queue

Computer A

Computer B

Computer C

• LAN: local area network

• WLAN: wireless local area network

• MAN: metropolitan area network

• WAN: wide area network

Computer networks can be classified according to their geographical coverage:

In Interconnecting multiple networks (internetworking), we are interested in the seamless integration of all these levels. Have in mind that different levels use different technologies!

Privately-owned networks Within a single building or campus of up to

a few kilometers in size

Covers a city A large antenna send the signal to the

host

Large geographical area Often a country or continent

Physical

Presentation

Session

Transport

Network

Data link

Application

The protocol stack:

The idea behind the model: Break up thedesign to make implementation simpler. Each layer has a well-defined function. Layers pass to one another only the information that is relevant at each level. Communication happens only betweenadjacent layers.

• Physical: Transmit raw bits over the medium.• Data Link: Implements the abstraction of an error free medium (handle losses, duplication, errors, flow control).• Network: Routing and Addressing. IP• Transport: Break up data into chunks, send them down the protocol stack, receive chunks, put them in the right order, pass them up.• Session: Establish connections between different users and different hosts.• Presentation: Handle syntax and semantics of the info, such as encoding, encrypting.• Application: Protocols commonly needed by applications (cddb, http, ftp, telnet, etc).

data data

AH data

PH data

SH data

TH data

NH data

DH DTdata

BITSPhysical

Presentation

Session

Transport

Network

Data link

Application

Physical

Presentation

Session

Transport

Network

Data link

Application

receiversender

Physical

Data link

IPARP RARP

ICMP IGMP

Transport TCP UDP

Network

Session

Presentation

Application

FTP HTTP DNS NFS …

Questions??