1. COM211 Communications and Networks CDA College Pelekanou Olga Email: fe_olga@mail.rufe_olga@mail.ru www.cdacollege.ac.cy/site/info-com-technology-ll/

2. 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

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

4.  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

5. A collection of autonomous computers interconnected by a single technology. Two computers are said to be interconnected if they are able to exchange information.

6.  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.

7. Is not a single network but a network of networks

8. (a) (b) point-to-point multiple-access Geographical coverage and scalability are limited. Each node needs one interface for each link.

9.  Transmission technology ◦ Broadcast links ◦ Point – to – point links

10.  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

11.  Broadcasting: addressing a packet to all destinations  Multicasting: transmission to a subset of the machines  Unicasting: one sender and one receiver

12.  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

13.  As a general rule ◦ Smaller geographically localized networks tend to use broadcasting ◦ Large networks usually are point – to -point

14.  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.

15.  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

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

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

18.  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.

19.  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

20.  Physical Media  Network Devices  Computers  Protocols  Services

21.  Twisted Pair  Coaxial  Fiber Optics  Wireless Transmissions

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

23.  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 )

24.  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

25.  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

26.  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).

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

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

29.  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.

30. 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

31. 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!

32.  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

33. Physical Presentation Session Transport Network Data link Application The protocol stack: The idea behind the model: Break up the design 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 between adjacent layers.

34. 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).

35. data AH data PH data SH data TH data NH data DH DT data BITS Physical Presentation Session Transport Network Data link Application Physical Presentation Session Transport Network Data link Application receiver sender

36. Physical Data link IP ARP RARP ICMP IGMP Transport TCP UDP Network Session Presentation Application FTP HTTPDNS NFS …

38. Questions??