Communication Software and Protocols 17.1 The Need for Communication Software Advantages of communicating through computer network Refer to textbook
Class work / Homework If part of the file transfer is corrupted, what will be done? Resend If part of the streamed video message is corrupted, what will be done? Will not resend. Compare Whatsapp and email. Compare SMS and whatsapp Suggest one possible application for each type of the communication at school. 17.2 The need for communication Protocols Communication protocol is a set of digital rules for data exchange.
Refer to http://www.protocols.com/protocols.htm for more information about different protocols My domain is managed via http://www.enomcentral.com 17.3 Concept of OSI model (Layer by Layer) Concept of packet switching, message switching, circuit switching. Circuit switching: a communication channel is established before 2 nodes communicate. (e.g. Traditional telephone line, the channel become idle even if you DO NOT talk after making the call)  Message switching Each message (e.g. file) is transmitted together. Packet switching (using in Internet, TCP/IP) Each message is divided into packets and each packet can use a different route to the destination. It utilities the network more efficiently. Details of OSI (7 layers model) are not mentioned in Syllabus. Concept of layering is important 17.4 TCP/IP (The protocol suites used in Internet) Transmission Control protocol (TCP) The file (message) is divided into packets. The packets are numbered. The TCP layer reassembles the received data packets into a single data file (message) Error checking, request for resend of loss packet. Internet Protocol It handles the actual delivery of data packets (routing) Refer to P.232-233 of textbook for more details
17.5 IP address IP address v4: 32 bits, usually written as 4 octet e.g. 192.168.1.1 Each octet: 0000 0000 - 11111111 (0-255) Network address, host address, subnet mask: default gateway
Our school have 14 usable public IP only. Public IP, private IP For a home network, ONE public IP is assigned by ISP. Each public IP is unique in the Internet. If we have several devices needed to connect to Internet, a router is added. (Router connect a LAN to Internet) For the computer connects to the LAN ports, it use private IP. (usually 192.168.1.X or 192.168.0.X) IPv6 : 128 bits address. MAC address may appear as part of the IPv6 address (latter 64 bits) Check the owner of an IP address: http://www.ip-lookup.net/ Compare DHCP, fixed IP (Static IP) Pro and cons of using DHCP over IP situation of using static IP and dynamic IP 17.6 Domain Names and Domain Name system Domain name : easy to remember. Must be translated to an IP address for accessing. Domain name are resolved from the end. Top level domain: -country code top level domain: e.g. cn, hk, jp, mo - Generic code top level domain: .com .net .org The domain of HK are managed by http://www.hkdnr.hk Six domain name category in HK
Domain name space  (Image from wikipedia.com) How Dynamic DNS work. - run a program in computer or home-router. Inform the DDNS server your IP address and update their record. Uniform Resource Locator URL consists of 4 parts:
Some common port: 25 FTP 22 SSH (Remote access, encrypted) 23 Telnet (not encrypted, remote access) 80 HTTP 17.7 HyperText Transfer Protocol (HTTP) Windows Command: tracert, nslookup Chapter 16 MC Test on Next Tuesday (Nov 8) Chapter 17 Communication and Protocols MC (40 marks)
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