1. Characteristics of Communication Systems
Core 3: Communication Systems

2. Characteristics of Communication Systems
We are going to look at the following…
Overview of Protocol Levels
How a message is passed from source to destination
Examples of Protocols at each level
Transmission Speed
Common Error Checking.

3. Overview of Protocol Levels
We said before that in IPT we will utilise 3 Protocol levels where as in actual fact there exists 7 in the OSI model. We refer to these 7 as the OSI ‘stack’. Each packet must descend the stack, be transmitted and then ascend the stack on the receiving computer.

4. Overview of Protocol Levels

5. Overview of Protocol Levels
IPT Application Level (1)
The IPT application level includes the following from the OSI Model…
7. OSI Application Layer – the actual data to be transmitted is created by a software application.
6. OSI Presentation Layer – The data is organised into a form suitable for transmission eg. Compressing an image and representing it as ASCII characters.
Protocols operating at this level include HTTP, DNS, FTP, SMTP, POP and SSL.

6. Overview of Protocol Levels
IPT Communication and Control Level (2)
The IPT communication and control level includes the following from the OSI Model…
5. OSI Session Layer – this is where communication with the network is established, commences and is maintained. Eg a net banking ‘session’.
4. OSI Transport Layer – the transport layer manages the correct transmission of each of data and organises retransmission if packets fail to reach their destination. Eg TCP (Transport Control Protocol).
3. OSI Network Layer – this is where packets are directed to their destination. IP (Internet Protocol) operates here. Routers direct traffic based on addresses.

7. Overview of Protocol Levels
IPT Transmission Level (3)
The IPT communication and control level includes the following from the OSI Model…
2. OSI Data Link Layer – this layer defines how the transmission media is actually shared. Device drivers that control physical transmission operate here. Switches and Ethernet Protocol operate at this level directing messages based on their MAC address. (Media Access Controller address).
1. OSI Physical Layer – here is where the actual physical transfer occurs, hence this layer is composed entirely of hardware. It converts bits into signals to be transmitted by twister pair in a LAN, copper line for ADSL, coaxial, fibre optic or wirelessly.

8. Characteristics of Communication Systems
We are going to look at the following…
Overview of Protocol Levels
How a message is passed from source to destination
Examples of Protocols at each level
Transmission Speed
Common Error Checking.

9. How a message is passed from source to destination
Message Creation
The message is compiled at the source for sending. Involves a software application eg…
A user write and using outlook
Pressing delete to remove a file stored on a file server
Speaking during a VOIP conversation

10. How a message is passed from source to destination
Organising Packets at the interface between source and transmitter
When a message is prepared for transmission it descends the stack of protocols from the Application level down to where it is ready for physical transmission by hardware at the Transmission level. Each protocol wraps the data packet from the layer above with its own header and trailer. The header and trailer contain information relevant to the protocol operating at the layer.

11. How a message is passed from source to destination
Contd…
For example...
Outlook prepares an at the Application level – this includes calling on the SMTP protocol.
Passing to the Communication C&A level involves firstly the TCP protocol followed by the IP protocol. TCP creates a connection between source and destination and then IP routes the data to its destination.
CC&A passes each IP datagram is passed to the Transmission level that operates the physical sending of data.
The process is reversed at the receiving end, stripping off headers and trailers, passing the packet back up the stack.

12. How a message is passed from source to destination
Signal Generation by the transmitter
The transmitter is the physical hardware that generates or encodes the data on the medium creating a signal. The transmitter represents individual bits as a wave that is transmitted along a medium. For example light waves over optic fibre or radio waves over wireless.

13. How a message is passed from source to destination
Transmission
Transmission occurs as the signal travels through the medium. Each bit of pattern of bits moves from transmitter to receiver as a particular waveform.

14. How a message is passed from source to destination
Synchronising the Exchange
In order for the receiver to decode the signal the receiver and transmitter must sample the signal with precisely the same timing. This synchronisation occurs using a common clock so that sampling occurs exactly the same at both ends.

15. How a message is passed from source to destination
Addressing and Routing
During transmission data packets will pass through many different and varied nodes and networks. Ethernet and Transmission Control protocols use the MAC address to determine the path to the receiver.

16. How a message is passed from source to destination
Error detection and correction
Error checking starts as the packet descends the protocol stack when checksum or CRC (Cyclic Redundancy Checking) values are established. These are compared at the destination to determine whether an error has occurred. If an error has occurred then the packet can be resent.

17. How a message is passed from source to destination
Security and Management
Protocols can restrict messages based on usernames and password, others can encrypt messages during encryption.

18. How a message is passed from source to destination
How a message is passed (overview)
Message Creation
Organisation of Packets
Signal Generation by the Transmitter
Transmission
Synchronising the Exchange
Addressing and Routing
Error Detection and Correction
Security and Management

19. How a message is passed from source to destination
1. Message Creation
8. Security and Management
2. Organising Packets
7. Error Checking and Correction
6. Addressing and Routing
4. Transmission
3. Signal Generation
5. Synching Exchange

20. Characteristics of Communication Systems
We are going to look at the following…
Overview of Protocol Levels
How a message is passed from source to destination
Examples of Protocols at each level
Transmission Speed
Common Error Checking.

21. Examples of Protocols
Protocol A protocol is a set of rules or procedures that must be observed for two devices to transfer data successfully. Handshaking Before two devices can communicate they must agree on the protocol or series of protocols they will use. This is called ‘handshaking’. Handshaking commences when one device asks to communicate with another and can take place either when a device is powered on OR prior to a communication session.

22. Examples of Protocols
In this section we will look at the following protocols…
Application Level Protocol – HTTP
Communication Control and Addressing Protocols – TCP and IP
Transmission Level Protocol - Ethernet

23. Examples of Protocols Application level - HTTP
The Hyper Text Transfer Protocol operates within the IPT application level and within level 6 of the OSI. HTTP is the primary protocol used by web servers to communicate and retrieve web pages from web servers.
There are 3 primary HTTP commands used by browsers – GET, HEAD and POST.

24. Examples of Protocols Application level - HTTP
HTTP GET – Retrieves entire documents eg. HTML files, images, videos etc. GET requests a document from a web server and the server returns the file to the browser.

25. Examples of Protocols Application level - HTTP
HTTP HEAD – Retrieves ONLY the header information for the file. This is used to check whether the file has been updated since the browser last retrieved the file. If NOT the browser can return a cached version for display.

26. Examples of Protocols Application level - HTTP
HTTP POST – POST sends data from a browser to a webserver. Commonly this is used to send data inputs created by users within a web based form. Eg. Signing up for a facebook account – user details are sent to FB servers using POST.

27. Examples of Protocols Communication C&A level - TCP
Transmission Control Protocol. TCP & IP are the protocols responsible for most data on the internet. TCP is primarily about ensuring messages send correctly. TCP requires IP (Internet Protocol) to be operating which is why TCP is often referred to as TCP/IP. In TCP each packet is a segment or string of bytes ready to be sent. TCP error checks in addition to sending packets.

28. Examples of Protocols Communication C&A level - TCP
Each TCP segment includes a header that contains the sequence of bytes contained within the segment AND a checksum. The checksum is calculated at the start and recalculated at the end and if the checksum matches the bytes within the segment are acknowledged.

29. Examples of Protocols Communication C&A level - IP
Internet Protocol is the protocol that sends packets from sender to receiver. IP does not guarantee packets reach their destination, nor does it acknowledge received info. It is simply all about sending data packets one after another. IP uses IP addresses and determines the best path for packets to take.
An IP address is a series of 4 bytes (32 bits) e.g Every piece of hardware connected to the web has a unique IP address within its network.

30. Examples of Protocols Transmission level – Ethernet
Ethernet operates at the transmission level meaning it operates at a physical level. This means it is built into various hardware designed to send and receive. Ethernet packets (known as frames) include a MAC (media access controller) address in the header. Every node on an Ethernet network must have a unique 6 – byte MAC address.