Communication Systems

Characteristics of Communication SystemsCore 3: Communication SystemsCharacteristics of Communication SystemsWe are going to look at the following

Overview of Protocol LevelsHow a message is passed from source to destinationExamples of Protocols at each levelTransmission SpeedCommon Error Checking.

Overview of Protocol LevelsWe 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.

Overview of Protocol Levels

Overview of Protocol LevelsIPT 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.

Overview of Protocol LevelsIPT 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.

Overview of Protocol LevelsIPT 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.

Characteristics of Communication SystemsWe are going to look at the following

Overview of Protocol LevelsHow a message is passed from source to destinationExamples of Protocols at each levelTransmission SpeedCommon Error Checking.

How a message is passed from source to destinationMessage Creation

The message is compiled at the source for sending. Involves a software application eg

A user write and email using outlookPressing delete to remove a file stored on a file serverSpeaking during a VOIP conversation

How a message is passed from source to destinationOrganising 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.

How a message is passed from source to destinationContd

For example...Outlook prepares an email 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.

How a message is passed from source to destinationSignal 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.

How a message is passed from source to destinationTransmission

Transmission occurs as the signal travels through the medium. Each bit of pattern of bits moves from transmitter to receiver as a particular waveform.

How a message is passed from source to destinationSynchronising 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.

How a message is passed from source to destinationAddressing 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.

How a message is passed from source to destinationError 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.

How a message is passed from source to destinationSecurity and Management

Protocols can restrict messages based on usernames and password, others can encrypt messages during encryption.

How a message is passed from source to destinationHow a message is passed (overview)

Message CreationOrganisation of PacketsSignal Generation by the TransmitterTransmissionSynchronising the ExchangeAddressing and RoutingError Detection and CorrectionSecurity and Management

How a message is passed from source to destination

1. Message Creation2. Organising Packets3. Signal Generation4. Transmission5. Synching Exchange6. Addressing and Routing7. Error Checking and Correction8. Security and ManagementCharacteristics of Communication SystemsWe are going to look at the following

Overview of Protocol LevelsHow a message is passed from source to destinationExamples of Protocols at each levelTransmission SpeedCommon Error Checking.

Examples of ProtocolsProtocol

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.

Examples of ProtocolsIn 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

Examples of ProtocolsApplication 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.

Examples of ProtocolsApplication 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.

Examples of ProtocolsApplication 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.

Examples of ProtocolsApplication 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.

Examples of ProtocolsCommunication 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.

Examples of ProtocolsCommunication 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.

Examples of ProtocolsCommunication 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. 10.0.0.138. Every piece of hardware connected to the web has a unique IP address within its network.

Examples of ProtocolsTransmission 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.