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3.1.2 Network Protocols - Describe the Open System Interconnection (OSI)…
3.1.2 Network Protocols - Describe the Open System Interconnection (OSI) network organisation model and each of its component layers
Layers
Transport Layer - Layer 4 Divides information into convenient sized packets. Can also check for errors. It communicates with the session layer above and the network layer below.
Network Layer - Layer 3 It works out the best route for the packets to use. Every packet that the Transport Layer has created needs to be sent to the correct machine across the network. Every computer has an IP address allowing this layer to add the correct address to each packet. Also reads the addresses of incoming packets and if they are destined for the computer then it allows them through. Packets not addressed are blocked (principle of the network 'firewall') Communicates with the Transport Layer above and Data Link Layer below.
Session Layer - Layer 5
Opens a temporary 'channel' between an application and a WAN to allow for commination. You can have more than one session running at a time. Can communicate with the presentation layer above and the transport layer below
Data Link Layer - Layer 2 Converts outgoing packets into the correct series of bits and converts incoming bits back into complete packets. Bits can be corrupted as they travel along the network, This layer will attempt to spot these reversals and fix them. At this layer you will find Ethernet switches and Network Cards. Communicates with Network Layer above and Physical Layer below.
Presentation Layer - Layer 6 Main tasks are converting information, data encryption, data compression or decompression. Communicates with the application layer above and the session layer below.
Physical Layer - Layer 1 This is the hardware level. The physical layer transmits and receives streams of bits. It specifies voltages levels and timings. It communicates with the Data Link Layer above.
Application Layer - Layer 7 Presents information/data to the end user. Deals with functions such as data transfer, messaging and distributed databases. Can only communicate with the presentation layer.
Definitions
Packet: A group of bits made up of control signals, error control bits, coded information and destination address of the data.
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Packet Switching: A method of sending data over a Wide Area Network. Packet Switching networks are available for general use in most Countries.
Packet Switching Example
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If packets get lost a request will be made for it to sent again. If a packet never arrives then the router deletes it.
Key Points
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Represented by 7 layers. 7 being at the top and closest to user. 1 being at the bottom for communication medium.
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Layer 7 - Application Layer
Layer 6 - Presentation Layer
Layer 5 - Session Layer
Layer 4 - Transport Layer
Layer 3 - Network Layer
Layer 2 - Data Link Layer
Layer 1 - Physical Layer
Rules
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Grouped into two sets
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Application Set
Mainly concerned with controlling how the various applications currently running are making use of network