Layers of The OSI Model
OSI MODEL CLASSIFICATIONS
- Application Support - Network Support
APPLICATION SUPPORT
- Application, Presentation, and Session. - Connecting software programs to a network is the primary responsibility of this functional block.
MAC SUB-LAYER
- Defines how packets are transferred into media. - In contention based networks, such as Ethernet, the MAC sub-layer is responsible for the contention scheme, for instance CSMS & CD.
OSI
- Developed by ISO. - Consists of a hierarchy of 7 layers, each with a specific set of functions involved in transmitting data from 1 device to another. - Also references network protocols typically used to support communication, positioning each protocol at the relevant layer based on it's function.
TRANSPORT LAYER or L4
- End to end data transmissions occur here. - Also implements mechanisms for ensuring reliable data transfer & data integrity, & for correcting transmission errors. - Manages connections with 2 layers adjacent to it: Session above & Network below. - Protocols segment data received from each session & pass it to the Network layer in the appropriate size & format. - Common protocols are UDP & TCP
SESSION LAYER or L5
- Establishes connections between devices & apps, maintains the connection, & then terminates or reestablishes it when required. - Specifies procedures for synchronizing data transfer between 2 devices with different transmission rates. - TCP function of establishing sessions occurs here. - Sockets operate at this layer. - Protocols include: RADIUS & TACACS+
PHYSICAL LAYER PROTOCOLS
- Ethernet - Fast Ethernet - ATM - Token Ring - FDDI - 802.11
PRESENTATION LAYER FILE FORMATS
- GIF: a graphic interchange format primarily used on the Internet - JPEG: a compressed graphical file format that reduces image file size, - TIFF: a digital format for storing images that's popular with photographers and in the publishing industry
DATA LINK LAYER or L2
- Handles hardware addressing so that physically connected devices can be connected. - Responsible for transferring data packets between adjacent network nodes without errors & after being sent, it awaits confirmation from receiving devices. - Network switches operate at this layer.
NETWORK LAYER PROTOCOLS
- IP - IGMP - ICMP - ARP - RIP
OSI BENEFITS
- It divides the network communication process into smaller, simpler stages & functions. - It encourages industry standardization by defining what functions occur at each network layer. - It standardizes network components, making it possible for multiple vendors to create components that can communicate. - It supports different types of network hardware and software, and - It makes applications easier to program because changes can be made at one layer without affecting all other layers
DATA LINK SUB-LAYERS
- LLC - MAC
PHYSICAL LAYER or L1
- Lowest layer in OSI model - Defines physical media & specs a network uses when transferring data. - Network adaptors & APs are some devices that operate at this layer. - Protocols receive fully formatted data frames from Data Link Layer & places them on the physical communications medium. - Layer determines the mode & medium of data transmission, which are factors that affect transmission speeds.
DATA LINK LAYER PROTOCOLS
- PPP - SLIP - SONET - Frame Relay
PRESENTATION LAYER or L6
- Protocols ensure that data represented by 1 system can be interpreted by another system, eg. ASCII. - Most programs contain data such as names, identification numbers, & passwords. - These items can be represented as characters, integers, or floating numbers, & each device on a network may use a different code to represent the same data. - Standard data formats are used to enable devices with different representation techniques to communicate with each other. This translation is only intermediary format, & will change at lower levels. - Also adds data compression and security-related services like encryption and decryption. - Protocols include SSL, TLS, & ASN.1
LLC SUB-LAYER
- Responsible for encapsulating protocols in upper layers so that multiple upper-layer protocols can share the same media.
NETWORK LAYER or L3
- Specifies how to route data from hosts on 1 network to host on another network, based on the host's network addresses. - Contains several protocol families, categorized based on functions they perform. These include protocols for: * Reliability * Established & managing connections * Routing
APPLICATION LAYER or L7
- The highest layer in the OSI model. - Provides network services to end users, according to the applications they're using. It's the only layer that users interact with directly. - Includes protocols for tasks such as transferring files, sending e-mails, and saving data to network servers. - Examples of protocols that operate at the Application layer are HTTP, SNMP, SMTP, DNS, and FTP. - Other protocols at this layer include Gopher, NFS, NTP, X.400 directory services, Telnet, and FTAM.
NETWORK SUPPORT
- Transport, Network, Data Link, and Physical. - Moving data on a network is the primary responsibility of this functional block.
SEVEN LAYERS OF OSI
1. Physical 2. Data Link 3. Network 4. Transport 5. Session 6. Presentation 7. Application
DATA ENCAPSULATION STEPS
1. user information is converted to data 2. Data is converted to segments 3. Data segments are converted to packets or datagrams 4. Datagrams or data packets are converted to frames for transmission, and 5. Frames are converted to bits
DATA ENCAPSULATION PROCESS
When a host transmits data across a network to another device, the data is wrapped - or encapsulated - in protocol and delivery information as it passes downward through each of the layers of the OSI model. At the receiving end, information that was added to the data is removed as it passes up the layers of the OSI stack. This process is called de-encapsulation.
