The OSI Models
Presentation Layer
Protocol conversion, data translation ASCII, JPEG
Physical Layer
Protocols at this layer identifies How digital data (bits) are converted to electric pulses, radio waves, or pulses of lights Specifications for cables and connectors The physical topology, including wireless access points
Data link Layer
Provides for the flow of data Mac addresses
Network Layer
Provides routing decisions IP, IPX
Session Layer
Provides the control functions necesssary to establish, manage, and terminate the connections (examples of technologies working in this layer are Network File System (NFS) and Structured Query Language (SQL)
Physical Layer
Provides the electrical and mechanical connection to the network
The physical layer
Transmission technique: determines whether the encoded bits will be transmitted by baseband (digital) or broadband (analog) signaling.
Physical Layer
(Layer 1)
Data Link
(Layer 2) Logical Link Control (LLS) / Media Access Control (MAC)
Transport
(Layer 4)
Session
(Layer 5)
Presentation
(Layer 6)
Application
(Layer 7)
transport layer
Unlike the lower "subnet" layers whose protocol is between immediately adjacent nodes, the transport layer and the layers above are true "source to destination" or end-to-end layers, and are not concerned with the details of the underlying communications facility.
Application
HTTP, Telnet, FTP, TFTP, SNMP
Data Link Layer
Handles error recovery, flow control (synchronization) and sequencing (which terminals are sending and which are receiving). It is considered the "media access control layer" and is where MAC addressing is defined, MAC address is sometimes called the Ethernet address
TRANSPORT LAYER
can accept relatively large messages, but there are strict message size limits imposed by the network (or lower) layer. Consequently, the transport layer must break up the messages into smaller units, or frames, prepending a header to each frame.
NETWORK LAYER
controls the operation of the subnet, deciding which physical path the data should take based on network conditions, priority of service, and other factors.
TRANSPORT LAYER
ensures that messages are delivered error-free, in sequence, and with no losses or duplications.
Presentation
formats or presents data into a compatible form for receipt by the Application layer or the destination systems
PRESENTATION LAYER
formats the data to be presented to the application layer. It can be viewed as the translator for the network. This layer may translate data from a format used by the application layer into a common format at the sending station, then translate the common format to a format known to the application layer at the receiving station.
transport layer
header information must then include control information, such as message start and message end flags, to enable the transport layer on the other end to recognize message boundaries. In addition, if the lower layers do not maintain sequence, the transport header must contain sequence information to enable the transport layer on the receiving end to get the pieces back together in the right order before handing the received message up to the layer above
APPLICATION LAYER
serves as the window for users and application processes to access network services.
Network
Layer 3
APPLICATION LAYER
Directory services Network virtual terminals
Transport
Connection services, either reliable or unreliable delivery of data
Data Link
Data at this layer is referred to as a frame
Networks
Data at this layer is referred to as a packet
Transport
Data at this layer is referred to as a segment , end to end flow control and port and socket numbers are added
Physical Layer
Data at this layer is referred to as bits
Data Link
Defines how host on the network are identified, how and when devices can transmit on the network medium, how to verify that the data received from the physical layer is error free
Data Link
Defines the rules and procedures for host as they access the Physical Layer.
Network
Describes how data is routed across networks and on to the destination
Presentation Layer
Accepts and structures the messages for the application. It translates the message from one code to another if necessary , This layer is responsible for data compression and encryption. Examples of technologies working in this layer are American Standard Code for information interchange (ASCII) and Joint Photographic Experts Group (JPEG)
Network layer
Accepts outgoing messages and combines messages or segments into packets, adding a header that includes routing information, examples of protocols working in this layer are IP (internet protocol) and (IPX) Internetwork Packet Exchange
Networks
Determines the next network point to which data should be sent, routers use a routing protocol to take into account various factors such as the number of hops in the path, link speed, and link reliability to select the optimal path for data.
PRESENTATION LAYER
Character code translation: for example, ASCII to EBCDIC. Data conversion: bit order, CR-CR/LF, integer-floating point, and so on. Data compression: reduces the number of bits that need to be transmitted on the network. Data encryption: encrypt data for security purposes. For example, password encryption
Presentation
Encapsulation of data into message envelopes by encryption and compression
Transport Layer
Ensures error-free packets TCP, UDP
Session Layer
Establishes, manages, and terminates NFS, SQL
Presentation
Formatting and translation of data between systems, Negotiation of data transfer syntax between systems, through converting character sets to the correct format
DATA LINK LAYER
Frame delimiting: creates and recognizes frame boundaries. Frame error checking: checks received frames for integrity. Media access management: determines when the node "has the right" to use the physical medium
NETWORK LAYER
Frame fragmentation: if it determines that a downstream router's maximum transmission unit (MTU) size is less than the frame size, a router can fragment a frame for transmission and re-assembly at the destination station. Logical-physical address mapping: translates logical addresses, or names, into physical addresses.
Network
Identifies hosts and networks using logical addresses, Maintains a list of known networks and neighboring routers
Application Layer
Interacts with application programs that incorporate communication component such as your internet browser and email. This layer is responsible for logging the message in, interpreting the request, and determining what information is needed to support the requestion Examples include HTTP for web browsing, ftp for transferring files, and SMTP for email transmission
Transport Layer
Is concerned with message integrity between source and destination. It also segments/reassembles (the packets) and handles flow control. Examples of protocols working in this layer are Transmission Control Protocol (TCP and User Datagram Protocol (UDP)
The physical layer
It describes the electrical/optical, mechanical, and functional interfaces to the physical medium, and carries the signals for all of the higher layers
The physical layer
It determines: What signal state represents a binary 1 How the receiving station knows when a "bit-time" starts How the receiving station delimits a frame
The physical layer
It provides: Data encoding: modifies the simple digital signal pattern (1s and 0s) used by the PC to better accommodate the characteristics of the physical medium, and to aid in bit and frame synchronization
TRANSPORT LAYER
It relieves the higher layer protocols from any concern with the transfer of data between them and their peers.
Session
Management of multiple sessions, assignment of the session ID number, setup maintain and tear down communication sessions
TRANSPORT LAYER
Message acknowledgment: provides reliable end-to-end message delivery with acknowledgments. Message traffic control: tells the transmitting station to "back-off" when no message buffers are available. Session multiplexing: multiplexes several message streams, or sessions onto one logical link and keeps track of which messages belong to which sessions (see session laye
TRANSPORT LAYER
Message segmentation: accepts a message from the (session) layer above it, splits the message into smaller units (if not already small enough), and passes the smaller units down to the network layer. The transport layer at the destination station reassembles the message.
The physical layer
Physical medium attachment, accommodating various possibilities in the medium: Will an external transceiver (MAU) be used to connect to the medium? How many pins do the connectors have and what is each pin used for?
The physical layer
Physical medium transmission: transmits bits as electrical or optical signals appropriate for the physical medium, and determines: What physical medium options can be used How many volts/db should be used to represent a given signal state, using a given physical medium
APPLICATION LAYER
Resource sharing and device redirection Remote file access Remote printer access Inter-process communication Network management
NETWORK LAYER
Routing: routes frames among networks. Subnet traffic control: routers (network layer intermediate systems) can instruct a sending station to "throttle back" its frame transmission when the router's buffer fills up.
SESSION LAYER
Session establishment, maintenance and termination: allows two application processes on different machines to establish, use and terminate a connection, called a session.
SESSION LAYER
Session support: performs the functions that allow these processes to communicate over the network, performing security, name recognition, logging, and so on
Physical Layer
Signals and media NICs, twisted-pair cable, fiber
NETWORK LAYER
Subnet usage accounting: has accounting functions to keep track of frames forwarded by subnet intermediate systems, to produce billing information.
Session
The layers primary function is managing the sessions in which data is transferred
TRANSPORT LAYER
The size and complexity of a transport protocol depends on the type of service it can get from the network layer. For a reliable network layer with virtual circuit capability, a minimal transport layer is required. If the network layer is unreliable and/or only supports datagrams, the transport protocol should include extensive error detection and recovery.
Application
This layer integrates network functionality into the host operating system, and enables network services.
Physical Layer
This layer of the OSI model sets standards for sending and receiving electrical signals between devices
NETWORK LAYER
This layer relieves the upper layers of the need to know anything about the data transmission and intermediate switching technologies used to connect systems. It establishes, maintains and terminates connections across the intervening communications facility (one or several intermediate systems in the communication subnet)
SESSION LAYER
allows session establishment between processes running on different stations.
network layer
in this layer and the layers below, peer protocols exist between a node and its immediate neighbor, but the neighbor may be a node through which data is routed, not the destination station. The source and destination stations may be separated by many intermediate system
DATA LINK LAYER
provides: Link establishment and termination: establishes and terminates the logical link between two nodes. Frame traffic control: tells the transmitting node to "back-off" when no frame buffers are available. Frame sequencing: transmits/receives frames sequentially. Frame acknowledgment: provides/expects frame acknowledgments. Detects and recovers from errors that occur in the physical layer by retransmitting non-acknowledged frames and handling duplicate frame receipt.
Transport
provides a transition between the upper and lower layers of the OSI model making the upper and lower layers transparent for each other
DATA LINK LAYER
provides error-free transfer of data frames from one node to another over the physical layer, allowing layers above it to assume virtually error-free transmission over the link.
transport layer
software (and software above it) on the source station carries on a conversation with similar software on the destination station by using message headers and control messages.
NETWORK LAYER
software must build headers so that the network layer software residing in the subnet intermediate systems can recognize them and use them to route data to the destination address
The physical layer
the lowest layer of the OSI model, is concerned with the transmission and reception of the unstructured raw bit stream over a physical medium
