Osi Layers And Protocols

ISO Layers and Protocols The ISO-OSI 7 layer Reference Model (officially known as ISO Standard 7498, 1984, 74981:1994. and CCITT standard X.200) was developed by the Internet Architecture Board and drafted by the IETF. “It provides a common basis for the coordination of standards development for the purpose of systems interconnection, while allowing existing standards to be placed into perspective within the overall Reference Model. The model identifies areas for developing or improving standards. It does not intend to serve as an implementation specification.”

Layer Application User Interface

Function • • • • •

Presentation Translation

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Session "syncs and sessions"

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Protocols

Network Components

used for applications specifically written to run over the network allows access to network services that support applications; directly represents the services that directly support user applications handles network access, flow control and error recovery Example apps are file transfer, e-mail, NetBIOS-based applications

DNS; FTP; TFTP; Gateway BOOTP; SNMP;RLOGIN; SMTP; MIME; NFS; FINGER; TELNET; NCP; APPC; AFP; SMB

Translates from application to network format and vice-versa all different formats from all sources are made into a common uniform format that the rest of the OSI model can understand responsible for protocol conversion, character conversion, data encryption / decryption, expanding graphics commands, data compression sets standards for different systems to provide seamless communication from multiple protocol stacks not always implemented in a network protocol

Gateway

establishes, maintains and ends sessions across the network responsible for name recognition (identification) so only the designated parties can participate in the session provides synchronization services by planning check points in the data stream => if session fails, only data after the most recent checkpoint need be transmitted manages who can transmit data at a certain time and for how long Examples are interactive login and file transfer connections, the session would connect and re-connect if there was an interruption; recognize names in

Redirector

NetBIOS Names Pipes Mail Slots RPC

Gateway

sessions and register names in history

Transport packets; flow control & error-handling

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• • • •

Network addressing; routing

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•

additional connection below the session TCP, ARP, RARP; Gateway layer manages the flow control of data SPX Advanced Cable between parties across the network Tester divides streams of data into chunks or NWLink packets; the transport layer of the receiving computer reassembles the message from packets NetBIOS / "train" is a good analogy => the data is NetBEUI divided into identical units provides error-checking to guarantee error-free data delivery, with on losses ATP or duplications provides acknowledgment of successful transmissions; requests retransmission if some packets don’t arrive error-free provides flow control and errorhandling translates logical network address and names to their physical address (e.g. computer name ==> MAC address) responsible for o addressing o determining routes for sending o managing network problems such as packet switching, data congestion and routing if router can’t send data frame as large as the source computer sends, the network layer compensates by breaking the data into smaller units. At the receiving end, the network layer reassembles the data think of this layer stamping the addresses on each train car

IP; ARP; RARP, ICMP; RIP; OSFP; Router IGMP; IPX

Frame Relay Device

NWLink

ATM Switch

NetBEUI

Advanced Cable Tester

OSI DDP DECnet

Data Link data frames to bits

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turns packets into raw bits 100101 and Logical Link Bridge at the receiving end turns bits into Control packets. Switch handles data frames between the • error Network and Physical layers correction and ISDN Router the receiving end packages raw data flow control from the Physical layer into data frames • manages link for delivery to the Network layer control and Intelligent Hub responsible for error-free transfer of defines SAPs frames to other computer via the NIC Physical Layer 802.1 OSI Model this layer defines the methods used to Advanced Cable transmit and receive data on the network. It consists of the wiring, the 802.2 Logical Link Tester devices use to connect the NIC to the Control wiring, the signaling involved to Media Access transmit / receive data and the ability to Control detect signaling errors on the network

media • •

communicates with the adapter card controls the type of media being used:

802.3 CSMA/CD (Ethernet) 802.4 Token Bus (ARCnet) 802.5 Token Ring

Physical hardware; raw bit stream

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802.12 Demand Priority transmits raw bit stream over physical IEEE 802 cable defines cables, cards, and physical IEEE 802.2 aspects defines NIC attachments to hardware, ISO 2110 how cable is attached to NIC defines techniques to transfer bit stream ISDN to cable

Repeater Multiplexer Hubs TDR Oscilloscope Amplifier

ISO/OSI Layer 7: Application Written by and for users. This is what pays for the other 6 layers!

Layer 6: Presentation Usually based on some kind of standard, such as HTML.

Layer 5: Session Deals usually with opening and closing a connection of some sort, or with an organized exchange of individual messages. This is the obvious place for • • •

login usage charges checkpointing and restarting in case of failure

Layer 4: Transport Deals with carrying data from one computer to another. We will consider primarily the two protocols TCP Used to operate a pair of byte streams between the computers. UDP Used to send individual messages between the computers.

Layer 3: Network This layer concentrates on getting data across a catenated network, using network layer addresses to determine the LANs that should be traversed, and the gateways to be used. We will focus exclusively on IP.

Layer 2: Link This is the "layer of the LAN". There is a wide range of link technology, with protocols to match. Here we have addresses that reflect a particular LAN, and the job of this layer as a provider is to format/parse data from/to the network layer in order to get it across the LAN, from network source to gateway to gateway ... until it reaches the network destination. This layer is usually subdivided into logical link control ( LLC) media access control (MAC)) which deals with how a particular station obtains access to the transmission medium, and approprate addresses at the link level, and identification of the peer provider at this level. Some of the Link technologies we will consider are: ethernet serial FDDI ATM wireless

Layer 1: Hardware This layer basically transfers on getting bits on to and off from a transmission medium, synchronized with some kind of clock so that the bits may be framed into bytes, and the butes may be framed in various ways. Some of the media used are twisted pair and coaxial cable / NIC fiber optics