ISO OSI Network Reference Model Layers

The International Standards Organization (ISO) has devised the Open Systems Interconnect (OSI) network model. The OSI model divides network functionality into seven layers. Each layer may communicate only with its surrounding layers within a given host. Between hosts, each layer communicates to its corresponding layer on the other machine, but only through the lower layers on both machines.

7) Application - user Programs (ftp, telnet, netx, doom, nfs, etc...)
6) Presentation - data translation(Bit Order, Byte Order, Character Code, File Syntax), Encription. (https, https, etc...)
5) Session - control of sessions between hosts Establish(binding), Transfer, Release. (TCP connection, NetBIOS, ASP)
4) Transport - transmission control service, error recovery, segmentation. (TCP socket, UDP, NetBEUI, SPX, VIPC, X.25)
3) Network - move data from point A to point B through multiple systems. Usually software (IP, IPX, DDP, X.121) in Routers and Gateways
- Paths, Logical Addressing (IP tuples, X.121), Switching,
- Routing (Discovery and Selection),
- Control (Congestion management, Sequencing, Gateways) (ICMP)
2) Data Link - maintain and release data in packets or datagrams, error detection- , Usually firmware (SONET/SDH(FDDI, SMDS, ATM), SLIP, PPP, FDDI, Kermit, Z-Modem, IEEE 802) in (Bridges, Intelligent hubs, NICs) like:
- Logical Link Control (LLC)
  - Synchronization - Async (Standard Serial), Sync (SPI, IIC), Iso ()
  - Services - Flow Control (X-On/X-Off, RTS/CTS, Centronix BUSY), Error Checking(CRC)
- Media Access Control (MAC)
  - Protocol - Contention (Ethernet: CSMA/CD or CA), Token Passing (Token: IEEE 802.4 or .5), Polling
  - Physical Address - Hardware (MAC) Address
1) Physical - physical media definition. Move Bits from Host IO Ports through Repeaters to Remote Systems. Always hardware:
- Cables(CAT-5, RG-58, twisted pair), Fiber Optic or RF links,
- Connected (BNC, RJ-45, DB9/25, Antennas) to
- Drivers(Volts, Amps, Transducers(LEDs, Lasers, OpAmps, MAX232,), TX/RX), that produce
- Signals according to Standards(RS232, Ethernet 10/100Base2/T, modem,V.24, V.35, X.21, X.22),

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