OSI (Open Systems Interconnection) is the major architectural model for how data is transmitted between software applications on a network or inter-network. OSI was developed by the ISO (International Organization for Standardization) in 1978 with a major revision in 1984. It is currently ITU (International Telecommunication Union) Recommendation X.200.

The OSI reference model is a conceptual model that is implemented with miscellaneous protocols. The OSI reference model divides network responsibilities into seven ordered layers:

  • 7 Application
  • 6 Presentation/Syntax
  • 5 Session
  • 4 Transport
  • 3 Network
  • 2 Data Link
  • 1 Physical

[You may want to utilize some mnemonic phrase. EG: "Please Do Not Throw Sausage Pizza Away" or "All Parents Should Teach New Dads Parenting".]

Each layer has these characteristics:

  • Each layer is relatively independent. This means that tasks can be performed within a layer without too much concern for the other layers.
  • Each layer can communicate with a layer immediately next to it but only Physical Layer 1 can communicate between computers.
  • Each layer is said to have a corresponding peer layer on the computer it is communicating with. One of the main goals of a layer is to communicate with its peer layer.
  • Each layer can act as a service user by requesting services from a service provider (an adjacent layer). The conceptual point where the two layers interface is said to be the SAP (Service Access Point).
  • Layer n can encapsulate data from layer n+1 by prepending the data with a header and appending the data with a trailer. Layer n-1 can then treat the header-data-footer from layer n as data which can also be encapsulated.

Typically a user has software application that transmits data to Application Layer 7 and works through to the Session Layer 5. The user's host then goes thru the bottom four layers. Data is encapsulated by each layer as it goes down. The data is transmitted through the physical medium in the Physical Layer 1. When the other user gets the message, the process is reversed, i.e. each layer strips of the headers and trailers as the data goes up.

OSI Layers in 10 Words or Less

In ten words or less, here is a description of each layer.

  • Application: Provides applications with an interface to network services.
  • Presentation: Application specific data is put in generic format for transmission.
  • Session: Initiates, manages, and terminates communication sessions.
  • Transport: Manages flow control of data across networks.
  • Network: Addressing and routing of data.
  • Data Link: Converts bits into structured data frames and vice-versa.
  • Physical: Defines electrical and mechanical parameter of bits in transmission.

OSI Layers in Detail

Here are the layers in greater detail:

Application Layer 7

This is the highest layer. Provides applications with an interface to network services. Here are some application protocols:

  • SFP (AppleTalk File Protocol). Remote file management for the AppleTalk suite.
  • NCP (NetWare Core Protocol). Novell's redirectors and client shells.
  • TCP/IP (Transmission Control Protocol/Internet Protocol) suite applications. This includes the following:
    • FTP (File Transfer Protocol). Enables file transfer services.
    • SMTP (Simple Mail Transport Protocol). For e-mail transfer.
    • SNMP (Simple Network Management Protocol). Manages and monitors network devices.
    • Telnet. Enables logon and control of remote hosts (computers).
  • OSI suite applications. This includes FTAM, VTP, and CMIP.

Presentation Layer 6 (aka Syntax Layer 6)

Application specific data is put in generic format for transmission. Provides coding and conversion functions. Outgoing data in the application layer is converted into a standard format for transmission. Incoming files in standard formats can be converted into formats the application understands. This includes:

  • Common data representation formats. Formats images, sound, and video, eg QuickTime,  MPEG, GIF, JPEG, and TIFF.
  • Conversion schemes. Text and data representation, eg EBCDIC and ASCII.
  • Standard data compression schemes. EG: GIF and JPEG can both compress.
  • Standard data encryption schemes.

Session Layer 5

Initiates, manages, and terminates communication sessions. Communication occurs between nodes on the network in packets of time called sessions. The session layer also deals with identifying communicating parties and enabling those with proper security clearance. Session layer protocols include:

  • ZIP (Zone Information Protocol). Used by the AppleTalk protocol suite.
  • SCP (Session Control Protocol), the DECnet Phase IV protocol for sessions.

Transport Layer 4

Manages flow control of data across networks. Handles inter-network data transport services that are transparent to the upper layers. This includes:

  • Flow Control Output. This ensures that a transmitting device does not send more data than the receiving device can handle.
  • Packet Sizing. Makes sure large streams of data are broken into packets of acceptable sizes.
  • Error Checking. Checks for errors by various means, including packet counting.
  • Error Recovery. Resolves errors by various means, including asking for retransmission.
  • Multiplexing. This enables data from different applications to use the same physical link.
  • Virtual Circuit Management. The initiation, maintenance, and termination of virtual circuits.

Some transport layer protocols include the following:

  • ATP (AppleTalk Transaction Protocol) and NBP (Name Binding Protocol). Used by the AppleTalk protocol suite.
  • NetBIOS (Network Basic Input/Output System). Microsoft's and IBM's LAN protocol suite.
  • NWLink. (NetWare Link). Microsoft's implementation of NetWare protocol.
  • SPX (Sequenced Packet eXchange). Novell's connection-oriented protocol.
  • TCP (Transmission Control Protocol). The eminently popular transport control that is part of the TCP/IP suite that makes the Internet go.

Network Layer 3

Addressing and routing of data. Enables multiple simultaneous communication links in an intra-network. Addressing translates logical addresses and names into their corresponding physical counterparts. Routing decides the path data takes. Routing decisions are based on quality of service, alternative routes, and delivery priority.

This layer also does packet switching, data routing, and network congestion control. sdThis layer supports both connection-oriented and connectionless services from the upper layers. This includes the following routing protocols:

  • BGP (Border Gateway Protocol). Used for Internet inter-domain routing.
  • DDP (Delivery Datagram Protocol). The data transport protocol used in AppleTalk.
  • IP (Internet Protocol). The eminently popular addressing protocol that is part of the TCP/IP suite that makes the Internet go.
  • IPX (Internetwork Packet eXchange). Novell's NetWare protocol.
  • NetBIOS (Network Basic Input/Output System). Microsoft's and IBM's LAN protocol suite.
  • NWLink. (NetWare Link). Microsoft's implementation of NetWare protocol.
  • OSPF (Open Shortest Path First). A link-state, interior gateway protocol used in TCP/IP networks.
  • RIP (Routing Information Protocol).

Data Link Layer 2

Converts bits into structured data frames and vice-versa. It sends data frames between the network and physical layers. Provides reliable transit of data by specifying network and protocol characteristics, including the following:

  • Physical Addressing.
  • Network Topology. This includes different arrangements such as bus or ring.
  • Error Notification. Alerts upper layers that an error has occurred.
  • Data Frame Sequencing. Reorders data transmitted out of sequence.
  • Flow Control Input. This ensures that a receiving device does not take more data than it can handle.

The Data Link Layer 2 is further subdivided into two sublayers by the IEEE:

  • LLC (Logical Link Control). This is the upper sublayer, IEEE specification 802.2. It manages communications between devices over a single link. This supports both connection-oriented and connectionless services. This includes the usage of SAPs as mentioned earlier.
  • MAC (Media Access Control). This is the lower sublayer, IEEE specifications 802.3-5 and 802.12. It does two basic things:
    • It manages protocol access to the Physical Layer 1.
    • It enables MAC addresses which allows devices to be uniquely identified.

Physical Layer 1

This is the lowest layer. It defines the electrical, mechanical, procedural, and functional specs for activating, maintaining, and deactivating the physical hardware connection. It converts electrical signals into bits and vice versa. This includes voltage levels that define the binary 0s and 1s, the timing of voltage changes, the maximum media distances, the media connectors, and the media specs. EG: This layer defines the physical characteristics of the CAT 5 cables and how it is used in Ethernet LANs.

OSI and Protocols

The top three layers are considered the upper layers or "Application" layers. The bottom four layers are considered the lower layers or "Data Transport" layers.

The top five layers are typically implemented only in software whereas the bottom two layers are implemented as both hardware and software.

OSI is implemented by communications protocols such as the TCP/ICP suite. There are four kinds of communications protocols:

  • LAN. These usually operate in the bottom two layers.
  • WAN. These usually operate in the bottom three layers.
  • Routing. These usually operate in Network Layer 3.
  • Network. These usually operate in the upper layers (7 through 4) as part of a protocol suite.

The IEEE (Institute of Electrical and Electronics Engineers) 802 Specifications zoom in on the lower layers of the OSI Reference Model. The 802 Project was started in February 1980, hence the name. The 802 specs have 12 categories covering network topologies, interface cards, and connections:

  • 802.1 Internetworking.
  • 802.2 LLC (Logical Link Control).
  • 802.3 Ethernet LANs (Local Area Network), i.e. CSMA/CD (Carrier-Sense Multiple Access with Collision Detection) or 10BASE-T. See also my definition of Ethernet.
    • 802.3z 1000BASE-T or gigabit Ethernet.
  • 802.4 Token Bus LAN.
  • 802.5 Token Ring LAN. See also my definition of Token Ring.
  • 802.6 MAN (Metropolitan Area Network).
  • 802.7 Broadband Technical Advisory Group.
  • 802.8 Fiber Optic Technical Advisory Group.
  • 802.9 Integrated Voice and Data Networks.
  • 802.10 Network Security.
  • 802.11 Wireless Networks.
  • 802.12 Demand Priority Access LAN, 100.

Different NICs (Network Interface Cards) can utilized different network protocols to communicate.

  • Novell and Apple developed ODI (Open Datalink Interface). ODI allows IPX/SPX (or Microsoft's NWLINK) to be bound with multiple NIC drivers.
  • Microsoft and IBM developed NDIS (Network Device Interface Specification). NDIS resides between the NIC drivers (that is the MAC) and the protocol stacks in the Data Link layer. NDIS can bind:
    • Multiple NICs to one protocol stack.
    • Multiple protocol stack to one NIC.
    • Multiple NICs to multiple protocol stack.

Network Protocols

Here are the most popular network protocols.


NetBIOS Extended User Interface and Network Basic Input/Output System. One of the most popular LAN enabling suites. It is used in Ethernet, token ring, and Windows NT networks. It provides services to the Session and Transport layers, including error detection and recovery.

It does not however provide routing support so it cannot extend into WANs or MANs. NetBIOS is frequently used in conjunction with IPX or TCP/IP to reach outside of the LAN.

NetBEUI extends NetBIOS so as to include data frame formatting as is needed by the Data Link layer.

NetBIOS has two communication modes:

  • Session. Provides connection-oriented service. This is good for large, sequential messages.
  • Datagram. Provides connectionless service. This is good for messages that can be sent in non-sequential packets.


Transmission Control Protocol/Internet Protocol. The most popular protocol suite, especially because it is the suite for the Internet. The OSI Layers are often reduced to 4 layers for the TCP/IP stack:

Layer Most common protocols Other protocols
Network IP IPv6
Link Ethernet Wi-Fi, Token ring, PPP, SLIP, FDDI, ATM, Frame Relay, SMDS

HTTP (Hyper Text Transfer Protocol) is a request/response protocol. HTTP is considered stateless since it cannot maintain application information from session to session. The 8 methods or "verbs" of HTTP requests are as follows:

  1. GET. The most common. Requests a representation of the resource. EG:
    GET /index.html HTTP/1.1
  2. HEAD. Like GET but requests just the header. Headers are always processed before the rest. An HTTP header may be plugged into HTML via code like this:
    <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
  3. POST. Submits user data (usu. via a form) as part of the request.
  4. PUT. Submits a representation of a resource.
  5. DELETE. Deletes the representation of a resource.
  6. TRACE. Echoes a received request. For confirmation and meta data about the intermediate or target servers or both.
  7. OPTIONS. Requests which HTTP methods are supported by the server. Most servers do GET, HEAD, and OPTIONS.
  8. CONNECT. For proxies and SSL options.

The 4 key verbs of HTTP are an important concept that is often related to SQL, CRUD, and BARED (or BREAD but the order is different):

PUT       INSERT    Create    Add
GET       SELECT    Read      Read
POST      UPDATE    Update    Edit
DELETE    DELETE    Delete    Delete

Other famous Application layer protocols of the TCP/IP suite include the following :

  • FTP (File Transfer Protocol). Enables file transfer services.
  • SMTP (Simple Mail Transport Protocol). For e-mail transfer.
  • SNMP (Simple Network Management Protocol). Manages and monitors network devices.
  • TELNET. Enables logon and control of remote hosts (computers).

TCP (Transmission Control Protocol) itself provides Transport layer services especially by breaking data into packets and managing the packets. TCP establishes connections, transfers data, and terminates connections. TCP establishes the ports that correspond to where applications send and receive data. TCP is "reliable" in that it ensures that packets are uncorrupted, in sequence, not duplicated, and all accounted for.

IP (Internet Protocol ) itself provides Network layer services. Globally unique IP addresses are assigned to each device connected to the network. This is how the whole planet is actually a giant network. IP follows the packet switching paradigm instead of the circuit switching paradigm. IP is "unreliable" in that packets may be corrupt, out of sequence, duplicates, or lost.

In addition, the TCP/IP suite provides cross-platform support, routing capabilities, and support for many other services including the following:

  • DHCP (Dynamic Host Configuration Protocol). Dynamically leases IP address to different users and computers on a network as needed. DHCP comes with the NT OS.
  • DNS (Domain Name Service). Maps IP addresses into user friendly Internet domain names. DNS servers are distribute throughout the Internet that share their information so that users can access virtually any domain name.
  • WINS (Windows Internet Name Service). Maps IP addresses to workstation names.


Enables communication with and between Macintosh computers. Windows can support AppleTalk but it needs to have NT Services for Macintosh installed first. The Macs are limited to accessing data on NTFS partitions.

This suite includes:

  • SFP (AppleTalk File Protocol). Remote file management, an Application layer protocol.
  • ZIP (Zone Information Protocol). A Session layer protocol.
  • ATP (AppleTalk Transaction Protocol) and NBP (Name Binding Protocol). A Transport layer protocol.
  • DDP (Delivery Datagram Protocol). The data transport protocol used in AppleTalk. A network layer protocol.


Advanced Program-to-Program Communication protocol. A protocol used in IBM's SNA (Systems Network Architecture). This enables peer-to-peer communication between mainframes such as the S/390 and mid-ranges such as the AS/400.


High-Level Data Link Control. A flexible, bit-oriented Data Link layer protocol that is still frequently used with mainframes. It is an ISO standard. HDLC supports:

  • both half- and full-duplexing transmission
  • both circuit-switched and packet-switched networks
  • both peer-to-peer and client/server networks
  • both wired and wireless media.

HDLC is based on SDLC (Synchronous DLC), IBM's protocol for mainframe communications. Some of the variations of HDLC include the following:

HDLC Variation Uses
NRM (Normal Response Mode) Multipoint networks that typically use SDLC
LAP (Link Access Procedure) Early X.25 implementations
LAPB (LAP Balanced) Current X.25 implementations
LAPD (LAP for the ISDN D channel) ISDN D channel and frame relay
LAPM (LAP for Modems) Error-correcting modems (specified as part of V.42)


Internetwork Packet Exchange/Sequenced Packet Exchange. A networking protocol suite for Novell's NetWare clients and servers, just as TCP/IP is to the Internet. IPX/SPX was based on XNS (Xerox Network System), the Ethernet protocol by Xerox.

IPX breaks manages and forms data packets in the Network Layer. IPX is analogous to IP of the TCP/IP suite.

SPX handles the sequencing, error checking, retransmission, and reassembly of said packets in the Transport Layer. SPX is analogous to TCP of the TCP/IP suite.

IPX/SPX also includes other protocols such as

  • NCP (NetWare Core Protocol). Novell's redirectors and client shells. Application layer.
  • RIP (Routing Information Protocol).
  • SAP (Service Advertising Protocol).
  • NLSP (NetWare Link Services Protocol).

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