Intro

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:

[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:

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.

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:

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:

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:

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:

Some transport layer protocols include the following:

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:

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:

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

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:

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:

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

Network Protocols

Here are the most popular network protocols.

NetBEUI/NetBIOS

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:

TCP/IP

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
Application HTTP DNS, TLS/SSL, TFTP, FTP, IMAP, IRC, NNTP, POP3, SIP, SMTP, SNMP, SSH, TELNET, ECHO, BitTorrent, RTP, rlogin, ENRP
Transport TCP UDP, DCCP, SCTP, IL, RUDP
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
Host: www.fake.com
  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):

                              Browse
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 :

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:

AppleTalk

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:

APPC

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.

HDLC

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:

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)

IPX/SPX

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

Page Modified: (Hand noted: 2007-09-13 18:43:06Z) (Auto noted: 2007-11-17 06:38:48Z)