Understanding How Wireless Fits into the OSI System Model

Posted by mekichan on Friday 29 August 2008 at 7:21 am

Wireless technology, as a networking component, is guided by the same standards processes and organizations defined for all other networking components in the industry. Although working in the networking industry can be difficult at best, there are many components to a network that can either make or break the system. In order to help standardize and define the areas a manufacturer must build their equipment to service, the International Organization for Standardization (ISO) created the Open Systems Interconnection (OSI) reference model. This model is a seven-layer approach to data networking. Each layer encompasses a specific set of tasks or standards that must be met in order for the network to function.We’ll review each layer in greater detail because this is a very important concept to understand.A comprehensive understanding of the OSI system model is of paramount importance for the internetworking designer, installer, or supportteam.

The seven layers to the OSI system model are as follows:

Physical layer
Data-link layer
Network layer
Transport layer
Session layer
Presentation layer
Application layer

We start at the bottom with the Physical layer.The Physical layer of the OSI system model is responsible for defining the electrical and mechanical aspects of networking.Topics such as cabling and the methods for placing the 0’s and 1’s of binary data on the medium are covered in great detail here. Standards such as Category 5, RS-232, and coaxial cable fall within the realm of the Physical layer.

The next layer is the Data-link layer.The Data-link layer defines the protocols that control the Physical layer. Issues such as how the medium is accessed and shared, how devices or stations on the medium are addressed, and how data is framed before transmission on the medium are defined here. Common examples of Data-link layer protocols are Ethernet,Token Ring, FDDI, and PPP.

Within the Data-link layer are two sublayers: the Media Access Control (MAC) and Logical Link Control (LLC).These two sublayers each play an important role in the operation of a network.We start with the MAC first.The MAC sublayer is responsible for uniquely identifying devices on the network. As part of the standards of the OSI system model, when a network interface in a router, switch, PC, server, or other device that connects to a LAN is created, a globally unique 48-bit address is burned into the ROM of the interface.This address must be unique or the network will not operate properly. Each manufacturer of network interfaces has been assigned a range of addresses from the Institute of Electrical and Electronics Engineers (IEEE).The MAC sublayer is considered the lower of the two sublayers and is also responsible for determining the access method to the medium, such as token passing (Token Ring or FDDI) or contention (CSMA/CD). Figure 1.5 shows an example of MAC addresses “on the wire” after being passed from the MAC layer to the Physical layer and being converted to 0’s and 1’s.

The next sublayer is the LLC layer.The LLC sublayer is responsible for handling error control, flow control, framing, and MAC sublayer addressing.The most common LLC protocol is IEEE 802.2, which defines connectionless and connection-oriented variants. IEEE 802.2 defines Service Access Points (SAPs) through a field in the Ethernet,Token Ring, or FDDI frame.Two SAPs are associated with LLC: the Destination Service Access Point (DSAP) and the Source Service Access Point (SSAP).These SAPs in conjunction with the MAC address can uniquely identify the recipient of a frame.Typically LLC is used for protocols such as SNA that do not have a corresponding network layer.

The next layer defined by the OSI reference model is the Network layer.The Network layer is responsible for addressing a network above the Data-link layer. The Network layer is where protocols such as Transmission Control Protocol/Internet Protocol (TCP/IP), Internetwork Packet Exchange (IPX) and AppleTalk tie into the grand scheme of things. Routing functions are also performed at the Network layer.TCP/IP routing protocols such as Routing Information Protocol (RIP), Open Shortest Path First (OSPF), and the Border Gateway Protocol (BGP) operate at the Network layer.We focus more on TCP/IP in the upcoming “Review of TCP/IP Basics” section.

The three previous layers we covered, Physical, Data-link, and Network, are considered the lower level protocols in the OSI reference model.These are the protocols that will more than likely consume the majority of your time as a network engineer. However, that does not mean that the next four layers are not important to the operation of a network.They are equally important, because without the next four layers, your network doesn’t even need to be in existence.

The fourth layer of the OSI system model is the Transport layer.The Transport layer defines the protocols that control the Network layer, similar to the way the Data-link layer controls the Physical layer.The Transport layer specifies a higher level of flow control, error detection, and correction. Protocols such as TCP, User Datagram Protocol (UDP), Sequenced Packet Exchange (SPX), and Name Binding Protocol (NBP) operate at this layer.These protocols may be connection-oriented, such as TCP and SPX, or connectionless, such as UDP.

The fifth layer of the OSI system model is the Session layer.The Session layer is responsible for establishing, managing, and terminating communication sessionsbetween Presentation layer entities and the Transport layer, where needed. Lightweight Directory Access Protocol (LDAP) and Remote Procedure Call (RPC) are examples of Session layer protocols.

The sixth layer of the OSI system model is the Presentation layer.The Presentation layer is responsible for ensuring that data sent from the Application layer of one device is comprehensible by the Application layer of another device. IBM’s Network Basic Input Output System (NetBIOS) and Novell’s NetWare Core Protocol (NCP) are examples of Presentation layer protocols.The ISO also developed a Presentation layer protocol named Abstract Syntax Notation One(ASN.1), which describes data types independent of various computer structures and representation techniques.ASN.1 was at one time thought to be the Presentation layer protocol of choice, when the ISO’s protocol stack was going to sweep the networking industry. Now we know that some components of ISO, such as Intermediate System to Intermediate System (IS-IS) as a routing protocol, and the X.500 directory services protocol have been widely deployed, while the majority of the protocol stack has been neglected.

The seventh, and final, layer of the OSI system model is the Application layer. The Application layer is responsible for providing network services to applications such as e-mail, word processing, and file transfer, which are not implicitly defined in the OSI system model.The Application layer allows developers of software packages to not have to write networking routines into their program. Instead, developers can utilize programming functions to the Application layer and rely upon Layer 7 to provide the networking services they require. Some common examples of Application layer protocols include Simple Mail Transfer Protocol (SMTP), Hypertext Transfer Protocol (HTTP), and Telnet.