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3-3
Internetworking at the Physical and Data Link Layers

DAVID KOEHLER

Assembling a departmental LAN has become, thanks to such standards as 10Base-T and robust network operating systems, a matter of assembling the appropriate off-the-shelf hardware and software. The next challenge is connecting individual LANs together in a corporate internet, a job that is not yet as simple as creating the local network.

This chapter focuses on the different internetworking options that are available to the network administrator or manager. Internetworking devices are products that allow processors on remote LAN segments to communicate. Throughout, the term LAN segment means a group of processors that communicate using a single LAN protocol, over a shared physical medium, with no modification of the generated bit pattern occurring between the originating and receiving stations. There are four primary internetworking devices: LAN repeaters, WAN transmission units, bridges, and routers. The concept of a layered communication protocol is fundamental to internetworking, and these internetworking devices may be differentiated according to the Open Systems Interconnection (OSI) protocol layer at which the LAN-to-LAN connection is established. However, it should be noted that although the OSI communications stack provides the model for most modern communications protocols, many modern protocols do not follow the exact layer definitions in OSL.

A REVIEW OF OSI

The OSI model comprises seven layers: physical, link, network, transport, session, presentation, and application. Each of the protocol layers has a unique autonomous function and a well-defined interface point to each of the adjacent layers.

The Physical Layer

The lowest layer of the stack is the physical layer. The functions within this layer initialize, maintain, and condition a circuit between the data terminal equipment (DTE) and the data communications equipment (DCE). This level of the protocol can only interpret electrical signals, either binary 1s and 0s in digital signals or continuous voltages in analog signals. Many physical layer standards exist for both point-to-point circuits and the multiple access media that are typical of LANs. Common point-to-point interfaces are EIA-232, V.35, and V.42. A common LAN physical protocol, used for Ethernets, is carrier-sense multiple access with collision detection (CSMA/CD). This protocol is transmitted over the shared Ethernet media using Manchester coding.

The Link Layer

The link layer transfers data across the link and performs bridging in LANs and synchronous data link control (SDLC) transmission over serial lines. The link layer has a defined interface to both the physical layer and the network layer. The well-defined interface between the physical and link layers provides flow control to ensure that the DTE does not become overburdened, and error checking on inbound data. The link layer is also responsible for bit positioning within the data unit. The defined link data unit has a link header (i.e., a prefix) and link trailer (i.e., a suffix) that encapsulates the raw data. It is this defined data unit that is transferred to the network layer.

The Network Layer

The network layer is the place in the protocol model where the concept of the network is introduced. It is also responsible for network addressing and datagram routing between networks; the network layer datagram has a network header for network based addressing. The entire network layer datagram, encapsulated in the link data unit, provides addressing information to the transport layer. Encapsulation is the operation that enables routers to transparently operate between different link layer protocols (e.g., Ethernet and Token Ring).

The Transport Layer

The transport layer is the boundary between the lower and higher network-oriented layers. Higher layer functions are more visible to the end user. The initial few bytes of the network layer datagram are the transport layer header, which contains all the transport layer controls. The transport layer handles end-to-end transport of network layer diagrams traversing an internet, maintaining the flow control and acknowledgement mechanisms. The transport layer also has a well-defined interface to the session layer.

The Session Layer

The session layer is not defined in all communications protocols, but is defined in OSI. A session layer header is passed through the underlying transport layer as data, becoming functional once the transport layer is stripped off. The session layer organizes the exchange of data between workstations, attending to simultaneous transmission, alternate transmission, and resynchronization of user data flow between user applications. An example of a session layer function would be the setup of a terminal or file transfer synchronization process, which is required before a terminal or file transfer application can commence. The session layer has an upper layer interface to the presentation layer that facilitates the transfer of application based data over the established session.

The Presentation Layer

The presentation layer ensures that application-generated data structures are accurately mapped or translated to meet the requirements of the specific applications. A presentation header is incorporated as part of the data unit at the session layer boundary. The presentation layer is concerned with data syntax and the negotiation functions that facilitate peer-to-peer data structure specifications. The presentation layer is responsible for the organization of the data generated through the interface from the application layer.

The Application Layer

The application layer supports end-user applications. It defines the semantics for the representation of user data in graphical user interfaces and supports such process as job management, file transfer, and electronic messaging. This is the protocol layer that affects the end user most directly.

The point of the OSI model is that independent layers (i.e., software modules) communicate through defined interfaces, thereby minimizing the complexity of any single layer. This layered approach to network protocols also simplifies the integration of code enhancements, because the code in all layers except where the change occurs will remain unaffected. Although each layer adds a separate header or trailer, layer encapsulation provides for a well-ordered communication mechanism.

Internetworking devices usually function in the physical, link, and network layers. LAN based repeater and WAN transmission devices (e.g., data service units/channel service units (DSU/CSUs) and modems) operate at the physical layer. Bridges and WAN packet switches function at the link layer. Routers and a portion of the X.25 packet switch function at the network layer.


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