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The layers can be viewed as a trickle down hierarchy of software procedures but must also include a trickle up hierarchical view as well.
While a detailed description and analysis of each OSI model layer is beyond the scope of this book, a knowledge of the basic purpose of each layer is useful. In the ATM and ADSL literature, there is often reference to the OSI model and the interworking relationships of the layers. The following discussion serves only as an introduction to the OSI model. The interested reader is encouraged to seek additional information and understanding.
The Physical Layer interface of the OSI model is concerned with the various physical interfaces of the equipment. Some of the issues this layer is concerned with are: voltages, electrical currents, frequencies, connectors, and transmission media, such as fiber, coaxial, or twisted pair. This layer is responsible for the physical generation and transmission of information and control signals. The Physical Layer interacts with the layer immediately above, the Data Link Layer. Control and data information are passed between layers through a software and hardware combination called a low level driver (LLD). The LLD is an electrical circuit that responds to low level (basic) software commands.
The Physical Layer controls the physical link between nodes of the communication path. The connections are called physical service access points (PSAPs). It also supervises the specific medium—coaxial, fiber, RF wave, twisted pair—of the transmission path. And it transmits the bits between nodes. The controlling aspect of the Physical Layer’s job includes such mundane activities as turning things off and on, as appropriate. The supervisory aspect includes monitoring the path and the data to detect conditions conducive to error generation. The transmission portion of its job is to cause the transmission medium to physically emit or receive the signal, either light wave, electrical, or RF wave, that represents the data.
Figure 4-3. OSI Data Link Layer
The Physical Layer responsibilities include the following functions:
Some of the specifications that define Physical Link Layers for specific types of networks include:
The Physical Layer interface may attach a trailer, header, or both to the data element. The trailer/header includes control information about the source and destination address, and error control information.
The Data Link Layer is responsible for establishing connection between networks, framing the data and control bits, and ensuring data integrity. The data integrity function provides flow and error control of the data transmitted over the physical link. The Data Link Layer operates on the physical devices involved in the electrical transmission of the data.
A primitive is a term used to describe a request for circuit connection, request for circuit deactivation, or a request to transfer data. Primitives are software routines of the lowest kind, in a manner of speaking. The Data Link Layer uses a defined set of primitives (basic software routines) that control the physical devices. An LLD interprets the primitives and manipulates the appropriate control circuits, typically called registers, that activate the circuit devices involved in determining the actual physical transmission path.
The Data Link Layer includes the Media Access Control (MAC). The MAC is just what it says it is. It is responsible for loading data into the transmission medium and unloading data received by the transmission medium. The MAC includes buffers (bit buckets, or more appropriately nowadays, byte barrels) to hold data on the transmit and receive paths. The MAC is able to accommodate differences in speed between applications and mediums to some degree. Also, the MAC frames the bits into ATM cells, both coming and going. And the MAC can request retransmission of data, if required.
The Data Link Layer also includes the Logical Link Control (LLC) Sub-layer. The LLC is the logical element that controls acknowledged, unacknowledged, connection-oriented, and connectionless-oriented services for the network node. The LLC manages and controls the flow of information into and out of the node based on the type of network connection bought and paid for.
The Data Link Layer responsibilities include:
Figure 4-4. OSI Network Layer
The Data Link Layer attaches a trailer, header, or both to the data element. The trailer/header includes control information about the source and destination address, and error control information.
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