One of the more interesting facets of IEEE 802 standards was the initial subdivision of the ISO Open System Interconnection Model’s data link layer into two sublayers: logical link control (LLC) and medium access control (MAC). Figure 2.5 illustrates the relationship between IEEE 802 local area network standards and the first three layers of the OSI Reference Model.
The separation of the data link layer into two entities provides a mechanism for regulating access to the medium that is independent of the method for establishing, maintaining, and terminating the logical link between worksta-tions. The method of regulating access to the medium is defined by the MAC portion of each LAN standard. This enables the LLC standard to be applicable to each type of network.
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Figure 2.5 Relationship between IEEE standards and the OSI Reference Model.
Medium Access Control
The MAC sublayer is responsible for controlling access to the network. To accomplish this, it must ensure that two or more stations do not attempt to transmit data onto the network simultaneously. For Ethernet networks, this is accomplished through the use of the CSMA/CD access protocol.
In addition to network access control, the MAC sublayer is responsible for the orderly movement of data onto and off of the network. To accomplish this, the MAC sublayer is responsible for MAC addressing, frame type recognition, frame control, frame copying, and similar frame-related functions.
The MAC address represents the physical address of each station connected to the network. That address can belong to a single station, can represent a predefined group of stations (group address), or can represent all stations on the network (broadcast address). Through MAC addresses, the physical source and destination of frames are identified.
Frame type recognition enables the type and format of a frame to be recognized. To ensure that frames can be processed accurately, frame control prefixes each frame with a preamble, which consists of a predefined sequence
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of bits. In addition, a frame check sequence (FCS) is computed by applying an algorithm to the contents of the frame; the results of the operation are placed into the frame. This enables a receiving station to perform a similar operation.
Then, if the locally computed FCS matches the FCS carried in the frame, the frame is considered to have arrived without error.
Once a frame arrives at a station that has the same address as the destination address in the frame, that station must copy the frame. The copying operation moves the contents of the frame into a buffer area in an Ethernet adapter card.
The adapter card removes certain fields from the frame, such as the preamble and start of frame delimiter, and passes the information field into a predefined memory area in the station into which the adapter card is inserted.
Refer to Chapter 4 for detailed information concerning Ethernet frame for-mats, as well as information concerning how the MAC layer controls the transmission and reception of data on an Ethernet local area network.
Logical Link Control
Logical link control frames are used to provide a link between network layer protocols and media access control. This linkage is accomplished through the use of service access points (SAPs), which operate in much the same way as a mailbox. That is, both network layer protocols and logical link control have access to SAPs and can leave messages for each other in them.
Like a mailbox in a post office, each SAP has a distinct address. For the logical link control, a SAP represents the location of a network layer process, such as the location of an application within a workstation as viewed from the network. From the network layer perspective, a SAP represents the place to leave messages concerning the network services requested by an application.
LLC frames contain two special address fields, known as the destination services access point and the source services access point. The destination services access point (DSAP) is one byte in length and specifies the receiving network layer process. The source services access point (SSAP) is also one byte in length. The SSAP specifies the sending network layer process. Both DSAP and SSAP addresses are assigned by the IEEE. Refer to Chapter 4 for detailed information concerning LLC frame formats and data flow.
Additional Sublayering
As previously mentioned, the standardization of high-speed Ethernet resulted in an additional sublayer at the data link layer, and the subdivision of the physical layer. Figure 2.6 illustrates the relationship between the first two layers of the ISO Reference Model and the IEEE 802.3µFast Ethernet sublayers.
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Logical link control Media access control Reconciliation sublayer Medium-independent interface
Physical coding sublayer
Physical medium attachments
Physical medium dependent Physical
layer Data link layer
Figure 2.6 IEEE 802.3µsublayering.
The additional sublayering illustrated in Figure 2.6 became necessary, as it was desired to support different media with one standard. To accomplish this required the physical layer to be independent from the data link layer, because there can be different coding schemes used to support transmission on different types of media.
To retain the CSMA/CD access protocol while supporting the use of different media required the use of different connectors, resulting in the introduction of a physical medium-dependent (PMD) sublayer. Because different data coding schemes are required to support 100 Mbps on different types of media, a physical coding sublayer was introduced. This sublayer defines the coding method used for transmission on different types of media. To map messages from the physical coding sublayer onto the transmission media resulted in those functions being performed by the physical medium attachment sublayer.
Thus, the physical layer was subdivided into three sublayers.
Although not shown on Figure 2.6, it should be noted that an Auto-Negotiation function resides under the PMD. The Auto-Auto-Negotiation function was added to provide an ease of migration from 10 Mbps Ethernet to 100 Mbps Ethernet and results in the Media-Independent Interface (MII) supporting both 10 and 100 Mbps data transfer. To accomplish this the MII clock is capable of operating at 2.5 MHz and 25 MHz.
At the data link layer an additional sublayer, known as the reconciliation sublayer, was introduced. This sublayer is responsible for reconciling the MII from the physical layer, with the MAC signal.
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Logical link control
Reconciliation Gigabit media-independent
interface Physical coding sublayer Physical medium attachment Physical medium dependent Physical
layer Data link layer
Media access control
Figure 2.7 Subdivision of the physical layer of Gigabit Ethernet.
Recognizing that Gigabit Ethernet would operate on different types of media also resulted in the subdivision of its physical layer. That subdivision is illustrated in Figure 2.7.
The reconciliation sublayer represents a transparent interface between the MAC sublayer and the physical layer which decouples the MAC layer from the physical layer. The Gigabit Media Independent Interface (GMII) includes transmit and receive data paths that are 8 bits in width, which, when coupled with a clock that now operates at 125 MHz, results in a data transfer capability of 1 Gbps.
From a comparison of Figure 2.6 and Figure 2.7, you will note that the sublayering of Gigabit Ethernet is similar to that of Fast Ethernet. However, the sublayers perform different functions. For example, under Fast Ethernet coding is based on the FDDI specification. In comparison, under Gigabit Ethernet reliance is shifted to the physical sublayers previously specified for the Fiber channel, as the latter operates at approximately Gigabit data rates and was selected for use by the 802.38 project members.
2.4 Internet Standards
The Internet as we know it dates to 1967 when the Advanced Research Projects Agency (ARPA), operating as a part of the United States Office of the
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Secretary of Defense, issued a Request for Proposal (RFP) for the creation of a packet switching network. The result of the RFP was a contract issued to Bolt, Beranek and Newmann (BBN), a then small company based in Cambridge, MA, whose efforts resulted in a network that enabled scientists and educators to share information. That network was known as ARPAnet.