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U Format The unnumbered command/response U format may be used in type 1 or type 2 operation depending on specific functions used to provide additional data link control functions. Logical Link ControlMedia Access Control Interface. Two primitives are used for unacknowledged connectionless (type 1) service: request and indication (see Exhibit 1-1-8). The request primitive is used to pass a link service data unit frame to the logical link control. The indication primitive is used to pass a frame from the logical link control on reception. Type 1 operation is suitable for individual, group, global, and null destination-service-access-point addressing and applications that require no data link layer acknowledgment or flow control. IEEE 802.2 recommends a set of command and response protocol datal units for type 1 service, shown in Exhibit 1-1-9.
Connection-oriented (type 2) service supports a virtual circuit between service access points. A set of service primitives supply the mechanism for a user to request or be notified of the establishment or termination of a data link connection. These primitives are shown in Exhibit 1-1-10.
The sets of command and response protocol data units supported in type 2 operation are given in Exhibit 1-1-11. Exhibit 1-1-8 illustrates the operation of the request, indication, and confirm primitives. Exhibit 1-1-12 illustrates the frame types, commands, and responses appropriate to each frame type, and their meanings.
MEDIA ACCESS CONTROL The media access control sublayer allows network stations to share a single transmission facility. It provides the logic necessary to access the network for frame transmission and reception. Media access control capabilities can be grouped by function into data link activities and media management activities. The data link activities consist of both transmit data encapsulation and receive data decapsulation functions. The media management activities consist of both transmit and receive media access management functions. IEEE 802.3 Carrier-Sense Multiple Access with Collision Detection The first IEEE 802.3 CSMA/CD recommendation, 10Base-T, was issued in 1985. Since then, users demands have changed. The original rate of 10M bps is proving inadequate, and bit rates of 100M bps and higher are being demanded. At the same time, the industry wishes to keep as much of the old standards as possible and to ensure backwards compatibility. Thus, the media access control (MAC) and higher layers are left unchanged. Continued use of installed cables is desirable. Several fast 802.3 standards have been proposed, referred to as 100Base-T4, 100Base-X, 100Base-TX, however none has been approved. The IEEE 802.3 CSMA/CD recommendation covers the lowest two layers of the OSI reference model and comprises the media access control layer frame format, physical layer, and media specification. It also includes the media access control-physical layer interface and the medial access control-logical link control protocol interface. Together, the logical link control and media access control sublayer correspond to the OSI data link layer. The IEEE 802.3 domain is shown in Exhibit 1-1-13.
The interface between the logical link control and media access control layers is comparable to the LAP-B service request specification. It supports facilities required to transmit and receive frames, relay frame-processing parameters, and provide error and status information on an operation-by-operation basis to the network (and higher) layers.
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