Under the 802.2 standard, there are three types of service available for send-ing and receivsend-ing LLC data. These types are discussed in the next three paragraphs. Figure 4.9 provides a visual summary of the operation of each LLC service type.
Type 1
Type 1 is an unacknowledged connectionless service. The termconnectionless refers to the fact that transmission does not occur between two devices as if a logical connection were established. Instead, transmission flows on the channel to all stations; however, only the destination address acts upon the data. As the name of this service implies, there is no provision for the acknowledgment of frames. Neither are there provisions for flow control or for error recovery. Therefore, this is an unreliable service.
Despite those shortcomings, Type 1 is the most commonly used service, because most protocol suites use a reliable transport mechanism at the trans-port layer, thus eliminating the need for reliability at the link layer. In addition, by eliminating the time needed to establish a virtual link and the overhead of acknowledgments, a Type 1 service can provide a greater throughput than other LLC types of services.
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A
A
A
PDU
PDU
PDU
PDU = Protocol data unit
PDU
B
B
B ACK
ACK Type 3 acknowledged connectionless source Type 1 unacknowledged connectionless service
Type 2 connection-oriented service
Legend:
ACK = Acknowledgment A,B = Stations on the network
Figure 4.9 Local link control service types.
Type 2
The Type 2 connection-oriented service requires that a logical link be estab-lished between the sender and the receiver before information transfer. Once the logical connection is established, data will flow between the sender and receiver until either party terminates the connection. During data transfer, a Type 2 LLC service provides all of the functions lacking in a Type 1 service, using a sliding window for flow control. When IBM’s SNA data is transported on a LAN, it uses connection-oriented services. Type 2 LLC is also commonly referred to as LLC 2.
Type 3
The Type 3 acknowledged connectionless service contains provision for the setup and disconnection of transmission; it acknowledges individual frames using the stop-and-wait flow control method. Type 3 service is primarily used in an automated factory process-control environment, where one central computer communicates with many remote devices that typically have a limited storage capacity.
Classes of Service
All logical link control stations support Type 1 operations. This level of support is known as Class I service. The classes of service supported by
frame operations 181
LLC indicate the combinations of the three LLC service types supported by a station. Class I supports Type 1 service, Class II supports both Type 1 and Type 2, Class III supports Type 1 and Type 3 service, and Class IV supports all three service types. Because service Type 1 is supported by all classes, it can be considered a least common denominator, enabling all stations to communicate using a common form of service.
Service Primitives
The LLC sublayer uses service primitives similar to those that govern the exchange of data between the MAC sublayer and its peer LLC sublayer. In doing so, the LLC sublayer supports the Request, Confirm, Indicate, and Response primitives described in Section 4.2 of this chapter. The major difference between the LLC and MAC service primitives is that the LLC sublayer supports three types of services. As previously discussed, the available LLC services are unacknowledged connectionless, connection-oriented, and acknowledged connectionless. Thus, the use of LLC service primitives varies in conjunction with the type of LLC service initiated. For example, a connection-oriented service uses service primitives in the same manner as that illustrated in Figure 4.7. If the service is unacknowledged connectionless, the only service primitives used are the Request and Indicate, because there is no Response nor Confirmation.
4.4 Other Ethernet Frame Types
Three additional frame types that warrant discussion are Ethernet-802.3, Ethernet-SNAP, and the IEEE 802.1Q tagged frame. In actuality, the first two types of frames represent a logical variation of the IEEE 802.3 frame, in which the composition of the data field varies from the composition of the LLC protocol data unit previously illustrated in Figure 4.8. The third type of frame provides the ability to form virtual LANs (vLANs) as well as to assign a priority level to a frame.
Ethernet-802.3
The Ethernet-802.3 frame represents a proprietary subdivision of the IEEE 802.3 data field to transport NetWare. Ethernet-802.3 is one of several types of frames that can be used to transport NetWare. The actual frame type used is defined at system setup by binding NetWare to a specific type of frame.
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Preamble
Start of frame delimiter
Destination address
Source
address Length Data
Frame check sequence
IPX header Information
Figure 4.10 Novell’s NetWare Ethernet-802.3 frame. An Ethernet-802.3 frame subdivides the data field into an IPX header field and an information field.
Figure 4.10 illustrates the format of the Ethernet-802.3 frame. Due to the absence of LLC fields, this frame is often referred to asraw 802.3.
For those using or thinking of using NetWare, a word of caution is in order concerning frame types. Novell uses the term Ethernet-802.2 to refer to the IEEE 802.3 frame. Thus, if you set up NetWare for Ethernet-802.2 frames, in effect, your network is IEEE 802.3–compliant.