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One of the main differences between IEEE s and Ethernet II s implementation of Ethernet is the framing used Recall from 2 that framing defines the format of information as it s carried across a data link layer medium A frame standardizes the fields in the frame and their lengths so that every device understands how to
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3: Layer 2 LAN Technologies
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FIGURE 3-1
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Ethernet 8023 Preamble Destination MAC address Source MAC address Length
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Ethernet frame types: Ethernet 8023 and Ethernet II
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IEEE s 8022 LLC encapsulated frame
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Ethernet II Preamble Destination MAC address Source MAC address Type
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read the contents of the frame The next sections discuss the framing that IEEE uses with the 8023 and 8022 standards and what Ethernet II uses
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IEEE 8023
As mentioned in Table 3-1, IEEE 8023 is responsible for defining the framing used to transmit information between two NICs The top part of Figure 3-1 shows the fields of an 8023 frame Table 3-2 explains the fields found in the frame
TABLE 3-2
Fields in the 8023 Frame
Field
Preamble Start of Frame (SOF) Destination MAC address Source MAC address Length Data FCS (field or frame checksum sequence)
Length in Bytes
Description
Identifies the beginning of the 8023 frame and is a string of 7 bytes of alternating 1s and 0s Indicates the following byte is the start of the frame The first 8 bytes are commonly referred to as the preamble, even though this is not quite true The MAC address to which the frame is to be sent The MAC address of the source of the frame Defines the length of the frame from this point to the checksum at the end of the frame The 8022 LLC encapsulated frame A checksum (CRC, or cyclic redundancy check) that is used to ensure that the frame is received by the destination error-free
6 6 2 Variable 4
Ethernet Evolution and Standards
The field checksum sequence (FCS) value is used to ensure that when the destination receives the frame, it can verify that the frame was received intact When generating the FCS value, which is basically a checksum, the NIC takes all of the fields in the 8023 frame, except the FCS field, and runs them through an algorithm that generates a 4-byte result, which is placed in the FCS field When the destination receives the frame, it takes the same fields and runs them through the same algorithm The destination then compares its 4-byte output with what was included in the frame by the source NIC If the two values don t match, the frame is considered bad and is dropped If the two values match, the frame is considered good and is processed further
IEEE 8022
IEEE 8022 (LLC) handles the top part of the data link layer Two types of IEEE 8022 frames are used: Service Access Point (SAP) and Subnetwork Access Protocol (SNAP) These 8022 frames are encapsulated (enclosed) in an 8023 frame when being sent to a destination Where 8023 (Ethernet) is used as a transport to get the 8022 frames to other devices, 8022 is used to define which network layer protocol created the data that the 8022 frame will include In this sense, it serves as a multiplexing function: it differentiates between TCP/IP, IPX, AppleTalk, and other network layer data types Figure 3-2 shows the two types of 8022 frames and Table 3-3 lists the fields found in an 8022 SAP frame
FIGURE 3-2
8022 SAP Destination SAP number Source SAP number Control
IEEE 8022 frame types: SAP and SNAP
Encapsulated upper-layer protocol packet
8022 SNAP Destination SAP number Source SAP number Control OUI ID
Type
Encapsulated network layer protocol packet
3: Layer 2 LAN Technologies
TABLE 3-3
Field
Destination SAP number Source SAP number Control field Data
Length in Bytes
1 1 1 2 Variable
Description
Identifies the network layer protocol to which this is to be sent Identifies the network layer protocol that originated this data Determines the fields that follow this field Contains the upper-layer network layer packet
8022 SAP Fields
When a destination NIC receives an 8023 frame, the NIC first checks the FCS to verify that the frame is valid and then checks the destination MAC address in the 8023 frame to make sure that it should process the frame (or ignore it) The MAC sublayer strips off the 8023 frame portion and passes the 8022 frame to the LLC sublayer The LLC examines the destination SAP value to determine which upper-layer protocol should have the encapsulated data passed to it Here are some examples of SAP values: TCP/IP uses 0x06 (hexadecimal) and IPX uses 0x0E If the LLC sees 0x06 in the SAP field, it passes the encapsulated data up to the TCP/IP protocol stack running on the device The second frame type supported by 8022 is SNAP, which is shown in the bottom portion of Figure 3-2 As you can see from this frame, two additional fields are included: OUI ID and Type Table 3-4 explains the 8022 SNAP fields One of the issues of the original SAP field in the 8022 SAP frame is that even though it is 8 bits (1 byte) in length, only the first 6 bits are used for identifying upper-layer protocols, which allows up to 64 protocols Back in the 1980s, many
TABLE 3-4
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