328 Practical Modern SCADA Protocols: DNP3, 60870.5 andRelatedSystems 12.8 MAC frame format The basic frame format for an 802.3 network is shown below. There are eight fields in each frame, and they will be described in detail. Figure 12.8 MAC frame format 12.8.1 Preamble This field consists of 7 octets of the data pattern 10101010. The preamble is used by the receiver to synchronize its clock to the transmitter. 12.8.2 Start frame delimiter This single octet field consists of the data 10101011. It enables the receiver to recognize the commencement of the address fields. 12.8.3 Source and destination address These are the physical addresses of both the source and destination nodes. The fields can be 2 or 6 octets long, although the six octet standard is the most common. The six octet field is split into two three octet blocks. The first three octets describe the block number to which all NICs of this type belong. This number is the license number and all cards made by this company have the same number. The second block refers to the device identifier, and each card will have a unique address under the terms of the license to manufacture. This means there are 2 48 unique addresses for Ethernet cards. There are three addressing modes that are available. These are: • Broadcast The destination address is set to all 1s or FFFFFFFFFFFF • Multicast The first bit of the destination address is set to a 1. It provides group restricted communications
12.8.4 Length Ethernet and TCP/IP networks 329 • Individual, or point-to-point First bit of the address set to 0, and the rest set according to the target destination node A two octet field that contains the length of the data field. This is necessary since there is no end delimiter in the frame. 12.8.5 Information 12.8.6 Pad 12.8.7 FCS The information that has been handed down from the LLC sub layer. Since there is a minimum length of the frame of 64 octets (512 bits or 576 bits if the preamble is included) that must be transmitted to ensure that the collision mechanism works, the pad field will pad out any frame that does not meet this minimum specification. This pad, if incorporated, is normally random data. The CRC is calculated over the data in the pad field. Once the CRC checks OK, the receiving node discards the pad data, which it recognizes by the value in the length field. A 32-bit CRC value that is computed in hardware at the transmitter and appended to the frame. It is the same algorithm used in the 802.4 and 802.5 standard. 12.9 Difference between 802.3 and Ethernet As has already been discussed, there is a difference between an 802.3 network and a Blue Book Ethernet network. These differences are primarily in the frame structure and are tabulated below. Table 12.1 Differences between IEEE 802.3 and Blue Book Ethernet (V2) The significant difference in the frame is the length field in 802.3 is interpreted as the higher protocol field in Ethernet. Since an 802.3 frame cannot be longer than 1500 bytes, the values in the protocol type field of the Ethernet V2 frame commence at 1500. This allows protocol analyzers to recognize one type of frame as opposed to the other.