13.4 Ethernet Frame Truncation

As noted in Chapter 2, Ethernet frames require a minimum length of 64 bytes, even when the frame carries just one information character. Thus, a query/response client/server communications session in which a workstation user on an Ethernet initiates queries by entering a few search characters results in the transmission of a frame with a large number of PAD characters to ensure that the minimum packet size of 64 bytes is reached.

13.4.1 Overhead

While the overhead associated with the use of PAD characters is relatively insignificant on a 10-Mbps 10BASE-T network, those PAD characters become more significant when frames are directed across a WAN transmission facility operating at a fraction of the LAN operating rate. Recognizing the effect of padded Ethernet frames on communications over WAN facilities, Advanced Computer Communications was among one of the first vendors to offer Ethernet frame truncation in its remote bridge and router products.

The use of frame truncation results in Ethernet padded frames having their PAD characters removed prior to transmission over a WAN. This results in the transmission of reduced-length frames that require less time to transmit. This technique also permits the remote bridge or router to strip PAD characters from minimum-length Ethernet frames as they are placed in memory, permitting more frames to be stored in memory as well as the faster servicing of frames when they are packetized and placed on the transmission line.

13.4.2 Utilization Example

As an example of the potential benefit obtained from the use of Ethernet frame truncation, consider a situation in which 20 stations on an Ethernet have users interactively working with a program located on a remote Token Ring network through remote bridges communicating at 19,200 bps. Let us assume a worst-case scenario in which each user simultaneously enters a query using a five-digit invoice number. Then, instead of five characters in the information field, each frame must contain 46 characters, including PAD characters to comply with the Ethernet minimum packet length standard. Without considering the overhead of the protocol used for transmitting frames from the Ethernet to the Token Ring network, this worst-case situation results in 20 * 46, or 920 bytes, presented to the remote bridge or router when only 20 * 5, or 100 bytes, actually require transmission. Then, the interactive traffic would use 920 * 8/19,200, or approximately 38 percent of the WAN bandwidth without Ethernet frame truncation. With Ethernet frame truncation, transmission would be reduced to 100 bytes, which would use 100 * 8/19,200, or approximately 4 percent of the WAN bandwidth.