Justifications for Choosing the C-4 Size

Optical Bandwidth and avoiding Fragmentation

Deciding on an SDH container size to accommodate payload requirements was one of the initial requirements of our project. The first consideration is that 95% of Canada's high capacity fibre optic lines are currently unlit, so bandwidth is not currently the main issue on this type of medium. The data rate of our PHY device was specified at OC-48, so we felt we qualified as Path Terminating Equipment of a high-capacity fibre optic line. Consequently, we simply chose an SDH container of sufficient size to contain the largest possible packet we might have to eliminate the need to handle packet fragmentation.

HDLC Bandwidth Expansion

The PPP protocol passed to our device from the LINK layer over the PL3 interface allows a maximum payload of 1500 bytes per packet. However, as the PPP is HDLC-framed, certain bytes in the PPP packets must be escaped. For example, as described in HDLC Bytestuffing, the HDLC framing character 0x7D must be replaced with two bytes in order to avoid framing errors at the receiver. Thus, a bandwidth expansion of 100% is possible between the receipt of PPP packets across the PL3 interface and the final HDLC-framed PPP packets. The implication that follows is that the final HDLC-framed packets may be just over 3000 bytes long.

However, barring malicious intent, this is a highly unlikely scenario. Thus, our final decision for container size was C-4 with a capacity of 2340 bytes. This allows for a PPP packet bandwidth expansion of just over 50% for the maximum size PPP packet and is a convenient size in general as it requires the least subsequent mucking about to produce an STM-16 frame. Any HDLC-framed packets that end up being longer than 2340 bytes are dropped on the floor. This problem is something we would change if we were to redesign.