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.
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.
HDLC Bandwidth Expansion