Chapter 1. Introduction1-4 MX2800 STS-1 User Manual 61200659L1-1compensation. Pointer adjustments allow the synchronous payloadenvelope (SPE) to float with respect to the SONET frame. Thepointer is contained in the H1 and H2 bytes of the line overhead(LOH), and is a count of the number of bytes the J1 byte is awayfrom the H3 byte, not including the transport overhead. Whentiming differences exist, dummy bytes are inserted into the SPEwithout affecting data. The receiving end can recover the payloadsince the pointer is adjusted to indicate where the payload overhead(POH) starts.Transport Overhead (TOH)The first three columns are the TOH, which contains the overheadbytes for the Section and Line layers. Section overhead iscomprised of 9 bytes and Line overhead is comprised of 18 bytes.STS-1 Synchronous Payload Envelope (SPE)The SPE has 87 columns and 9 rows of bytes for a total of 783bytes. Column 1 contains the STS Path Overhead and has 9 bytes.The remaining 774 bytes are available for payload.Virtual Tributaries (VT)The SPE is divided into seven virtual tributary groups, made up of12 columns each. Smaller tributaries are multiplexed together toform these groups. For example, there can be four VT1.5 tributariesin a VT group or three VT2 tributaries. A VT1.5 tributary consistsof three columns per STS-1 frame and has a bit rate of 1.728 Mbps,allowing transport of a T1 at 1.544 Mbps plus required overhead. AVT2 uses four columns per STS-1 frame and has a bit rate of 2.304Mbps, allowing transport of an E1 at 2.048 Mbps plus requiredoverhead.
