1-4 61204659L1-1ASection 1, Introduction MX2800 STS-1 User ManualSTS-1 OverviewVirtual Tributaries (VT)The SPE is divided into seven virtual tributary groups, made up of 12 columns each. Smaller tributaries aremultiplexed together to form these groups. For example, there can be four VT1.5 tributaries in a VT groupor three VT2 tributaries. A VT1.5 tributary consists of three columns per STS-1 frame and has a bit rate of1.728 Mbps, allowing transport of a T1 at 1.544 Mbps plus required overhead. A VT2 uses four columnsper STS-1 frame and has a bit rate of 2.304 Mbps, allowing transport of an E1 at 2.048 Mbps plus requiredoverhead.STS-1 PointersThere are several mechanisms that allow for timing compensation between the STS-1 and its low-speedtributaries. The MX2800 STS-1 multiplexer uses pointer adjustments to achieve timing compensation.Pointer adjustments allow the synchronous payload envelope (SPE) to float with respect to the SONETframe. The pointer is contained in the H1 and H2 bytes of the path overhead (POH), and is a count of thenumber of bytes that the J1 byte is away from the H3 byte, not including the transport overhead. Whentiming differences exist, dummy bytes are inserted into the SPE without affecting data. The receiving endcan recover the payload since the pointer is adjusted to indicate where the payload overhead (POH) starts.Unit TimingThe MX2800 STS-1 multiplexer allows four timing methods:• Loop Mode• Free-Running Mode• External Nonrevertive Mode• External Revertive ModeLoop ModeThe multiplexer derives timing from the incoming STS-1 signal.A failure in this signal, such as a loss-of-signal (LOS) or a loss-of-framing (LOF) will cause the unit toenter Holdover (HO) mode for the duration of the receive signal defect. During HO, the unit will continuetransmitting at the same rate as the receive signal prior to entering its defect condition. Stability of thetransmit during HO is guaranteed to ± 4.9 ppm over 24 hours. The defect condition must be cleared beforeL OOP time mode can be resumed.Free-Running (FR) ModeIn this mode, an internal oscillator is used to generate transmit timing. FREE - RUNNING mode provides aSONET minimum clock specification of ± 20 ppm.External Nonrevertive and Revertive ModesE XTERNAL timing mode allows the MX2800 STS-1 to derive its transmit timing from the BITS clocksthrough wire-wrap pins located on the back of the chassis. Two sources can be specified to provide exter-nal timing: a primary and a secondary source. Failure of the primary source (due to LOS or AIS) will cause
