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Anleitung Zusammenfassung
This frame is transmitted row by row, from left to right, and with the most significant bit (MSB) of each byte transmitted first. It is defined to operate at the basic rate of 8 kHz or 125 microseconds per frame, or 8,000 frames per second at a bit rate of 51.84 Mbps. The frame is divided into two parts to physically segregate the layers. The first 3 columns make up the transport overhead (TOH); the remainder is the synchronous payload envelope (SPE). The TOH has 3 rows for the section overhead (SOH) and 6 rows for the line overhead (LOH). The SPE has one column for the path overhead (POH). The remaining 86 columns are for payload data (49.536 Mbps). STS-1 Pointers There are several mechanisms that allow for timing compensation between the STS-1 and its low speed tributaries. The MX2800 STS-1 multiplexer uses pointer adjustments to achieve timing 61200659L1-1 MX2800 STS-1 User Manual Chapter 1. Introduction compensation. Pointer adjustments allow the synchronous payload envelope (SPE) to float with respect to the SONET frame. The pointer 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 away from the H3 byte, not including the transport overhead. When timing differences exist, dummy bytes are inserted into the SPE without affecting data. The receiving end can recover the payload since 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 overhead bytes for the Section and Line layers. Section overhead is comprised 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 783 bytes. 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 of 12 columns each. Smaller tributaries are multiplexed together to form these groups. For example, there can be four VT1.5 tributaries in a VT group or three VT2 tributaries. A VT1.5 tributary consists of 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. A VT2 uses four columns per STS-1 frame and has a bit rate of 2.304 Mbps, allowing transport of an E1 at 2.048 Mbps plus required overhead. MX2800 STS-1 User Manual 61200659L1-1 Chapter 1. Introduction Unit Timing SNMP The MX2800 STS-1 multiplexer allows three timing methods. The first is LOOP timed, in which the 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 to enter HOLDOVER (HO) mode for the duration of the receive signal defect. During HO, the unit will continue transmitting at the same rate as the receive signal prior to entering its defect condition. Stability of the transmit during HO is guaranteed to +/- 4.9 ppm over 24 hours. The defect condition must be cleared before LOOP time mode can be resumed. The second mode of operation is FREE-RUNNING (FR) mode. In this mode, an internal oscillator is used to generate transmit timing. FREE-RUNNING mode provides a SONET minimum clock specification of +/- 20 ppm. The third timing operation mode is EXTERNAL. EXTERNAL timing mode allows the MX2800 STS-1 to derive its transmit timing from any one of the 28 DSX-1/E1 physical ports. The ports selected may be data carrying ports. Two sources can be specified to provide external timing: a primary and a secondary source. Failure of the primary source (due to LOS or AIS) will cause the unit to switch to the secondary source, if available. In the event that both the primary and secondary sources are lost, the unit will enter HOLDOVER mode as described previously. Holdover mode will be exited upon recovery of at least one clock source. The MX2800 STS-1's embedded SNMP feature allows the unit to be accessed and controlled by a network manager through the 10BaseT local area network (LAN) port. The MX2800 STS-1 61200659L1-1 MX2800 STS-1 User Manual Chapter 1. Introduction supports the MIB-II standard, RFC 1213, RFC 1595, and the ADTRAN Enterprise Specific MIB. MIB files are available at The term SNMP broadly refers to the message protocols used to exchange information between the network management system (NMS) and the managed devices, as well as to the structure of device management databases. SNMP has three basic components, the network manager, the agent, and the MIB. Network Manager The network manager is a set of control programs that collect, control, and present data pertinent to the operation of the network devices. These programs reside on a network management station. Agent The agent is a control program that resides in every network device. This program responds to queries and commands from the network manager, returns ...
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