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NE40E-M2 V800R010C10SPC500 Configuration Guide - WAN Access 01

This is NE40E-M2 V800R010C10SPC500 Configuration Guide - WAN Access
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Overview of CPOS Interfaces

Overview of CPOS Interfaces

This section describes packet over SONET/SDH (POS) and channelized POS (CPOS) interfaces in terms of the basic concepts of synchronous optical network (SONET) and synchronous digital hierarchy (SDH), channelization and non-channelization, and SDH frame structure.

Introduction to SONET and SDH

SONET is a synchronous digital transmission standard defined by the American National Standards Institute (ANSI) and mainly used in North America and Japan. Clocks at each network level are synchronized with a very precise master clock.

SONET defines the line rate hierarchical structure of synchronous transmission for the optical transmission system. The basic SONET transmission rate is 51.84 Mbit/s :
  • An electrical signal with the transmission rate is called Level 1 Synchronous Transport Signal (STS-1).
  • An optical signal with the transmission rate is called Level 1 Optical Carrier (OC-1).

SONET, which uses synchronous signals, can easily multiplex signals.

SONET-based SDH is an international standard defined by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) and mainly used in Europe. The corresponding SDH standard is based on the proposals from G.707 to G.709 passed in 1988 and the proposals added in 1992.

SDH is similar to SONET to a great extent. The basic transmission rate of SDH is 155.52 Mbit/s, which is called Level 1 Synchronous Transfer Module (STM-1). This rate equals the OC-3 rate in SONET.

SDH, which uses synchronous multiplexing and flexible mapping, can multiplex or demultiplex low-speed tributary signals from SDH signals without using many multiplexing or demultiplexing devices. SDH reduces signal consumption and equipment investment.

Table 5-1 lists the common SONET and SDH transmission rates. For convenience, the approximations of these transmission rates are placed in the parentheses.
Table 5-1 Common SONET and SDH transmission rates



Transmission Rate (Mbit/s)

Electrical Signal

Optical Signal

Optical Signal








155.520 (155)








622.080 (622)
















2488.320 (2.5 Gbit/s)








9953.280 (10 Gbit/s)

Channelization and Unchannelization

When SDH signals are formed by multiplexing low-speed tributary signals, these tributary signals are called channels.

Channelization means that multiple independent channels of data are transmitted over an optical fiber by using low-speed tributary STM-N signals. During the transmission, each channel has its own bandwidth and start and end points and follows its own monitoring policy.

Unchannelization means that all data is transmitted through a single channel over an optical fiber by using all STM-N signals. During the transmission, all the data has the same ID and start and end points and follows the same monitoring policy.

Channelization can utilize bandwidth more efficiently by transmitting multiple channels of low-speed signals. Unchannelization is used to transmit a single channel of high-speed signals.

Introduction to Channelized Serial Interfaces

The serial interface formed by a CPOS interface is called a channelized serial interface, which has the same logical features as a synchronous serial interface.

An E1 channel of a CPOS interface can work in either clear channel mode (unframed mode) or channelized mode:

  • In clear channel mode, an E1 channel without timeslot division forms a 2.048 Mbit/s serial interface.

    The naming convention for a serial interface is "serial interface-number/e1-number:0," where interface-number specifies the name of the CPOS interface, and e1-number specifies the number of the E1 channel.

  • In channelized mode, 31 timeslots across an E1 channel, excluding timeslot 0, can be randomly bundled into serial interfaces.

    The naming convention for a channelized serial interface is "serial interface-number/e1-number:set-number," where interface-number specifies the name of the CPOS interface, e1-number specifies the number of an E1 channel, and set-number specifies the number of the interface formed by bundling the timeslots of the E1 channel.

The NE40E supports only STM-1 Channelized POS.

Pos andCPOS

POS applies to metropolitan area networks (MANs) and wide area networks (WANs) and supports packet data, such as IP packets.

CPOS interfaces make full use of the SDH system and have the following functions:
  • Perform refined division of bandwidth.

  • Reduce the number of low-speed physical ports required on a Router.

  • Enhance the aggregation capability of the low-speed interfaces of a Router.

  • Improve the dedicated line access capability of a Router.

SDH Frame Structure

This section describes the frame structure of an SDH signal, that is, the structure of an STM-N frame.

To add or drop low-speed tributary signals to or from high-speed signals, try to distribute tributary signals in the frame evenly and regularly. The ITU-T regulates that STM-N frames are rectangular and expressed in bytes, as shown in Figure 5-1.

Figure 5-1 STM-N frame structure

STM-N is a frame with the dimension of 9 rows x 270 x N columns. Here, N is the same as that in STM-N, indicating how many STM-1 signals are multiplexed to this STM-N signal.

An STM-N frame consists of the following parts:
  • Section overhead (SOH): includes regenerator section overhead (RSOH) and multiplex section overhead (MSOH).
  • Administration unit pointer (AU-PTR): is the pointer that specifies the first byte of the payload. The receiver can correctly extract the payload based on the location of the pointer.
  • Payload

Related Concepts

  • Multiplexing units: Basic SDH multiplexing units include container (C-n), virtual container (VC-n), tributary unit (TU-n), tributary unit group (TUG-n), administrative unit (AU-n), and administrative unit group (AUG-n). Here, n stands for the number of the unit level.

  • Container: It is used to carry service signals that are transmitted at different rates. G.709 defines specifications for five types of standard containers: C-11, C-12, C-2, C-3, and C-4.

  • VC: It is an information terminal of SDH channels and is used to support connections between SDH channel layers. VCs are classified as lower-order VCs or higher-order VCs. VC-3 in AU-3 and VC-4 are higher-order VCs.

  • TU and TUG: A TU provides adaptation between lower-order and higher-order path layers. A collection of TUs, occupying a fixed position in the payload of the higher-order VC, is called a TUG.

  • AU and AUG: An AU provides adaptation between higher-order channel layer and multiplex section layer. A collection of AUs, occupying a fixed position in the payload of STM-N, is called an AUG.

Multiplexing E1 to STM-1

In the SDH multiplexing process outlined in the G.709 recommendation, there is more than one multiplexing path from a valid payload to STM-N. Figure 5-2 shows the multiplexing process from E1 to STM-1.
Figure 5-2 E1 to STM-1 multiplexing process

In practice, different countries and regions may use different multiplexing paths.

Calculation of E1 Path Sequence Numbers

CPOS interfaces use the byte interleave multiplexing mode so that the lower-order VCs in a higher-order VC are not arranged in sequence. To facilitate configuration, the following section uses E1 in CPOS with the AU-4 multiplexing path as an example to demonstrate the TU number calculation method.

The multiplexing process in Figure 5-3 shows that the 2 Mbit/s multiplexing structure is 3-7-3 when the AU-4 multiplexing path is used. The following formula shows how to calculate the TU-12 numbers located in different positions in a VC-4:

VC-12 number = TUG-3 number + (TUG-2 number - 1) x 3 + (TU-12 number - 1) x 21

In a VC-4, all TUG-3s are numbered the same; all TUG-2s are numbered the same; two TU-12s with a number difference of 1 are adjacent.

  • The numbers listed in the preceding formula stand for the positions in a VC-4 frame.
  • The TUG-3 number ranges from 1 to 3; the TUG-2 number ranges from 1 to 7; the TU-12 number ranges from 1 to 3.
  • The TU-12 number indicates which one of the 63 TU-12s is in the VC-4 frame based on the sequence and also indicates the E1 channel number.
Figure 5-3 Sequence for arranging TUG-3, TUG-2, and TU-12 in a VC-4 frame

If the AU-3 multiplexing path is used, the TU-12 number calculation can be deduced in a similar manner.

If 63 E1 channels are configured on a CPOS interface, these channels can be directly numbered from 1 to 63. If a Router of Huawei is connected to a channelized STM-1 interface of a router from another vendor, take note of the differences in channel numbers.

Overhead Bytes

SDH provides monitoring and management at layers. Monitoring is classified as section monitoring or path monitoring. Section monitoring is classified as regenerator section monitoring or multiplex section monitoring. Path monitoring is classified as higher-order path monitoring or lower-order path monitoring. Different overhead bytes help to implement the monitoring functions.


This section describes only the SDH overhead bytes used in configuration.

  • SOH

    SOH consists of RSOH and MSOH.

    The payload of an STM-N frame contains the path overhead (POH) that monitors low-speed tributary signals.

    J0, the regenerator section trace byte, is contained in RSOH. This byte is used to transmit the Section Access Point Identifiers (SAPIs) repeatedly to check the connection between the receiver and the transmitter. The byte can be any character in a carrier's network, whereas the J0 bytes of the receiver and transmitter must match each other at the border of two carriers' networks. With the J0 byte, a carrier can locate and rectify faults in advance to speed up network recovery.

  • Path overhead

    SOH monitors section layers, whereas POH monitors path layers. POH is classified as lower-order path overhead or higher-order path overhead.

    The higher-order path overhead monitors the paths at VC-4 and VC-3 levels.

    J1, the higher-order VC-N path trace byte, is contained in the higher-order path overhead. Similar to J0, J1 is used to transmit SAPIs repeatedly to check the connection between the receiver and the transmitter. The J1 bytes of the receiver and transmitter must match each other.

    C2, the path signal label byte, is contained in higher-order path overhead. C2 is used to specify the multiplexing structure and the attributes of the information payload in a VC frame, including whether the path is loaded with services, service types, and the mapping mode. The C2 bytes of the receiver and transmitter must match each other.

CPOS Interface Types

As shown in Figure 5-4, some governmental agencies and enterprises use low-end and mid-range devices to access the transmission network over E1 leased lines. Users with bandwidth requirements between E1 and T3 (44 Mbit/s), such as data centers, use several E1 leased lines simultaneously.

The bandwidth of these users aggregates on one or more CPOS interfaces over the transmission network. CPOS interfaces are then connected to the high-end device that identifies each low-end device based on timeslots.

Figure 5-4 Networking diagram of typical CPOS implementation

In practice, there may be more than one level of transmission networks between CPOS interfaces and low-end devices. Other transmission devices are needed to relay communication between low-end devices and transmission networks. Logically, such an implementation is equivalent to the networking mode in which each low-end device is connected to Device A over an E1 leased line or N x E1 leased lines.

Updated: 2019-01-02

Document ID: EDOC1100058399

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