Export PDFWhat Is CPOS?
Channelized POS (CPOS) is a channelized interface technology based on SONET/SDH. It divides a POS link into multiple sub-channels through timeslot allocation, enabling concurrent transmission of multiple services. Its core strength lies in fine-grained bandwidth partitioning, which reduces the need for numerous low-speed physical ports and enhances the device's capabilities in leased line access and service aggregation.
CPOS encapsulates low-speed tributary signals (such as E1) in SONET/SDH signals into independent channels and uses timeslot bundling to support parallel transmission of multiple data channels. The technology consists of CPOS physical interfaces, SONET/SDH service adapters, and channelization configuration software, and relies on the timeslot allocation mechanism of SONET/SDH networks.
Why Is CPOS Needed?
IP services are increasingly deployed across metropolitan area networks (MANs) and wide area networks (WANs). However, traditional Packet over SONET/SDH (POS) links, despite offering high-speed transmission, are constrained by fixed bandwidth allocation and cannot accommodate differentiated service requirements. In conventional POS-based networks, low-speed services such as E1 have to be delivered through dedicated physical ports, requiring high port density. At the same time, operators increasingly require fine-grained bandwidth management, stricter service isolation, and tighter cost control. Conventional POS can no longer support high-density leased line access or flexible bandwidth allocation.
Channelized POS (CPOS) addresses these challenges by partitioning a single POS link into multiple logical sub-channels through timeslot allocation. This allows the POS link to carry services of different rates (such as E1) across sub-channels. This technology resolves the following key issues:
- Low bandwidth utilization: Conventional POS uses fixed bandwidth allocation, often resulting in wasted resources. CPOS enables on-demand bandwidth allocation through timeslot bundling.
- Reliance on numerous ports: CPOS reduces reliance on numerous low-speed physical ports and lowers device deployment costs.
- Poor service isolation: CPOS assigns dedicated bandwidth and monitoring policies to each sub-channel, enhancing service security and quality.
Typical Application Scenarios of CPOS
Using CPOS Interfaces to Connect Devices Through Optical Fibers or WDM
CPOS interfaces can be used to connect DeviceA at a branch and DeviceB at the headquarters through an optical cable either directly or using wavelength division multiplexing (WDM).
Connecting CPOS interfaces using an optical cable either directly or using WDM
Using CPOS Interfaces for E1 Line Aggregation
Several enterprise branches connect to the SDH network using E1 lines. For branches with higher bandwidth requirements, a single E1 line is insufficient, so additional E1 lines are leased to expand capacity.
E1 lines of all branches are aggregated through one or more CPOS interfaces on DeviceA. DeviceA uniquely identifies each branch device by time slot. This is equivalent to each branch accessing DeviceA through one or multiple E1 lines.
Using CPOS interface for E1 line aggregation over an SDH network
How Does CPOS Work?
CPOS divides a single POS link into multiple logical sub-channels (such as E1 channels) using timeslot allocation. Its core mechanisms include:
- Timeslot bundling: Timeslots for low-speed services (such as E1 services) are grouped as required to form logical channels.
- SDH payload mapping: The SDH service adapter maps encapsulated IP packets to the payloads of SDH frames.
- Channelization configuration: By defining the timeslot assigned to each E1 channel, CPOS ensures that different services are carried over separate sub-channels.
The following uses E1 service transmission as an example to describe how CPOS works:
- Packet encapsulation: IP packets are encapsulated into link-layer frames using PPP or HDLC.
- Timeslot allocation: The encapsulated frames are placed into the timeslots allocated to the corresponding E1 channels.
- SDH payload mapping: The data transmitted over E1 channels is then mapped to the payloads of SDH frames, with Regenerator Section Overhead (RSOH) and Multiplex Section Overhead (MSOH) inserted to support section monitoring and fault detection.
- Transmission: The resulting SDH frames are transmitted over optical cables, either directly or using WDM, to the remote end.
- Reception and decapsulation: The receiving device parses the SDH frames to extract the data transmitted over the E1 channels, and decapsulates the data using PPP or HDLC to restore the original IP packets.
SDH Frame Structure
This section outlines the STM-N frame structure in SDH.
To add or drop low-speed tributary signals to or from high-speed signals, it is necessary to distribute the tributary signals in a frame evenly and regularly. According to ITU-T specifications, STM-N frames adopt a rectangular byte-based block structure.
STM-N frame structure
An STM-N frame consists of 9 rows and 270 × N columns, where N denotes the number of multiplexed STM-1 signals. This is what "N" refers to in STM-N.
This frame consists of three parts: SOH, Administrative Unit pointer (AU-PTR), and payload. SOH can be further classified into Regenerator Section Overhead (RSOH) and Multiplex Section Overhead (MSOH). AU-PTR is a pointer that identifies the starting byte of the payload within an STM-N frame, enabling the receiver to accurately locate and extract the payload.
E1-to-STM-1 Multiplexing
According to ITU-T Recommendation G.709, there are multiple SDH multiplexing paths for mapping payloads to STM-N signals. The following figure illustrates the multiplexing process from E1 to STM-1.
E1-to-STM-1 multiplexing
The multiplexing process involves the following concepts:
- Multiplexing unit: SDH defines several basic multiplexing units, including 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), where n denotes the level of the unit.
- Container: an information structure 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.
- Virtual Container (VC): an information structure that is used to support channel-layer connections in SDH and which functions as the information terminal of SDH channels. VCs are classified into lower-order VCs and higher-order VCs. VC-3 in AU-3 and VC-4 are both higher-order VCs.
- Tributary Unit (TU) and Tributary Unit Group (TUG): A TU is an information structure that provides adaptation between lower-order and higher-order path layers. A TUG is comprised of one or more TUs occupying designated positions in the payload of a higher-order VC.
- Administration Unit (AU) and Administration Unit Group (AUG): An AU is an information structure that provides adaptation between the higher-order path layer and the multiplex section layer. An AUG is comprised of one or more AUs occupying designated positions in the payload of an STM-N frame.
SDH Overhead Bytes
SDH supports monitoring at both the section and path layers. Section-layer monitoring includes regenerator section monitoring and multiplex section monitoring, while path-layer monitoring includes higher-order path monitoring and lower-order path monitoring. These monitoring functions are implemented through different overhead bytes.
- Author： Ling Ziwen
- Updated on： 2026-05-22
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