Overview of CPOS Interfaces
A CPOS interface is a channelized POS interface that allows a device to effectively converge low-speed access lines.
SONET and SDH
The synchronous optical network (SONET) is an ANSI-defined synchronous transport mode and a global standard transport protocol. It transmits data at a basic rate of 51.84 Mbit/s.
The synchronous digital hierarchy (SDH) is an ITU-T-defined international standard developed based on SONET. SDH uses synchronous multiplexing and flexible mapping structure to produce low-speed tributary signals from SDH signals and does not require a lot of multiplexing and demultiplexing devices. This reduces signal loss and device investment.
The basic 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.
Table 16-1 lists the common SONET and SDH transmission rates.
SONET |
SDH |
Transmission Rate (Mbit/s) |
|
|---|---|---|---|
Electrical Signal |
Optical Signal |
Optical Signal |
|
STS-1 |
OC-1 |
- |
51.840 |
STS-3 |
OC-3 |
STM-1 |
155.520 (155) |
STS-9 |
OC-9 |
STM-3 |
466.560 |
STS-12 |
OC-12 |
STM-4 |
622.080 (622) |
STS-18 |
OC-18 |
STM-6 |
933.120 |
STS-24 |
OC-24 |
STM-8 |
1244.160 |
STS-36 |
OC-36 |
STM-12 |
1866.240 |
STS-48 |
OC-48 |
STM-16 |
2488.320 (2.5 Gbit/s) |
STS-96 |
OC-96 |
STM-32 |
4876.640 |
STS-192 |
OC-192 |
STM-64 |
9953.280 (10 Gbit/s) |
Introduction to POS
A POS interface uses SONET/SDH to transfer IP data in MAN and WAN. The POS uses the link-layer protocol (FR, PPP and HDLC) to encapsulate IP packets, uses the service adapter at the SONET/SDH channel layer to map the packets to the payload of SONET/SDH signals, then uses the SONET/SDH transport layer and section layer to add a path overhead and segment overhead and encapsulate the payload in a SONET/SDH frame, and finally transmits it on the optical network.
The device provides POS interfaces with two rates and their signal levels are OC-3/STM-1 (155 Mbit/s) and OC-12/STM-4 (622 Mbit/s).
CPOS
Low-speed tributary signals can be multiplexed into SDH signals. These low-speed tributary links are known as channels. CPOS interfaces are channelized POS interfaces that use the SDH mechanism to perform refined division of bandwidth. A CPOS interface reduces the number of low-speed physical interfaces required in networking and improves the capability of a device to access leased lines. CPOS interfaces in Figure 16-2 are used in the regional headquarters to converge E1/T1 lines of each branch, improving the convergence ability of the device for low-speed access.
