# Overview

Packet over SONET (Synchronous Optical Network)/SDH (Synchronous Digital Hierarchy) is the serial transmission of data over SONET frames through the use of a protocol such as Point-to-Point Protocol (PPP).

Packet over SONET/SDH is an ideal feature for networks that are built for providing Internet or IP data. It provides superior bandwidth utilization and efficiency compared with other transport methods. For expensive WAN links, packet over SONET can provide as much as 25 to 30 percent higher throughput than networks based on Asynchronous Transfer Mode (ATM). By transporting frames directly into the SONET/SDH payload, the overhead required in an ATM cell header for IP over ATM encapsulation is eliminated.

The router supports PPP, Cisco High-Level Data Link Control (HDLC), and Frame Relay over SONET/SDH.

## POS Features

POS supports the following features:

- Payload scrambling
- Clock source configuration
- Maximum transmission unit (MTU) size configuration
- Maximum receive unit (MRU) size configuration
- POS framing
- Cyclic redundancy check (CRC) checking
- Loopback configuration

## SONET/SDH

SONET is an ANSI standard for transmitting bits over fiber-optic cable. SDH is the international standard defined by the International Telecommunication Union (ITU). SONET/SDH is the physical infrastructure of choice for carrier ATM networks operating at speeds above 50 Mbps.

SONET/SDH allows carriers to build high-speed international links without requiring conversion from one transmission protocol to another (for example, T1 to T3 or T1 to E3 conversion).

SONET transmission speeds start at 51.84 Mbps and are referred to as OC1. SDH transmission speeds start at 155.52 Mbps and are referred to as STM1. All other speeds are multiples of these base numbers. Table 20 shows the speeds of the most common SONET/SDH implementations.

Table 20: Most Common SONET/SDH Implementations

SONET | SDH | Transmission Speed
--- | --- | ---
OC1 | — | 51.84 Mbps
OC3 | STM1 | 155.52 Mbps
OC12 | STM4 | 622.08 Mbps
OC48 | STM16 | 2.4 Gbps
OC96 | STM32 | 4.876640 Gbps
OC192 | STM64 | 9.953280 Gbps