{{about, PL-4, a networking protocol, the PRC missile PL-4, PL-4 (missile) PL-4 or POS-PHY Level 4 was the name of the interface that the interface

SPI-4.2 SPI-4.2 is a version of the System Packet Interface published by the Optical Internetworking Forum. It was designed to be used in systems that support OC-192 SONET interfaces and is sometimes used in 10 Gigabit Ethernet based systems. SPI-4 is an i ...

is based on. It was proposed by

PMC-Sierra PMC-Sierra was a global fabless semiconductor company with offices worldwide that developed and sold semiconductor devices into the storage, communications, optical networking, printing, and embedded computing marketplaces. On January 15, 2016, ...

to the

Optical Internetworking Forum The Optical Internetworking Forum (OIF) is a prominent non-profit consortium that was founded in 1998. It promotes the development and deployment of interoperable computer networking products and services through implementation agreements (IAs) fo ...

. The name means Packet Over SONET Physical layer level 4. PL-4 was developed by PMC-Sierra in conjunction with the Saturn Development Group.

Context

There are two broad categories of chip-to-chip interfaces. The first, exemplified by

PCI-Express PCI Express (Peripheral Component Interconnect Express), officially abbreviated as PCIe or PCI-e, is a high-speed serial computer expansion bus standard, designed to replace the older PCI, PCI-X and AGP bus standards. It is the common mot ...

and

HyperTransport HyperTransport (HT), formerly known as Lightning Data Transport, is a technology for interconnection of computer processors. It is a bidirectional serial/parallel high-bandwidth, low- latency point-to-point link that was introduced on April 2 ...

, supports reads and writes of memory addresses. The second broad category carries user packets over 1 or more channels and is exemplified by the

IEEE 802.3 IEEE 802.3 is a working group and a collection standards defining the physical layer and data link layer's media access control (MAC) of wired Ethernet. The standards are produced by the working group of Institute of Electrical and Electronics Eng ...

family of

Media Independent Interface The media-independent interface (MII) was originally defined as a standard interface to connect a Fast Ethernet (i.e., ) media access control (MAC) block to a PHY chip. The MII is standardized by IEEE 802.3u and connects different types of PHYs ...

s and the Optical Internetworking Forum family of

System Packet Interface The System Packet Interface (SPI) family of Interoperability Agreements from the Optical Internetworking Forum specify chip-to-chip, channelized, packet interfaces commonly used in synchronous optical networking and Ethernet applications. A typic ...

s. Of these last two, the family of System Packet Interfaces is optimized to carry user packets from many channels. The family of System Packet Interfaces is the most important packet-oriented, chip-to-chip interface family used between devices in the

Packet over SONET Packet over SONET/SDH, abbreviated POS, is a communications protocol for transmitting packets in the form of the Point to Point Protocol (PPP) over SDH or SONET, which are both standard protocols for communicating digital information using lase ...

and Optical Transport Network, which are the principal protocols used to carry the internet between cities.

Applications

PL-4 was designed to be used in systems that support

OC-192 Optical Carrier transmission rates are a standardized set of specifications of transmission bandwidth for digital signals that can be carried on Synchronous Optical Networking (SONET) fiber optic networks. Transmission rates are defined by rate of t ...

SONET Synchronous optical networking (SONET) and synchronous digital hierarchy (SDH) are standardized protocols that transfer multiple digital bit streams synchronously over optical fiber using lasers or highly coherent light from light-emitting diode ...

interfaces and is sometimes used in

10 Gigabit Ethernet 10 Gigabit Ethernet (10GE, 10GbE, or 10 GigE) is a group of computer networking technologies for transmitting Ethernet frames at a rate of 10 gigabits per second. It was first defined by the IEEE 802.3ae-2002 standard. Unlike previous Eth ...

based systems. A typical application of PL-4 (SPI-4.2) is to connect a framer device to a network processor. It has been widely adopted by the high speed networking marketplace.

Technical details

The interface consists of (per direction): * sixteen LVDS pairs for the data path * one LVDS pair for control * one LVDS pair for clock at half of the data rate * two FIFO status lines running at 1/8 of the data rate * one status clock The clocking is

Source-synchronous Source-Synchronous clocking refers to a technique used for timing symbols on a digital interface. Specifically, it refers to the technique of having the transmitting device send a clock signal along with the data signals. The timing of the unidirec ...

and operates around 700 MHz. Implementations of SPI-4.2 (PL-4) have been produced which allow somewhat higher clock rates. This is important when overhead bytes are added to incoming packets.

Trivia

The name is an

acronym An acronym is a word or name formed from the initial components of a longer name or phrase. Acronyms are usually formed from the initial letters of words, as in ''NATO'' (''North Atlantic Treaty Organization''), but sometimes use syllables, as ...

of an acronym of an acronym as the P in PL stands for POS-PHY and the S in POS-PHY stands for SONET (

Synchronous Optical Network Synchronous optical networking (SONET) and synchronous digital hierarchy (SDH) are standardized protocols that transfer multiple digital bit streams synchronously over optical fiber using lasers or highly coherent light from light-emitting diodes ...

History

PL-4 is a descendant of PL-3 which itself is a descendant of the

ATM Forum The ATM Forum was founded in 1991 to be the industry consortium to promote Asynchronous Transfer Mode technology used in telecommunication networks; the founding president and chairman was Fred Sammartino of Sun Microsystems. It was a non-profit in ...

UTOPIA family of standards. The UTOPIA standards were developed by the

SATURN Development Group The SATURN Development Group was an important industry forum that enabled the specification of chip-to-chip interfaces for the communications industry. It was co-founded in 1992 by PMC-Sierra and Sun Microsystems. Several significant specificati ...

for use in ATM systems.

External links