The advantages of virtual channels in sRIO 2.0 and the technologies that accelerate their adoption
Serial RapidIO (sRIO) is a high-performance, packet-based technology that can be used in a growing number of applications, including wireless infrastructure, storage, medical imaging, and military industries.
Using the sRIO 2.0 standard, system designers can select link rates from 1.25Gbps to 6.25Gbps, and port widths from 1x to 16x, providing high granularity to select the port data rate that is most suitable for specific applications. In addition to the physical layer enhancements of sRIO 2.0, there are a series of higher-level functions that are specifically designed to provide unprecedented flow control of switch structures.
Virtual channels (Virtual channel, VC) can provide the ability to control different types of traffic in the system. VC helps the system designer control packet flow by dividing the link into several independent channels and assigning packets to a specific channel. The first VC is called VC0, which is a backward compatible VC that uses the sRIO 2.0 standard and operates the same as the sRIO 1.3 specification link. In addition, sRIO 2.0 also supports up to more than 8 VCs (VC1-VC8).
VC enhances the control of the data flow in the structure. Each VC can guarantee to occupy a part of the link bandwidth. System designers can control the interaction of multiple traffic types, and in fact can isolate them from each other by allocating bandwidth. Delay-sensitive traffic (such as streaming video) can be guaranteed to be allocated part of the bandwidth in the entire switch structure.
Since all VCs have less available bandwidth than guaranteed, in order to maximize the use of links, sRIO 2.0 can be used for any available bandwidth. In essence, the allocation of bandwidth is intelligent, and at the same time, it can ensure that the greedy VCs that urgently need more bandwidth than it can not seize the bandwidth from other VCs, and at the same time ensure that no bandwidth is idle as long as packets are sent.
Figure 1 shows the advantages of VC bandwidth reservation. The three packet streams share a link, 10% of the bandwidth is allocated to VC_A, 60% is allocated to VC_B, and 30% is allocated to VC_C. In the first part of the simulation, only VC_A and VC_C have packets to transmit, so their bandwidth growth exceeds their respective allocation values ​​to utilize the unused portion allocated to VC_B. As shown in the figure, VC_C can occupy 75% of the available link bandwidth, while VC_A occupies the remaining 25%. sRIO 2.0 allows VCs to use unused links, and packets appear in accordance with their corresponding bandwidth allocation. In the second half of the simulation, VC_B appears to transmit traffic through the shared link. sRIO 2.0 allows the switch to quickly respond to changes in traffic and change bandwidth utilization when it needs to match a programmed allocation. In this case, a discontinuous but delay-sensitive stream, such as VC_B, can quickly obtain 60% of its allocation. Once the packet from VC_B is transmitted, the link usage is once again redistributed to the remaining VCs with packets to transmit.
sRIO 2.0 can provide an additional layer of link partitioning control by providing a function unique to VC0. VC0 can be configured to obey bandwidth reservations, just like all other VCs; or it can be configured to automatically obtain any required bandwidth, and all remaining bandwidth is allocated to all other VCs that have packets to transmit. This helps to control the plane traffic transmitted through VC0, making the operation completely independent of the data plane traffic, and is only subject to the priority rules of the sRIO 1.3 specification.
VC provides two packet transmission modes-continuous transmission (CT) and reliable transmission (RT). The RT operation is similar to the earlier version of the sRIO specification. When the packet cannot be received, the packet is retransmitted to ensure lossless packet transmission. CT is optimized to reduce delayed traffic, which can accept packet loss by not sending packets again. VC0 supports all defined priorities and only operates in RT mode.
Higher VC (1 ~ 8) can run in CT and RT mode, which helps customers optimize the transmission method of different types of data. For example, given that control plane traffic may require the response and guaranteed delivery provided by RT, data plane traffic (such as from audio streams) may benefit from reduced CT latency, and if retransmissions are performed, they may be lost.
Challenges faced by switch suppliers
One of the challenges of adding functionality to a specification is to integrate new functionality into existing and next-generation systems. Backward compatibility is the cornerstone of the sRIO specification development process, but the promotion of the new standard is equally important. System developers will see more and more sRIO 2.0 compatible products in the near future.
Switch vendors are likely to be the first to adopt sRIO 2.0. The switch is the foundation of the embedded structure ecosystem, which essentially lies in the verification of the new specifications. Processing endpoints such as digital signal processors and field programmable gate arrays may lag behind in producing new devices that comply with the latest revised specifications. Therefore, switch vendors face a severe challenge when producing next-generation solutions, initially at least pushing these solutions to systems based on existing endpoints that use current technology. As endpoints supporting sRIO 2.0 become available, designers will transition to them as existing systems evolve or new systems are introduced. Inevitably, there will be an extended period in which sRIO 1.3 and sRIO 2.0 overlap. In addition, many systems may exist for a longer period of time, requiring existing sRIO 1.3 subsystems to communicate with sRIO 2.0 compatible subsystems.
However, without the support of existing endpoints, switch vendors can take advantage of the many advantages of sRIO 2.0, because these advantages are mainly concentrated in the switching structure. The advantages of VC are very competitive, and the support for the gradual adoption of this technology may prove highly attractive to system designers. Therefore, the challenge for switch vendors is to implement transitional support for VC functionality in the sRIO architecture of systems that may be dominated by sRIO 1.3 compatible endpoints. In addition, switch vendors must provide a simple transition mechanism so that as the system evolves to accommodate only sRIO 2.0 devices, the structure and use of VCs will also evolve.
Virtual channel for sRIO 1.3 system
Switches that support VC must provide internal circuits that specifically send VC packets and support sRIO 2.0 bandwidth allocation requirements. If the switch is intelligent and can send an incoming sRIO 1.3 packet as an sRIO 2.0 packet with a higher VC value, these paths can be utilized by sRIO 1.3 compatible packets. The switch supplier can provide customers with the ability to program a mapping protocol to process incoming sRIO 1.3 packets as sRIO 2.0 packets in terms of buffer utilization, switching algorithm decisions, load balancing, and bandwidth reservation. This provides system designers with control over the structure defined in sRIO 2.0, even if the ability to generate higher VC packages is not available. This capability helps the system designer to control the processing of different data streams in the system, just as the entire system is running on sRIO 2.0 devices.
Figure 2 describes this concept. Although the device only generates sRIO 1.3 compatible traffic, the switch can be configured to handle some incoming traffic as if it had a higher VC, mainly mapping the sRIO 1.3 package to the sRIO 2.0 package that programs the VC value. One method that can be used to communicate to the switch how to handle a particular packet is to use a unique target identifier. This enhancement helps the switch to send the packet to the correct endpoint, while limiting the outgoing traffic of the packet based on the bandwidth allocation to the mapped VC. As shown in the figure, with this feature, the switch can process three different endpoints as generators of VC_A, VC_B, and VC_C. The expected flow to the target can imitate the flexibility and response shown in Figure 1. No need to use sRIO 2.0 endpoints. Although it may limit the use of transmitter-based flow control, this VC mapping capability does help system designers to more accurately approximate the expected flow in the system, rather than restricting users to only follow sRIO 1.3 packet ordering rules.
Another key element of VC in sRIO 2.0 is the ability to run in CT mode, which helps to drop packets when delays need to be reduced. Any effort to transition to VC must face this key feature. Unlike sRIO 2.0, sRIO 1.3 devices will not accept packages if there is no buffer space. The switch supplier can provide a pseudo CT mode option to operate with sRIO 1.3 endpoints close to sRIO 2.0 CT mode. In this mode, the packet to be resent is actually replaced by a new packet. This mode helps customers take advantage of the delay of VC running in CT mode in the sRIO 1.3 system.
Communication between sRIO 1.3 and sRIO 2.0 systems
As new sRIO 2.0 endpoints enter the market, it is necessary to provide a solution that can connect the two specifications so that the subsystems designed for sRIO 2.0 can take advantage of all of their advantages while continuing to work with existing sRIO 1.3 Subsystem communication.
A powerful feature is to provide a switch that operates as a converter between sRIO 1.3 existing traffic and sRIO 2.0 VC traffic. sRIO provides built-in backward VC value conversion, because the header portion that specifies the VC value reuses priority and critical request traffic areas. Although this does help sRIO 1.3 devices to forward sRIO 2.0 packages with a specified VC value, this is a static conversion and may not meet the needs of system designers who must support both sRIO versions in the same system.
However, the switch can operate as an intelligent interconnection between the two subsystems. By providing a programmable conversion function, each subsystem operates in a different sRIO version. Suppose a system includes an existing sRIO 1.3 system board, an sRIO 2.0 system board, and a switching board with a conversion function. As shown in Figure 3, the switch can have a programmable conversion function between the two systems. Feel free to perform packet conversion. This helps system designers to transition to new specifications when modifying subsystems, introducing new subsystems, and enabling the entire sRIO 2.0 ecosystem to use all necessary components. This solution does not simply map the packets to the VC and change the way the switch handles each packet. Instead, it changes the header of the incoming packet, recalculates the cyclic redundancy check (CRC), and eventually generates a new packet. System designers can use this solution with minimal impact and huge advantages.
Accelerate the success of sRIO 2.0
sRIO 2.0 will be an embedded structure with powerful functions, supporting various data flows. VC is the main element that will change the data flow that a system designer can define and control through its structure. The challenge for switch vendors is to provide their customers with powerful sRIO 2.0 solutions that can quickly take advantage of VC's superior capabilities, and as the ecosystem continues to evolve, provide a simple way to evolve into a complete sRIO 2.0 solution.
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