Hello, everyone!
In this post, I would like to share with you the LCAS technology.
The concept of cascading and virtual concatenation is defined in ITU-T G.7070, and these two concepts play an important role in MSTP technology. The VC cascading technology can realize the rate adaptation of the Ethernet bandwidth and the SDH virtual channel, thereby achieving flexible configuration of bandwidth, and in particular, the virtual cascading technology can fully utilize the bandwidth.
1. Virtual cascading technology principle
Virtual concatenation technology can be thought of as cascading multiple small containers and assembling them into a larger container to transport data traffic. This technology can cascade containers from different speeds, such as VC-12 to VC-4. Cascading with small containers can achieve very small particle bandwidth adjustment, and the corresponding maximum bandwidth after cascading can only be small. In the range. For example, if you do a cascade of VC-12, the maximum bandwidth it can provide is only 139 Mbit/s. For example, the IP data packet is carried by three virtual concatenated VC-3s, and then the three VC-3s are independently transparently transmitted to the destination by the network, and because they are independently transmitted to the destination, they reach the destination. The delay is also different, which requires reordering at the destination and restoring to the original packet.
The H4 byte in the SDH frame carries information on how to reassemble these VCs to restore them to the original information. This 16-byte H4 byte mainly includes two important pieces of information: multi-frame indicator (MFI), serial number. The multi-frame indicator is dynamic and automatically increments by 1 whenever there is a new frame. These three VC-3s have a unique MFI number because they carry the same packet. Thus, when they reach the end point with different delays respectively, the end points can recombine these independent VCs according to the same MFI number.
The original node will give the corresponding sequence number of the different VCs of the same virtual cascade channel. The serial number of a VC-xv channel is 0 to x-1, and the sequence number is gradually increased in order. In this way, the original data packets can be correctly reassembled, and it also avoids the complicated process that the network management must perform sequential monitoring of the dispersed VCs.
2. The characteristics of virtual cascade technology
The biggest advantage of virtual concatenation is that it can enable SDH to provide the right size channel for data services, avoiding the waste of bandwidth. Virtual concatenation technology allows the bandwidth to be adjusted to a small granularity to suit the user's needs. The minimum allocatable granularity defined in G.7070 is 2M. Since the transmission paths of each virtual concatenated VC on the network are independent, when the physical link has an interruption in one direction, it will not affect the VC transmitted from the other direction, when the virtual concatenation and the LCAS protocol phase When combined, data transmission can be guaranteed, thereby improving the reliability and stability of the entire network.
As a multi-link PPP technology that can also utilize multiple SDH virtual containers for data transmission, it is also used in the market. It is a point-to-point transport layer adaptation technology. The main principle of Multi-link PPP is to split the upper-layer service flows in parallel and perform PPP encapsulation separately. The PPP packet must have a frame header identifier, and the data stream in the PPP packet should be bit inserted to prevent the data packet from being identical to the frame identifier, and each PPP packet must have its own serial number so that the receiving end can be correct. Reorganization. In terms of implementation ideas, it has similarities with virtual concatenation technology, but since Multi-link PPP is not designed specifically for SDH, virtual concatenation is superior to Multi-link PPP in terms of transmission performance and bandwidth allocation granularity; This combination of virtual cascade technology and GFP technology is more prominent.
LCAS improves and complements virtual concatenation technology. LCAS dynamically adjusts the number of virtual containers required for service mapping to meet the bandwidth requirements of various services. Through LCAS, the bandwidth utilization increases and the robustness of virtual concatenation is enhanced.
That's all, I hope you like it.
Thank you!
