Application and Development of MSTP

1. MSTP and Its Key Technologies

Currently, MSTP is a hot topic in the world. In fact, the earliest concept in foreign countries is MSPP, that is, multi-service allocation platform. The concept of "Platform" is more appropriate, that is, it relies on a certain technical platform to expand and derive new functions and applications. MSTP is based on the SDH technology platform to Expand functions of data and other new businesses. As we all know, SDH is the most suitable for carrying time-division multiplexing (TDM) services, and it is not highly efficient to carry data services. After being transformed into MSTP, the situation will be very different. SDH seems to have gained a new life, and many operators no longer build a pure SDH transmission network. In addition, MSTP technology and network will be studied and applied in a nest of places. SDH vendors also repackage their MSTP products. Even so, MSTP is still an unstandardized term, which is not defined in terms defined by the optical transport standard organization, and ITU-T SG15 only defines a series of standards based on Ethernet transport networks, MSTP is not officially mentioned. The reason may be that we believe that MSTP belongs to the category of communication products or devices, and similar terms are inappropriate in rigorous technical standards, but in fact, the content of technical standard specifications is consistent with the actual MSTP product features.

Whether it's ITU-T or China's industry standard [1-6], MSTP integrates data business functionality, including asynchronous transmission mode (ATM), Ethernet, elastic grouping ring (RPR) multi-Protocol Label Switching (MPLS) and other technologies. The MSTP product launched by the manufacturer also covers more or less the above features.

So far, MSTP has few ATM functions, Ethernet passthrough is the most widely used, and Ethernet bridging (or layer-2 switching) is the second; RPR applications are receiving strong attention; MPLS applications have not yet been formally emerging, but have received wide attention and the prospects are consistently optimistic.

Based on the SDH platform, MSTP can be implemented based on multiple SDH line rates, including 155 Mb/s, 622 Mb/s, 2.5 Gbit/s, and 10 Gbit/s. On the one hand, MSTP retains the inherent cross capabilities of SDH and the traditional PDH business interfaces and low-speed SDH business interfaces to continue to meet the needs of TDM business; on the other hand, MSTP provides functions such as ATM processing, Ethernet passthrough, L2 Ethernet Switching, RPR processing, and MPLS processing to meet the data service aggregation, sorting, and integration requirements. Currently, most MSTP prefer the general frame Forming Procedure (Green Code) as the excellent encapsulation procedure, while the virtual cascade and link capacity adjustment strategy (LCAS) adapt to different bandwidth particle needs, in addition, the link capacity can be adjusted within a certain range. In addition to the Ethernet function, the mstp rpr function module overcomes the shortcomings of the original Ethernet Switching speed, and can implement fast Protection Switching within 50 ms. In addition, RPR also provides fair algorithms to ensure reasonable utilization of link bandwidth and prevent link congestion to the maximum extent.

The MPLS function can be used to extend the MSTP Networking capability from the ring network to the GRID network, and multiple client services (including Ethernet, ATM, and frame relay) can be implemented through pseudo-line (PW) access and convergence, and then converge to the core data network through Tunnel, finally forming a full network-wide MPLS, bringing the advantages of MPLS to the extreme.

For a long time, the data leased line/private network services of major customers have been an important source of income for some operators. MSTP devices can be used to achieve point-to-point, point-to-point, and multi-point-to-Multi-Point networking of data services, this service can be used to provide services such as an Ethernet leased line (EPL), an Ethernet virtual leased line (EVPL), an Ethernet private network (EPLAN), and an Ethernet Virtual Private Network (EVPLAN, based on the customer's specific needs, the Service classification (CoS) and service quality (QoS) capabilities are provided. In particular, the layer-2 VPN established by using the MPLS function is opposite to the traditional TDM leased line, the price/performance ratio is much higher.

2. Notes for MSTP applications

2.1relationship between MSTP and data devices

There is no doubt that MSTP and data devices are associated with the network and coexist for a long time. MSTP devices are mainly used in the convergence and access layer networks of metro networks. They compete with low-end data devices, such as ATM switches, Ethernet switches, and RPR switches at the access layer. However, in most cases, the carrier selects system devices based on their network conditions. Despite the advantages of MSTP over pure data equipment, such as space saving, cost-effective, and integrated network management, if the operator's O & M system is not reformed, that is, the transmission equipment and data equipment are completely separated for construction, so the MSTP application effect will be greatly reduced.

On the other hand, the MSTP network management communication interface uses SDH Qx/Q3 and even the Corba interface, while data devices generally use Simple Network Management Protocol (SNMP ). With the gradual expansion of MSTP applications, more and more operators propose to connect low-end MSTP devices to the network management platform of medium and high-end data devices through SNMP for unified management. Obviously, this management only needs to manage the data boards and components in MSTP, and does not need to manage the SDH part in MSTP.

2.2MSTP Interconnection

The interconnection of MSTP involves multiple layers. First, at the business level, we need to consider the communication between the business leased line and the private network, for example, the intercommunication between EPL/EVPL and EPLAN/EVPLAN. If the application is embedded with MPLS, it is the interconnection problem between the virtual private line service (VPWS) and the virtual private network service (VPLS. Second, mainstream encapsulation protocols, such as the intercommunication of the green code, should be considered at the encapsulation level. Once again, at the data processing level, the intercommunication between ATM is rarely used and will not be considered much. RPR is only used in single-ring scenarios, so communication between multiple manufacturers is not considered, because the probability that devices from multiple manufacturers are configured on the same RPR ring network at the same time is almost zero. Ethernet communication is widely used and needs to be considered. However, Ethernet has been a technology that has been around for decades and MAC layer communication will not be too difficult. MPLS intercommunication includes static configuration and dynamic configuration. Static configuration relies on the network management system to actively establish the mark switching channel (LSP) or pseudo line (PW; dynamic configuration must take into account the intercommunication between the signaling protocol and the routing protocol. If you consider cross-domain applications and fault-related re-routing recovery, the problem will become very complex.

MPLS intercommunication must take into account the LSP and pseudo-line intercommunication. This issue is particularly important when combining VPWS and VPLS applications. Fourth, in the SDH bearer layer, considering the interconnection between virtual cascade and LCAS, of course, there are some standard usage of overhead bytes and protocol processing problems. Finally, on the SDH line side, we need to consider the STM-n interconnection problem. Of course, this is also the old problem and principle problem of SDH.

3. Next Development of MSTP

3.1MSTP and ASON Integration

The further development of MSTP is to load the Automatically Switched Optical Network (ASON) control plane. At present, ASON Control Plane processing particles are mainly VC-4/3 particles or VC-4/3 continuous cascade or virtual cascade particles. Because the data businesses connected to MSTP are mainly transmitted through virtual cascade of VC-n, for example, a Gigabit Ethernet (GE) businesses can be carried and transmitted through virtual cascade of eight VC-4. If the customer layer of Ethernet is strictly separated from the MSTP service layer, then a GE call corresponds to a GE connection, but corresponds to eight VC-4 connections, this is a problem where a call contains multiple connections or multiple layers of calls and connections. It is also a problem that must be considered and solved in a strict network hierarchy.

Since the GE business required by a customer's device can be carried and transmitted through a virtual cascade of eight VC-4, if the control plane is ignored, any VC-4 of the mstp transfer plane is faulty, under the control of the network management system, the LCAS mechanism can be used to adjust the bandwidth in the transmission plane. This is a mature technology that has been implemented. Considering that in the future, the application of ASON mainly provides the bandwidth (BoD) and virtual optical private network (O-VPN) as needed, we need to consider the realization of BoD function by the control plane, and not necessarily in the case of a fault. For example, the actual traffic at the GE interface has been reduced to 800 Mb/s or even 600 Mb/s, and the actual traffic is no longer full of GE traffic. After detecting this situation, can it pass through the user network interface (UNI) initiate a new connection request to reduce the bandwidth of 8 VC-4 to 6 or even 4? When the transport plane supports LCAS, the control plane has no problem in implementing this function. The key to the problem is that if the traffic changes too much in real time, this will cause repeated bandwidth adjustment on the control plane. This "shock" will not bring risks to mature and stable ASON applications, but it will cause some problems in the early stages of ASON application. For example, the SDH circuit required by the original business equipment is clearly displayed on the network management interface. The establishment, modification, and release of the circuit are all under the control of the network management operator, the circuit matrix can also be obtained after network planning. However, in the ASON environment, the discovery of circuit resources is automatically performed, the establishment, adjustment, and release of circuits are applied by the customer's devices. In most cases, the network management does not need to participate in these processes, it is just a more macro "five management functions ". If the ASON Control Plane encounters the "Shock" mentioned just now, O & M personnel who have been relying on the network management for a long time may not be able to adapt, from "not adaptive" to "Adaptive" to "handy" is not a short-term process.

3.2relationship between MSTP and next-generation Bearer Network

The current telecom network follows the 7-layer mechanism of Open Systems Interconnection (OSI). SDH and WDM are divided into the physical layer; ATM and Frame Relay (FR) ethernet and RPR are classified into the data link layer, that is, the second layer. Therefore, it is often said that the Ethernet Switching is a layer-2 switch. MPLS is special and is divided into two layers, tunnel is a Tunnel technology, but in general, there are many components belonging to the second layer. The third layer is the IP layer, and the higher layer has little to do with the transport network of the physical layer, you do not need to go into details here. Speaking of the next generation of telecommunications networks, the public point of view is to streamline the level, the industry is generally recognized as three layers (the preliminary architecture of the ITU-T SG13 standard is also three layers ). First, the transport layer should be retained, but the main body is not SDH or MSTP, but the Optical Transport Layer Based on the WDM System. The bearer layer will combine the Layer 2 and Layer 3 in the current OSI, the corresponding Network is also called the Packet Exchange-based Bearer Network (Bearer Network). The next layer is the business layer. In the bearer network, the current industry's opinion is that MPLS will dominate the world, but whether it will combine the advantages of Ethernet, RPR, MPLS, and IP to create a new standard without a fixed number.

MSTP has been integrated into many layer-2 data technologies, such as ATM, Ethernet, RPR, and MPLS, which have successively become an important function module of MSTP. Operators are increasingly available to choose when building a Metro Transmission Network. Of course, these standards and functional modules are not mutually exclusive, but complementary advantages. Despite its tenacious vitality, MSTP has two kinds of changes in the "Next Generation Network" wave: First, it gradually exits from the core layer of the transmission network and plays a role in the edge network; second, MSTP gradually increases the proportion of data processing and evolves into a real bearer network device with group switching as the core.

The control plane of the next generation network can be considered: due to the gradual exit of the SDH standard, the rise of the true light layer, the increasing integration of the data layer, the birth of the bearer layer, the streamlining of the network layer, and the comprehensive maturity of the control plane technology, the concept of ASON will be gradually reduced, the overlapping model will gradually give way to the peer-to-peer model, and the general multi-protocol mark Exchange (GMPLS) or some other mature control plane technology will uniformly control business layer equipment, bearer layer equipment and Optical Transport Layer equipment.

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