1 Introduction
The metropolitan transport network is a network that covers urban areas, suburbs, or some smaller cities and counties, and provides an integrated transport platform for multi-city services. It is a basic transport network that carries fixed, mobile, and data services within the metropolitan area. , It is generally based on multi-service optical transmission network, supplemented by multiple access technologies, and provides an integrated transmission bearer platform for multiple services and communication protocols. The metropolitan transport network is connected upwards to the inter-provincial and intra-provincial trunks, and is responsible for the introduction of integrated services, and completes the tasks of business access and circuit leasing for group users, commercial buildings, and intelligent communities.
2 Features of Metropolitan Transport Network
The metropolitan transport network is a very complex network. Each city and each city is different due to the different status quo. From the perspective of the network layered structure, the metro transport network is generally divided into a core transport layer, an aggregation layer and an access layer. Floor. For cities with small networks, the network hierarchy can be simplified according to the actual situation. The characteristics of the metropolitan transport network are analyzed from a general perspective.
Multi-business. The metropolitan transport network needs to support multiple services at the same time. The feature that a single platform supports multiple protocols and handles mixed services is the key factor for the metro optical transport network to obtain sufficient competitive advantage, and it is also the most important feature. Multi-service support is the cornerstone of the metro optical transport network, which can provide operators with many competitive advantages, such as backward compatibility (such as SDH over WDM), significant cost reduction (reduced network layering and equipment), and simplified network management And configuration workload reduction.
Security survivability and augmentability. Metropolitan transport network involves a large number of customers and services, and the security and reliability of the network directly affects customers. The transport network should support network node backup and line protection, and provide network security measures. Provide better business definition. At the same time, the metropolitan transport network should fully consider the service expansion capability, and can provide rich broadband value-added services for different user needs, so that the network can continue to make profits.
Dynamic. Compared with the backbone transport network, the metropolitan transport network is more dynamic. The dynamic and unpredictable nature of multiple data services have strengthened the demand for the metro transport network. The current development trend is that more and more customers need Business with more flexible bandwidth. They need fast service configuration, more short-term, flexible service contracts and QoS-based prices. In the future, there may be demand for new services such as bandwidth allocation on demand.
Network scalability. Due to the impact of user needs and the dynamic changes in geographic distribution, metropolitan data services are volatile. The metropolitan transport network should be built into a complete, unified, flexible networking, and easily expandable flexible network platform, leaving sufficient room for expansion As requirements change, operators can be allowed to continuously increase bandwidth according to business requirements without the need for overall network upgrades.
3 Analysis of related technologies in metropolitan area networks
SDH multi-service transmission platform. SDH Multi-Service Transport Platform (MSTP) is a widely used product. In order to meet the needs of multiple services in the metropolitan area network, SDH has evolved from simply supporting voice service interfaces such as 2Mb / s, 155Mb / s to supporting multiple service interfaces such as Ethernet and ATM, and maps many different services through YC or VC cascade Enter the SDH time slot for processing. The SDH multi-service platform integrates the transmission node with various service nodes, and the manufacturers only have different degrees of integration.
The starting point of MSTP is to complete the functions of layer 2 or layer 3 as additional functions of SDH. The processing of layer 2 or ATM layer is separated from SDH processing, but they can all be mapped to SDH VC time slots for reorganization . From a functional point of view, in addition to the SDH function, MSTP also has a layer 2, MAC layer, and ATM function.
MSTP is more suitable for operating companies that have laid a large number of SDH networks. It can conveniently and effectively support packet data services and realize the transition from circuit-switched networks to packet networks. , At the same time, it can ensure the uniformity of network management.
Flexible packet ring technology. The Resilient Packet Ring (RPR) technology being developed by the IEEE 802.17 working group has absorbed the economy of Gigabit Ethernet and the 50ms ring protection feature of the SDH system. RPR uses an Ethernet-like frame format, combined with silk-marking, based on MAC high-speed switching, simplifying IP forwarding. RPR technology can support finer bandwidth granularity, lower network cost, can carry bursty IP services, while supporting traditional voice transmission, has a better bandwidth fairness mechanism and congestion control mechanism. RPR ring is to implement a fair mechanism on the entire ring rather than on a separate link, and it is easy to implement a global fair mechanism. The service provider can use the rate at which the source node sends packets to control the rate of upstream and downstream nodes. The bandwidth strategy allows all the bandwidth between any two nodes on the ring to be allocated to these two nodes without congestion, without the inflexibility of fixed circuit systems like SDH, and at the same time more than point-to-point Ethernet More effective.
At present, the RPR standard has not been completed. One of the important issues is the transparent transmission of the clock. The RPR synchronization mechanism is different from SDH. It must ensure that the TDM clock can be transparently transmitted to the peer. The second challenge comes from the definition of RPR is a technology under a ring network structure, which cannot work in a complex network environment (even interconnection between rings), while the actual metro network environment is very complicated.
RPR technology is suitable for networks that are mainly based on data services and supplemented by TDM services. The scope of its applications will gradually expand and is suitable for new networks.
Metropolitan WDM optical network. WDM technology not only improves the utilization rate of optical fiber, but also has the transparency of the signal in the metropolitan area network where the service signal is complex and changeable. It can directly transmit the signal without adjusting the rate and frame structure of the signal from different devices. This gives users, especially those who rent wavelengths, maximum flexibility. At the same time, the signals between different wavelengths do not interfere with each other, and each wavelength can flexibly move up and down. WDM technology is mainly used in metropolitan backbone networks. Metropolitan OADM ring networks can carry a large number of customers with multiple protocols and multiple rates of services. The method of carrying one service per wavelength will quickly exhaust the wavelength. To improve the bandwidth utilization of each wavelength, try to avoid low The rate service occupies an optical wavelength channel alone. An emerging cost-effective method is to multiplex multiple low-rate customer signals into a wavelength channel. This technique is called sub-wavelength multiplexing, which enables each wavelength to carry multiple services. This sub-wavelength multiplexer reduces the application threshold of the metropolitan area network WDM system, can directly accommodate low-rate signals, and brings flexibility to the networking. WDM ring network solves two important problems: optical fiber shortage and transparent transmission of multiple services. Cost is an important factor limiting its application. At present, it is mainly used to protect those services that SDH cannot yet protect, such as ESCON, Fiber Channel, etc.
In the current optical network, the provision of data services needs to be processed through four layers: first map the services into IP packets and encapsulate them with ATM cells, then map the ATM cells into SDH frames, and finally convert them into optical signals in the optical network Upload (using WDM / DWDM). With the rapid development of IP services, the shortcomings of this structure are increasingly exposed. People began to study the ATM layer and SDH layer stripped from the 4-layer structure, and its function is integrated into the IP / MPLS layer and WDM / OTN (optical transport network In the) layer, the IP service is directly transmitted on the WDM optical path (that is, IP over OpTIcal, currently mainly IPover WDM / DWDM). In the traditional optical network, the signaling control and dynamic switching functions are introduced, the IP layer and the optical network layer are placed under the same control plane, and the configuration and connection management of the optical network is implemented. Under this idea, a new type that can automatically complete the network connection The network ASON (Automatically Switched Optical Network) came into being.
Automatically exchange optical networks. ASON is developed on the basis of IP over DWDM, the bottom layer is still OTN, the main difference is the introduction of a control plane on OTN. The control plane completes the dynamic control of the transmission plane through signaling exchange. The introduction of the control plane brings the following benefits: rapid implementation of service provision, allowing network resources to dynamically allocate routing and bandwidth; easy management, service providers do not need to develop and maintain operation support system software for the configuration management of new transmission technology systems; with extensions The signaling capability of the system adds supplementary services; it can achieve fast protection and recovery in the event of a failure, saving redundant capacity and resources compared to the usual transmission network; the control plane protocol has more abundant primitives than the management plane protocol Group can be used for various transmission technologies.
4 General Label Switching (GMPLS) technology
In order to adapt MPLS to different application environments such as time-division multiplexing and wavelength-division multiplexing to support the establishment of connections in circuit-switched networks, the IETF has expanded the concept and form of labels in MPLS accordingly. Covered in, launched the general label switching-GMPLS. It has many new features:
Time slots, virtual channels and wavelengths can be used as labels. The objects managed by GMPLS are not only packets, but also FR. ATM, SDH, WDM, etc., and the interfaces on these devices can also be subdivided into PSC (packet switching function), TSC (TDM switching function), LSC (wavelength switching Function) and FSC (fiber exchange function) and other types.
Bandwidth can be allocated for discrete units, because time slots, wavelengths, and optical fibers are all discrete units.
It has downlink on-demand label distribution and bidirectional LSP establishment capability using upstream labels, and can simplify the switching process and reduce the establishment delay of bidirectional LSPs by transmitting suggested labels from upstream nodes to downstream nodes.
You can set tag groups to narrow the selection of downstream tags. Of course, with the introduction of the GMPLS control plane, new requirements have also been placed on traditional data communication networks (DCN), especially circuit-switched networks. First, DCN must ensure that it can provide control information transmission between the controllers, and can provide channels for exchanging control information directly or indirectly for the two LSRs: Second, the provided channels must be reliable and safe: Finally, DCN It must support IP, and must have high reliability and QoS to avoid user data service errors affecting control data and ensure the smooth transmission of control information.
An ADSL filter separates the analogue voice-frequency signals from the ADSL (broadband) data signals(Broadband being defined as data transfer greater than 128KBPS.)
If ADSL filter/splitters are not used, the ADSL data signals are heard as "noise" on any equipment connected to the "normal" telephone-line. Apart from being annoying during a telephone conversation, there is the possibility that the additional ADSL signals may cause problems with alarm-units (etc) that may be connected across the line.
This ADSL-interference is illustrated in the diagram below that shows how the unfiltered ADSL data signals appear across the "standard" telephone equipment.
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