上世纪８０年代，很多用户在本地采用了局域网（ＬＡＮ）技术。一个公司、企业、机关以ＬＡＮ将本单位的多台个人计算机连接起来，共享本地网络资源，同时通过网桥或路由器接入公共电信网。这类用户的数据特点是数据量大、突发性高。除ＬＡＮ外，计算机辅助设计（ＣＡＤ）、计算机辅助制造（ＣＡＭ）以及图像传送业务也具有突发性特点，这是因为他们的文件数据量往往很大，比如一张普通的Ｘ光片就会有８兆的数据量。
用分组网为这些用户开放业务，由于用户要传送的数据量大，而分组网的接入速率低、传送时延长，用户收发信息要作长时间的等待，会令用户不满意。如果用数字数据网（ＤＤＮ）数字数据专线为这些用户开放业务，通信效率虽然提高了，但费用较贵。来自用户的新的通信需求促使人们考虑采取新的通信技术。At the same time, network technology has changed greatly. The intelligence of the user equipment general increase, the relay has been widely used transmission line fiber, high optical transmission performance, low error rate. In this case, the error correction and flow control problems can be solved by a high-level agreement on a user device, network protocols can be simplified. Thus, people packet switching protocol is simplified, resulting in a Frame Relay (FR) technology.
Frame relay (frame relay) is a new public data network communication protocols rise in 1992 and 1994 a rapid development. Frame relay is an efficient data transmission technology that can quickly and inexpensive transmission of digital information in one or many of the application. It can be used for voice, data communication, both for local area network (LAN) wide area network can also be used for communication (WAN) is. Each user will get a Frame Relay leased line to the frame relay node. FR network to the end user, it is often changed by a user and not visible to process the data transmission channel between the user and others.
It is a simplified X.25 WAN protocol. Statistical multiplexing protocol is a frame relay protocol, it is possible to provide a plurality of virtual circuits on a single physical line. Each virtual circuit data link connection identifier (Data Link Connection Identifier, DLCI) is identified, only the DLCI for effective local interface and the peer interface directly connected thereto, does not have a global validity, i.e., in the FR network same DLCI on different physical interfaces does not indicate a same virtual circuit. Frame relay network may be a public network or a private enterprise network may be directly connected between the data network of devices.
With X. 25 packet switching technology, it has the following characteristics:
(1) Frame Relay X.25 packet switching inherited characteristics, statistical multiplexing by multiplexing multiple virtual circuits on a single physical circuit, dynamically allocated among users data bandwidth, link utilization.
(2) greatly simplifies the Frame Relay X.25 communications protocols, network error detection only on the information processing, the error correction is not found to be an error frame is discarded, the end to end flow control to the user terminal to complete, reducing the processing load of network switches, reducing-end transmission delay of user information.
(3) the user provides a Frame Relay preferential charging policy, i.e. according committed information rate (CIR) to charge and to ensure that the information transmitted is less than the CIR; the same time, allowing the user to transfer data is higher than CIR information, which is not part of the information transfer fee, to be transmitted when the network is idle, to be discarded when congestion.
(4) Frame Relay frame length is longer (up to 4096 bytes), LAN data transfer in a long frame length (1500 bytes) of information frame when the high efficiency, suitable for implementing LAN interconnection.
Concepts
DTE: FR network between user devices (such as hosts and routers, etc.) for data communications capabilities, the user equipment is referred to as data terminal equipment (the Data Terminal Equipment's, the DTE);
the DCE: provision of access for a user equipment the device belonging to the network device, referred to as data circuit-terminating equipment (data circuit-terminating equipment, DCE );
UNI: The interface between the DTE and DCE are called user-network interface (the Network Interface the User, the UNI);
the NNI: Network interface between the network and the interconnection network is called an interface (Network-to-Network Interface, NNI).
VC describes
different ways according to the established virtual circuits, virtual circuits are divided into two types: PVC (Permanent Virtual Circuit, PVC) and SVC (Switched Virtual Circuit, SVC). VC generated manually set called PVC. VC consultations by agreement is called SVC, the VC is automatically created and deleted by the frame relay protocol. The most used in the frame relay permanent virtual circuit manner.
In the PVC mode is necessary to detect whether a virtual circuit is available. Local Management Interface (Local Management Interface, LMI) protocol is used to detect whether a virtual circuit is available. LMI protocol maintains frame relay protocol PVC table, including: notification PVC increasing, detecting PVC deleting, monitoring PVC status change, verify the integrity of the link. The system supports three local management interface protocols: ITU-T Q.933 Annex A, ANSI T1.617 Appendix D of compatible and non-standard protocols.
The basic mode of operation of the LMI protocol is: The DTE at a certain time interval sends a status request message (Status Enquiry packets) to query the status of the virtual circuit, the DCE receives the device status request packet, the packet with the state (status packet) notifies the current state of all the virtual circuits on the interface DTE.
For DTE side, PVC state is completely determined by the DCE side apparatus; DCE side to the device, state PVC is determined by the network. In the case where the two network devices are directly connected, the DCE side virtual circuit state device is set to the device administrator.
Frame Relay Address Mapping
FR address mapping protocol address of the peer device and the peer device frame relay address (local DLCI) association with the high-level protocol by the protocol address of the addressed terminal device to the peer device.
FR mainly used to carry IP, when transmitting IP packets, the routing table can only know the next-hop address of the packet, before sending its corresponding DLCI must be determined by the address. This process can be done by looking Frame Relay address mapping table, because the address mapping table is stored in the mapping relationship next hop IP address and the next hop corresponding DLCI.
Address mapping table may be configured manually or dynamically maintained by Inverse ARP (InARP).
Interworking Frame Relay network and ATM network
currently interworking PVC frame relay network and ATM network three ways:
A first type is between two frame relay networks over an ATM network interconnection. ATM network to receive information from a frame relay network, which will be adapted into a plurality of ATM cells transmitted within the ATM network, the network at the other end of the ATM cell resynthesis information to another FR FR network. ATM network relay transmission channel may be provided by the network in this manner, a frame; A second type is a frame relay protocol interworking between terminals FR online user equipment and the use of the ATM network. ATM interworking completed by an ATM adaptation layer 5 (the AAL5) protocol and the mapping function provides the core frame format, both devices can communicate; A third embodiment is the same FR online user equipment is not used in ATM networks interworking between terminals of the frame relay protocol. By function over ATM AAL5 SSCS for a specific service provided to complete the mapping of the core protocol and frame format when interoperability, interworking both devices.
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