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Introduction to Frame Relay Technology

In the 1980s, many users used LAN (local area network)  technology locally. A company, enterprise, or organization connects multiple personal computers of the unit with LAN, shares local network resources, and accesses the public telecommunication network through a bridge or router. The data characteristics of such users are large data volume and high burstiness. In addition to LAN, computer-aided design (CAD), computer-aided manufacturing (CAM), and image transfer services are also bursty because their file data is often large, For example, a normal X-ray film will have 8 megabytes of data.

The packet network is used to open services for these users. Because the amount of data to be transmitted by the user is large, and the access rate of the packet network is low and the transmission is prolonged, the user has to wait for a long time to send and receive information, which may make the user unsatisfied.If you use digital data networks(DDN) and digital data lines to open up services for these users, although the communication efficiency is improved, but the cost is relatively high. New communication needs from users have prompted people to consider adopting new communication technologies.

At the same time, network technology has changed a lot. The intelligence of user equipment is generally improved. The optical fibers have been commonly used in trunk transmission lines, and the optical fiber transmission performance is high and the bit error rate is low. In this case, the error correction and flow control problems can be solved by higher layer protocols on the user equipment, and the network protocol can be simplified. Thus, the packet switching protocol has been simplified, resulting in Frame Relay (FR) technology.

Frame relay is a new public data network communication protocol that emerged in 1992. It began to develop rapidly in 1994. Frame Relay is an efficient data transmission technology that can transmit digital information quickly and inexpensively in one-to-one or one-to-many applications. It can be used for voice and data communication, and can be used for both local area network (LAN) and wide area network (WAN) communication. Each Frame Relay user will get a dedicated line to the Frame Relay node. The Frame Relay network handles data transmission with other users through a channel that changes frequently and is invisible to the user.

It is a simplified X.25 WAN protocol. The Frame Relay protocol is a statistical multiplexing protocol that provides multiple virtual circuits on a single physical transmission line. Each virtual circuit is identified by a Data Link Connection Identifier (DLCI). The DLCI is valid only on the local interface and the peer interface directly connected to it. It does not have global validity, that is, in the frame relay network. The same DLCI on different physical interfaces does not mean that it is the same virtual circuit. A frame relay network can be either a public network or a private network of an enterprise, can also be a network formed by direct connections between data devices.

Same as X. Compared with the 25 packet switching technology, it has the following characteristics:

(1) Frame Relay inherits the characteristics of X.25 packet switching statistical multiplexing. By multiplexing multiple virtual circuits on one physical circuit, the data bandwidth resources are dynamically allocated among users, which improves the line utilization.

 (2) Frame Relay greatly simplifies the X.25 communication protocol. The network only detects errors and does not correct errors in information processing. It discards the error frame once it is found. It also delivers end-to-end flow control to the user terminal, reducing the processing load on the network switch and reducing the end-to-end transmission delay of user information.

(3) Frame Relay provides users with a preferential charging policy, that is, charging according to the committed information rate (CIR) and guaranteeing the transmission of information below CIR; meanwhile, allowing users to transmit data higher than CIR. Information, this part of the information transmission is not charged, the network will be transmitted when idle, and will be discarded when congestion occurs.

(4) Frame relay has a long frame length (up to 4096 bytes) and is more efficient when transmitting long-range LAN data information frames (about 1500 bytes), which is suitable for LAN interconnection.

Concept introduction

DTE: The frame relay network provides the ability to communicate data between user equipment (such as routers and hosts). The user equipment is called Data Terminal Equipment (DTE);

DCE: A device that provides access to a user equipment. It belongs to a network device and is called a Data Circuit-terminating Equipment (DCE).

UNI: The interface between DTE and DCE is called User Network Interface (UNI);

NNI: The interface between the network and the network is called the Network-to-Network Interface (NNI).

Virtual circuit introduction

According to different virtual circuit establishment methods, virtual circuits are classified into two types: permanent virtual circuits (PVCs) and switched virtual circuits (SVCs). The virtual circuit generated by manual setting is called a permanent virtual circuit. The virtual circuit generated by protocol negotiation is called a switched virtual circuit. This virtual circuit is automatically created and deleted by the Frame Relay protocol. The most widely used method in frame relay is the permanent virtual circuit mode.

In the permanent virtual circuit mode, it is necessary to detect whether a virtual circuit is available. The Local Management Interface (LMI) protocol is used to detect whether a virtual circuit is available. The LMI protocol is used to maintain the PVC table of the frame relay protocol, including: notifying the increase of PVCs, detecting the deletion of PVCs, monitoring the change of PVC status, and verifying the integrity of links. The system supports three local management interface protocols: ITU-T Q.933 Appendix A, ANSI T1.617 Appendix D, and non-standard compatible protocols.

The basic working mode of the LMI protocol is that the DTE device sends a status request packet (Status Enquiry packet) to query the status of the virtual circuit at a certain interval. After receiving the status request packet, the DCE device uses the status message immediately. (Status packet) Notifies the DTE of the status of all virtual circuits on the current interface.

For the DTE side device, the state of the permanent virtual circuit is completely determined by the DCE side device; for the DCE side device, the state of the permanent virtual circuit is determined by the network. In the case where two network devices are directly connected, the virtual circuit status of the DCE side device is set by the device administrator.

Frame Relay Address Mapping

The frame relay address mapping is to associate the protocol address of the peer device with the frame relay address (local DLCI) of the peer device, so that the higher layer protocol can be addressed to the peer device through the protocol address of the peer device.

The frame relay is mainly used to carry the IP protocol. When sending an IP packet, the next hop address of the packet can only be known according to the routing table. Before the transmission, the corresponding DLCI must be determined by the address. This process can be performed by looking up the frame relay address mapping table, because the address mapping table stores the mapping relationship between the next hop IP address and the DLCI corresponding to the next hop.

The address mapping table can be manually configured or dynamically maintained by Inverse ARP (Reverse Address Resolution Protocol).

Interworking between Frame Relay Network and ATM Network

Currently, there are three ways for interworking between a Frame Relay PVC network and an ATM network:

The first way is to interconnect two frame relay networks through an ATM network. The ATM network receives information from a frame relay network, adapts it to several ATM cells for transmission within the ATM network, and transmits ATM cell recombined frame relay information to another frame relay at the other end of the network. network. In this way, the ATM network can provide a relay transmission channel for the intranet;

The second way is the interworking between the user equipment on the frame relay network and the terminal using the frame relay protocol in the ATM network. Interworking between the two devices through the function completion protocol and the mapping of the core frame format provided by the ATM 5th Adaptation Layer (AAL5) of the ATM;

The third way is the interworking between the user equipment on the frame relay network and the terminal that does not use the frame relay protocol on the ATM network. During interworking, the protocol and the core frame format mapping are implemented through the functions provided by the SSCS of the ATM on the AAL5 of the ATM to implement interworking between the two devices.

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