Frame Relay, solutions for businesses and corporations (Part 2)

Addressing method

In Figure 6, assume two PVCs [11], one between Atlanta and Los Angeles and one between San Jose and Pittsburgh. Los Angeles uses DLCI 22 to refer to its PVC with Atlanta, while Atlanta refers to the same PVC as the DLCI number 82. Similarly San Jose uses the DLCI 12 number to reference its PVC with Pittsburgh and Pittsburgh. DLCI 62 application. The network uses mechanisms in (internal mechanism) to store two PVC identifiers of different local significance.

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Figure 6: An example of the use of DLCI in Frame Relay network.

The DLCI address area has a limit of 10 bits (see later). As such, it is capable of generating 1024 DLCI addresses. The usable part of these addresses is determined by the type of LMI used. Cisco LMI type supports a range of DLCI addresses from 16 to 992 for user data. The rest is for service providers. It includes LMI messages and multicast addresses.

LMI: Cisco's addition to Frame Relay

The main development in the history of Frame Relay development was 1990 when Cisco Systems, StraCom, Northern Telecom and Digital Equipment Corporation jointly formed a focus group on the development of Frame Relay technology and promoting the delivery of products. Its products can work together. This group has developed a specification that adheres to the base Frame Relay protocol. They also expanded it through the addition of capabilities to serve complex network environments. Frame Relay extensions all converge on LMI (local management interface).

The LMI extension

In addition to the basic Frame Relay functions for data transmission, Frame Relay specifically includes LMI extensions that allow for greater and more complex network connections. The following is a summary of LMI extensions:

- Virtual circuit status messages (common) - Provides communication and synchronization between the network and user devices, periodically reporting the existence of new PVC [11], removing existing PVC and General information about PVC integrity calculation. Virtual circuit status messages prevent sending data through PVCs that do not last long.

- Multicasting (optional) - Allows an entity to send a single frame (frame) but transfer it to multiple receiving entities. Thus multicasting supports efficient transport of routing protocol messages and address resolution protocols that must be sent simultaneously to multiple destinations.

- Global addressing (optional) - Giving a global (global) connection, allowing them to be used to identify a specific interface with Frame Relay network. Global addressing makes the Frame Relay network the same as a LAN for addressing. Address resolution protocols, so implementing through Frame Relay is exactly as they are done via LAN.

- Simple flow control (optional) - Fully prepared for a flow control mechanism XON / XOFF applied to the entire Frame Relay interface.

Here we will examine some of the characteristics of LMI:

+ Global addressing: In addition to the common features of LMI, optional LMI extensions (optional) are particularly useful in the network connection environment. The first important LMI extension option is Global Addressing . With this extension, the values ​​inserted in the DLCI field of a frame are the overall meaningful addresses of individual end-user devices (eg routers).

As mentioned, the base Frame Relay specification (not extended) only supports values ​​for the DLCI field that identifies PVC [11] with local significance. In this case, there are no addresses that identify network addresses or nodes assigned to these interfaces. Because these addresses do not exist, they cannot be detected by traditional addressing solutions and discovery techniques. That means that with normal Frame Relay addressing, we have to use static maps to refer to them. These static configurations tell routers which DLCIs are used to find a remote device and related network connections.

In Figure 6, note that each interface has its own identifier. Suppose that Pittsburgh must send a frame to San Jose. The DLCI value at the VC terminal San Jose is 12 and 62 at the Pittsburgh terminal. So Pittsburgh takes up the value 62 in the DLCI field and sends the frame into the Frame Relay network to go to San Jose. Each router interface has a distinctive value such as its network node identifier, so individual devices can be distinguished from each other. That allows routing in the overall environment. Global addressing provides meaningful benefits in a large and comprehensive network.

+ Multicasting and inverse ARP: Multicasting is another optional LMI feature. Multicast groups are designed with a series of four DLCI values ​​separately (1019 to 1022). Frames sent by a device that uses one of these reserved DLCIs are created with a replica by the network and sent to all output points in the set (set). Multicasting expansion also defines LMI messages that inform user devices about the addition, deletion and presence of multicast groups. In networks that take advantage of dynamic routing, routing information must be exchanged between routers. Routing messages can be effectively sent by using frames with a DLCI multicast. This allows messages to be sent to specific router groups.

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Figure 7: Inverse ARP.

Inverse ARP mechanism [13] allows the router to automatically construct the Frame Relay map , as described in the figure. The router receives the DLCIs that are used in the switch during the initial LMI exchange. The router then sends an Inverse ARP request to each DLCI for each protocol configured on the interface if the protocol is supported. Feedback from Inverse ARP is then used to build the Frame Relay map

+ Frame Relay map: The next router address specified from the routing table must be used to determine the DLCI as described in Figure 8. The solution is implemented through a data structure called a Frame Relay map . The routing table is then used to provide the next router routing address or DLCI for outbound traffic. This data structure can be statically configured on the router or Inverse ARP property used to automatically install the Frame Relay map .

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Figure 8: Frame Relay map

+ Frame Relay switching table ( Frame Relay switching table ): Frame Relay switching table consists of four inputs: two for incoming port and DLCI, and two for outgoing port and DLCI as described in picture. So DLCI can be rearranged while it passes through each switch.

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Figure 9: Frame Relay switching table

Here we know some basic methods of Frame Relay operation and can now summarize the implementation of Frame Relay as follows.

Steps to implement Frame Relay

Step 1 : Register Frame Relay service with a service provider (in Vietnam is VDC Company).
Step 2 : Connect each router, either directly or through a CSU / DSU to Frame Relay switches.
Step 3 : Once the CPE router [9] can work, send a 'status inquiry' message to the Frame Relay switch. This message informs the switch about the state of the router and requests that the switch provides the connection status of other remote routers that it wants to exchange data with.
Step 4 : When the device switches Frame Relay received the request, it responds with a 'Status' message containing both the DLCI of the remote routers for the router to transmit data.
Step 5 : Give each DLCI the router receives through an Inverse ARP message (if Inverse ARP does not work or if the remote router does not support Inverse ARP you need to configure the DLCI values ​​and IP addresses for remote routers), a reference input is created in the Frame Relay map table . It includes local DLCI, the network layer address of the remote router and the state of the connection. Note that the DLCI value is DLCI that has been configured locally by the router, not the DLCI that the remote router is using. There are three connection states that can be displayed in the Frame Relay map :

+ Active state : Indicates that the connection is active and the router can exchange data
+ Inactive state : Indicates that the local connection to Frame Relay switches is active, but the connection of the remote router to Frame Relay switches does not work.
+ Deleted state : Indicates that LMI is not received from Frame Relay switches or there is no service between the CPE router and Frame Relay switches appear.

Step 7 : Every 60 seconds, the routers exchange Inverse ARP messages once.
Step 8 : By default, every 10 seconds (or by configuration), the CPE router (9) sends a 'keepalive' message to the Frame Relay switch. The purpose of this 'keepalive' message is to check if the Frame Relay switch is still active.

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Figure 10: Steps to implement Frame Relay

Epilogue

Currently VDC has added 5 partners to provide Frame Relay services including NTT, Equant, KDDI, REACH and VITC to expand the ability to provide this service to 500 points globally. The Frame Relay service provided by the company has reached speeds of over 15 Mb / s with about 150 customer channels nationwide. Up to this point, VDC and NTT Communications have about 30 customers in use. Frame Relay service, including many domestic and foreign enterprises in export processing zones in Vietnam. In the near future, international bandwidth between Frame Relay, VDC and NTT Communications service providers will be upgraded to satisfy the growing demand for services by domestic and foreign businesses. Frame Relay is becoming one of the leading technology solutions for businesses and large corporations - organizations that need a large-scale data transmission network, cost savings, cost reduction and enhanced capabilities. competitiveness in production and business activities.

Royal Union

Note

[1] CCITT (Consultative Committee for International Telegraph and Telephone): Advisory committee on international telegraph and telephone.
[2] ANSI (American National Standards Institute): American National Standard Academy.
[3] OSI (Open System Interconnection): Open system connection model.
[4] TCP (Transmission Control Protocol): The control protocol transmitted in the TCP / IP protocol family has a mechanism for detecting errors and then replaying lost packets.
[5] HDLC (High-Level Data Link Control): This is a standard of data packaging on ISO wide area network (WAN); HDLC supports both configuration (point-to-point) (point-to-point) and multipoint (multiple points).
[6] CDU / CSU (Channel service unit / Data service unit): A digital interface device that connects end-user devices to local phone lines.
[7] DTE (Data terminal equipment): A device towards the end user of an interface between the user and the network is considered as a device that sends or receives data or both (usually a router). A DTE connects to a data network through a DCE (eg a modem) and specifically uses the clock signal generated by that DCE. DTEs include devices such as computers, protocol changers, multiplexers.
[8] DCE (Data circuit-terminating equipment): The device used to convert user data from DTE to an acceptable form for Wan service The commonly used DCE device is a modem.
[9] CPE (customer premises equipment): Termination equipment such as terminals, phones or modems provided by the telephone company, installed at the customer site and connected to the company's network phone.
[10] Data-link connection identifier ( DLCI ): A value indicating a PVC or an SVC in a Frame Relay network. In the basic Frame Relay specification, DLCIs are locally significant (it is understood: connected devices can use different values ​​to indicate the same connection. In the LMI expansion specification, The DLCI has overall meaning (it is understood: the DLCI indicates separate terminals).
[11] PVC (Permanent virtual circuit): A virtual channel is fixed. The PVC saves bandwidth related to channel setup.
[12] SVC (switched virtual circuit): A virtual channel is set to dynamically demand when the transmission has ended. SVCs are used in situations where data transmission is infrequent.
[13] Inverse ARP (Inverse Address Resolution Protocol)
[14] ISDN (Integrated Services Digital Network): This is a switch-based wide area network technology that allows telephone networks to transmit data and voice.
[15] MUX : In communication systems, MUX stands for multiplexing, a device that sends multiple signals on a transmission channel at the same time as a single, complex signal to one device. Other devices can recover (restore) the inherent separate signals on the receiving device. Receivers are sometimes called DEMUX.
[16] PBX (private branch exchange): General branch, switching system used in companies and organizations to handle outgoing and incoming calls.PBX is a device that provides private internal voice switching service and voice related services in a private network.

See Part I