Handbook of Local Area Networks, 1998 Edition:LAN Interconnectivity Basics Click Here! Search the site:   ITLibrary ITKnowledge EXPERT SEARCH Programming Languages Databases Security Web Services Network Services Middleware Components Operating Systems User Interfaces Groupware & Collaboration Content Management Productivity Applications Hardware Fun & Games EarthWeb sites Crossnodes Datamation Developer.com DICE EarthWeb.com EarthWeb Direct ERP Hub Gamelan GoCertify.com HTMLGoodies Intranet Journal IT Knowledge IT Library JavaGoodies JARS JavaScripts.com open source IT RoadCoders Y2K Info Previous Table of Contents Next WHAT IS A DATA LINK CONNECTION? The essence of a frame relay connection or service is a type of virtual circuit called a data link connection (DLC). A data link connection can be used to replace a leased line in many applications. In the majority of frame relay equipment, and therefore services, DLCs are permanent virtual circuits (PVCs), predefined by both sides of the connection. Switched virtual circuits (SVCs) are defined in the frame relay specifications but are not in widespread use today. To date, only a few vendors have announced availability of SVCs. (Frame relay SVCs are discussed in more detail later in this chapter.) Each DLC has an identifying number called the data link connection identifier (DLCI). These numbers identify logical circuits on frame relay lines. Something important to know about data link connections is that the DLCI has local significance only—that is, the DLCI of a frame relay circuit can be (and often is) different at the local and distant ends of the circuit (see Exhibit 3-4-3) Therefore, network managers need to know both the local and remote DLCIs for each data link connection they are using. The circuit definitions made in each intervening frame relay switch (in the carrier’s network or private network) take care of routing the frame relay frames to the proper destination. Exhibit 3-4-3.  Local Significance of DLCI There is a reason DLC numbers have local significance only—frame relay defines interface specifications, and the DLC numbers operate over point-to-point links between equipment. There may be several links (or hops) over which a DLC’s data may travel before it reaches the last hop and is delivered (as with any network). Global Addressing. When using global addressing, DLC numbers are administered such that they are not arbitrarily reused within a frame relay network, thus allowing the use of numbers for association with a particular site. If global addressing was applied to the example in Exhibit 3-4-3, all sites might use DLC 100 for data that is destined for site A. Site A would never be configured with a DLC 100, because that would mean the data was destined for itself. Global addressing is not part of the frame relay protocol, it is just a practice that eases management of many data link connections. For this reason, some carriers use global addressing in their networks. STANDARDS FOR FRAME RELAY SERVICE AND EQUIPMENT Frame relay moved rapidly from a proposed standard to a plausible, economical way to communicate data over the wide area in part because of the concentrated effort of several groups. These groups included a dedicated subcommittee of the American National Standards Institute (ANSI), the Interop organization (sponsor of the LAN/WAN internetworking trade show), the ITU, the Frame Relay Forum (a consortium of vendors and carriers), and Bellcore, the well-known telecommunications R&D organization. Frame relay’s basics are well established today, thanks to these groupsí cooperative activity. The Frame Relay Forum, or FRF, educates users about the benefits of employing frame relay technology. The FRF offers a fax-back information service, World Wide Web (WWW) and FTP servers, and a newsgroup on the Internet (set up and operated by Indiana University). During the initial stages of development of frame relay standards, a group of companies—namely, Cisco, Digital Equipment Corp., Northern Telecom, and StrataCom, who at the time acquired the moniker Gang of Four—agreed on a subset of the standards that had not been defined at that time: the management and status of active and inactive DLCs. The result was the local management interface (LMI) specification. LMI provides for a way to query a frame relay circuit (in-band) and receive information about what DLCs are available. Virtually every company involved in developing frame relay equipment supports part of all of the LMI interface. Subsequently, the working group within the ANSI T1.S1 subcommittee responsible for frame relay developed Annex D to its specification. It is similar in most respects to LMI (i.e., similar but not interoperable). Most frame relay equipment supports both LMI and Annex D and possibly also Annex A to the ITU-TSS version of frame relay, which is very similar to ANSI Annex D. Interoperating over Frame Relay: The RFC 1490 Standard Because of the growing popularity of LAN internetworking, the Internet Engineering Task Force (IETF)—the body that provides standards for the Internet that are used for developing internetworking devices worldwide—released RFC 1294 (Request for Comment number 1294), thus providing a way to encapsulate IP and other protocols in frame relay. This standard was later superseded by RFC 1490, which expands the definitions provided by the original RFC, essentially formalizing a number of previously open issues that arose as customers and vendors built multivendor networks and conducted interoperability testing for LAN internetworking over frame relay. The end result is that today customers can use frame relay networking equipment that conforms to RFC 1490 and have a high degree of success in interoperating over frame relay. Any problems that arise will most likely be a result of misconfiguration of non-frame relay related parameters. Previous Table of Contents Next Use of this site is subject certain Terms & Conditions. Copyright (c) 1996-1999 EarthWeb, Inc.. All rights reserved. Reproduction in whole or in part in any form or medium without express written permission of EarthWeb is prohibited. Please read our privacy policy for details.