SNA Over Frame Relay. ATG s Communications & Networking Technology Guide Series. This guide has been sponsored by

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1 SNA Over Frame Relay ATG s Communications & Networking Technology Guide Series This guide has been sponsored by

2 Table of Contents Introduction Systems Network Architecture The Basics Frame Relay An Overview When Frame Relay Applies Frame Relay Support for SNA RFC 1490 Encapsulation DLSw over Frame Relay Quality of Service in Frame Relay Networks The Benefits of Frame Relay ATM and Frame Relay Appendix A Frame Relay Details Glossary About the Editor Gerald P. Ryan is the founder of Connections Telecommunications Inc., a Massachusetts-based company specializing in consulting, education and software tools which address Wide Area Network issues. Mr. Ryan has developed and taught numerous courses in network analysis and design for carriers, government agencies and private industry. Connections has provided consulting support in the areas of WAN network design, negotiation with carriers for contract pricing and services, technology acquisition, customized software development for network administration, billing and auditing of telecommunications expenses, project management, and RFP generation. Mr. Ryan is a member of the Networld+Interop program committee. This book is the property of The Applied Technologies Group and is made available upon these terms and conditions. The Applied Technologies Group reserves all rights herein. Reproduction in whole or in part of this book is only permitted with the written consent of The Applied Technologies Group. This report shall be treated at all times as a proprietary document for internal use only. This book may not be duplicated in any way, except in the form of brief excerpts or quotations for the purpose of review. In addition, the information contained herein may not be duplicated in other books, databases or any other medium. Making copies of this book, or any portion for any purpose other than your own, is a violation of United States Copyright Laws. The information contained in this report is believed to be reliable but cannot be guaranteed to be complete or correct. Copyright 1996 by The Applied Technologies Group, One Apple Hill, Suite 216, Natick, MA 01760, Tel: (508) , Fax: (508) info@techguide.com Web Site:

3 Introduction This Technology Guide examines the successful transition of SNA traffic from traditional leased line environments to Frame Relay network services. Historically, SNA traffic, which, even today usually consists of mission critical corporate applications, has been highly susceptible to unpredictable network delays. Network designers had been reluctant to trust this important traffic to anything other than dedicated leased line services. Frame Relay service, however, has been an attractive option for supporting SNA. With mature Frame Relay services, SNA traffic can be managed and Frame Relay networks designed in ways that assure prioritization, guaranteed throughput, and protection against network congestion. This Technology Guide explores those issues in detail and supports the conclusion that Frame Relay is a suitable, attractive, and cost-effective way to support SNA, if provisioned through switch equipment that provides all of the necessary capabilities for success. Systems Network Architecture The Basics SNA has been IBM s architecture and strategic direction for communications for over twenty years. Conceived in the early 1970s, SNA was a means for integrating and consolidating a variety of incompatible communications products that IBM was supporting at the time. The initial market was the largest users of IBM processing systems (the large banks, for example) but SNA became so successful that today it is virtually universal among IBM customers. One expert has determined that some 50,000 SNA networks exist today, representing over 60% of all data network traffic. 2 SNA Over Frame Relay The term Systems Network Architecture and the SNA acronym refer both to the formally-defined formats and protocols, and to the various products that implement them. From its humble beginnings supporting video display terminals and banking devices, SNA has evolved and grown to include a wide range of network technologies, data streams, application types, processors, and user devices. Recently, SNA has been required to coexist with, and interconnect to a variety of other protocol environments such as TCP/IP and Novell s IPX. The terminals originally supported by SNA (the IBM 3270 family) are used mainly for realtime interactive dialogues involving human operators. Fast, consistent response times and high network up-time are fundamental to user satisfaction. SNA networks are usually deemed to be mission critical by their owners; hence, cost-effectiveness, ease-of-use, scalability, and adaptability are critical success factors. In short, users expect SNA to be fully capable of exceptional performance in the most demanding user environments. SNA s predictable and consistent performance is achieved as a result of its rich protocol suite. An important part of its operation is the manner in which it serves remote terminals with timed polls over dedicated transmission paths. This results in assured arrival or failure notification. This makes SNA a good transport for critical business applications, where transactions must either succeed or fail with certainty. The SNA architecture, although more stable and long-lived than its individual products, has also changed over the years. Today s network-centric, peer-to-peer SNA is dramatically different from the original hostoriented and controlled version. Four distinct stages in the transformation of SNA can be identified: 1. Host-based, hierarchical, interactive sessions over dedicated circuits with proprietary components (now called subarea SNA). Technology Guide 3

4 2. Multiple hosts, mesh-connected front end processors, a wider variety of user devices, multiple suppliers, and the use of shared networks (LANs and X.25 networks), but still host-oriented and controlled (Multi-Systems Networking). 3. Peer-to-peer networking combined with distributed network control and intelligent end systems, various underlying network types, and distributed control, but involving an essentially homogeneous implementation (i.e., IBM proprietary). 4. Peer-to-peer networking in a multiprotocol, multiservice, multivendor environment using both dedicated and shared networks (based on the IBM Open Blueprint for Networking). Today, a large percentage of commercial networks remain at stage 2 in the evolution. One of the major transformational changes that IBM has undertaken in the 1990s has been the positioning of SNA and APPN as open technologies. As a result, this highly critical and important tool, used by many businesses to support mission critical applications, has become much more standard based, with a high degree of interoperability with other standard based systems. SNA Issues and Alternatives As we examine the potential of Frame Relay as a vehicle to support SNA, we must consider support for: SNA using SDLC link layer protocols (point-topoint and multipoint), which are normally used with wide area circuits. Much traditional SNA service continues to operate on multi-point leased line circuits (a single host connection with multiple attached terminals on a bridged, leased circuit). SNA using LLC protocols in a shared LAN environment. 4 SNA Over Frame Relay SNA Polling A major issue to be considered in supporting SNA over Frame Relay is polling (as well as other protocol overhead). In subarea SNA, the host NCP controls activities by polling the user devices for input data. Polls are associated with system timers that depend on relatively consistent end to end network timing intervals in order to operate successfully. For example, for some SNA users, it is typical to set a polling time-out value to two seconds, which means that if there is no response to a poll within that time, the system begins a time-out recovery process. If valid responses arrive after the time out value expires, it could result in a thrashing condition in which error recovery procedures contribute to a system breakdown. Difficulties of this type could theoretically arise using Frame Relay if there are network conditions which periodically cause delays beyond those two seconds. These delays could be caused by Frame Relay network congestion or inappropriate discard rules for data frames. These potential difficulties are compounded by the natural variability in response times produced by congestion in shared networks, which is often associated with traffic queuing delays. Polling also increases the overall number of messages to be transferred across the network. Since polling messages are delivered to the network as if they were ordinary traffic, they could compete for network services in a way that exhausts the service resource. Furthermore, the pricing of some public Frame Relay services is volume sensitive, which could make polling protocols expensive to support. Technology Guide 5

5 Frame Relay An Overview Frame Relay Network The term Frame Relay refers to a set of standards and conforming products that improve upon the popular X.25-based interface to packet switching networks. Frame Relay standards, by assuming the availability of high quality physical links, can eliminate some of the error detection and recovery processing that is essential in an unreliable environment. This higher quality is generally available in the fiber optic media used in North America but is not yet available universally. A Frame Relay Network (FRN), a member of what is called the fast packet family of networks, is a wide area communications system that allows multiple users to share, in an effective manner, a set of common transmission resources. FRNs are available as public or private networks, with access speeds up to 45 Mbps. Frame Relay standards cover three areas: the network services provided, the user/network interface, and the signaling functions supported. At the core of all Frame Relay services is the data link connection (DLC), which is a form of virtual circuit connecting the source and destination users. DLCs are full-duplex, can be point-to-point or multicast, and can be permanent (a PVC) or switched (an SVC). Each DLC has identifying numbers, one at each endpoint, called Data Link Connection Identifiers (DLCIs). The DLCI is contained in the Address Field of the frame. 1 Physical Channel Multiple DLCIs FRN services are based on the exchange of frames between end users via intermediate switching nodes, with error-processing performed at the endpoints (rather than hop-by-hop as in X.25). A frame is a variable-length protocol message that is positioned in Layer 2 of the OSI Reference Model (see diagram below). The standards for frame relay define the interface protocols for accessing a frame relay network, not the protocols used within the network. X Multiple DLCIs 1 PVC Network Layer Data Link Layer Physical Layer Core 1 Physical Channel Multiple DLCIs Frame Relay Committed Information Rate (CIR) One of the key requirements of any shared network is to assure the independence of the data streams of different users (i.e., to prevent any interference of one user by another) and to prevent one traffic stream from constraining others (e.g. a file transfer shutting out terminal access). One of the unique features of Frame Relay standards is the provision of a rate enforcement procedure which provides the user with a committed 6 SNA Over Frame Relay Technology Guide 7

6 information rate for a PVC. This provides the ability to pre-assign specific bandwidth allocations for virtual circuits and according to certain conditions. These conditions in FRNs are the characterizations of the traffic in terms of importance, contracted volume, and burstiness. FRN users need to develop a service level agreement with their service provider to describe both traffic characteristics and the actions to be taken when certain limits are reached. The following parameters are used to define the traffic: The Committed Information Rate (CIR) is the rate at which the service provider guarantees to transfer user data under normal conditions. The Committed Burst Size (Bc) is the maximum number of bits, during a given time interval T, that will be accepted under normal conditions. The Excess Information Rate (EIR) is the rate above the CIR at which the service provider agrees to transfer data under best effort conditions. The Excess Burst Size (Be) is the maximum number of uncommitted bits above Bc, during time interval T, that will be accepted. The FRN guarantees that all committed information will be delivered as required. Any additional information is transported by the network with no guarantee that discarding will not occur. The FRN sets a bit in the frame control overhead called the Discard Eligibility (DE) bit, which allows a congested node to drop the frame if necessary. The source may, if it so chooses, set the Discard Eligibility bit voluntarily. Any traffic exceeding the Be limit can either be refused outright by the network or forwarded with no guarantees of delivery. When Frame Relay Applies The following is a characterization of network situations where a Frame Relay solution is most suitable: When traffic patterns fluctuate randomly (a generally continuous traffic flow, with some burstiness.) This is typical of an SNA terminal environment in which a large number of devices compete randomly for the SNA service. When the telecommunications facility (the telephone line) is of high quality and line errors are unlikely to occur. When the traffic pattern is between a finite set of end points. For example, where there are centralized hosts or servers supporting a distributed population of terminal users. When management support and simplification is a priority. Frame Relay reduces the number of ports required, through consolidation of physical networks and the use of intelligent Frame Relay Access Devices (FRADs). When security is a priority. When traffic is aggregated onto a Frame Relay service, the virtual private network is not visible or accessible to outsiders. When communication is needed for business associates outside of the corporate enterprise, it can be accommodated by establishing a Frame Relay service connection to them. This is a minimal cost connection, used only when there is traffic, which uses the same process that is used for internal communications. 8 SNA Over Frame Relay Technology Guide 9

7 The Value of Frame Relay At its most straightforward, Frame Relay provides a virtually transparent method for sending packets of data across a wide area network on an as-needed basis without requiring dedicated, point to point leased lines. When priced right, this option is a tremendous addition to corporate network alternatives. It can expand the reach of the corporate network to include all of its business partners and associations. It reduces the need for large numbers of dedicated leased lines. It provides bandwidth flexibility to absorb occasional bursts of traffic beyond the committed rates. It is generally less expensive than leased line alternatives. The downside of Frame Relay, however, is the potential for erratic performance and lost data. This potential is realized when the Frame Relay network is poorly designed and when the Frame Relay switches deployed in the network do not properly manage the traffic flow. If the Frame Relay network is properly designed, and if the switches do properly manage traffic, then Frame Relay is a superb solution for virtually all corporate data traffic requirements. The answer is yes. Frame Relay can support SNA quite successfully. There are, however, a variety of ways in which SNA can be integrated into Frame Relay. Some are more successful than others. In addition, the various Frame Relay switch manufacturers have implemented a variety of additional features, over and above the fundamental capabilities of the various integration methods. These provide varying degrees of added value for the transport of SNA traffic over a Frame Relay network. Furthermore, Frame Relay allows the simultaneous support of SNA, IP, and other protocol based traffic to share a single, protocol independent, network. IBM s Position IBM has designated Frame Relay as a strategic network technology and as a migration path from legacy SNA environments to the high speed core ATM networks planned for the future. In 1995, IBM delivered Front End Processor software capabilities to allow users to integrate SNA and LAN traffic over FRNs. IBM is able to support multiple downstream Physical Units (PUs) over a single Frame Relay link to a front end processor, as illustrated below. Frame Relay Network Frame Relay Support for SNA One important issue today is the question of whether or not Frame Relay can adequately support SNA/SDLC traffic so that business can take advantage of the flexibility and cost advantage provided by Frame Relay, while still protecting the corporate investment in legacy systems and applications. 10 SNA Over Frame Relay Front End Processor Virtual Multidrop Circuit Technology Guide 11

8 Frame Relay/SNA Methods In its simplest (but most expensive) SDLC implementation, all of the native SNA traffic, every poll, protocol message, and data packet, is simply packaged within separate Frame Relay frames in the data stream. Each remote concentrator, router, or controller communicates over its own individual PVC, and each PVC is given its own CIR. This approach is expensive because of the high volume of traffic represented by all of the polling and protocol overhead on the network. It is also most likely to have performance problems if the backbone network is under engineered, or performs poorly. However, if designed properly, this method will work to support SNA with the least amount of switch and CPU overhead. This takes more network overhead, but has less switch and CPU overhead. Several other techniques have been devised for supporting SNA over a FRN: SNA/SDLC is encapsulated directly into TCP/IP and transported over the FRN. This approach is not widely used because of the continued need to support the SDLC protocol demands, while overlaying it on to the protocol demands of TCP/IP and then integrating it with the Frame Relay protocol. This arrangement becomes unwieldy and difficult to manage. SNA/SDLC is converted to SNA/LLC, then encapsulated in TCP/IP and transported over the FRN. In this approach, the LLC2 (Logical Link Control Type 2) protocol replaces SDLC. The SDLC session is terminated, which reduces much of the protocol polling that has to be supported by the wide area network. This approach has merit, but again, there is the difficulty of maintaining several levels of protocol, with all of it s potential for conflict. The benefit is that the risk of SDLC time-outs is lessened. 12 SNA Over Frame Relay SNA/SDLC is converted to SNA/LLC2, then directly encapsulated onto a FRN based on the encapsulation techniques described in RFC This approach reduces the SDLC protocol overhead while offering an encapsulation method designed specifically for the Frame Relay interface. Data Link Switching (DLSw) is used for routing of SNA traffic, and also involves the use of TCP/IP, but as an external transport after the SDLC protocol issues have been dealt with. DLSw and RFC 1490 are considered to be the two contenders for supporting SNA over Frame Relay and are described in more detail below. RFC 1490 Encapsulation RFC 1490 defines a format for transporting SNA directly over a FRN. It is a sophisticated, native encapsulation technique for Frame Relay and is an efficient and powerful technique for a FRN backbone. RFC 1490 provides a highly optimized, very low overhead, cost-effective means for transporting both subarea SNA and APPN traffic. Since Frame Relay itself does not provide the specific transport mechanisms required by SNA, the RFC 1490 support for SNA uses LLC2 as part of the encapsulation to provide link-level sequencing, acknowledgments, and flow control. RFC 1490 does not require the additional protocol overhead and nodal processing for TCP used by other methods and hence is inherently more scalable. Bandwidth alone becomes the main limiting factor in adding more SNA sessions to a FRN. In general, the demands of even very large SNA networks can be met by simply increasing the numbers of PVCs and access links. Technology Guide 13

9 RFC 1490 is a framework for implementation but does not ensure interoperability in and of itself. It is essential that the vendors who elect to implement RFC 1490 have the capabilities and unique knowledge needed to assure system compatibility. DLSw over Frame Relay Data Link Switching (DLSw) (RFC 1795), originally developed by IBM itself, has since been adopted as an industry standard by the members of the APPN Implementors Workshop. DLSw is a transport method for SNA packets which intercepts the DLC layer, placing SNA packets into TCP/IP headers. As such, it can be overlaid onto a Frame Relay network. DLSw provides virtual pointto-point connections between pairs of Data link Layer MAC addresses. The DLSw specification deals with the encapsulation of SNA, APPN and NetBios in TCP/IP, which can then be transported over Frame Relay links. One of its key functions is to terminate the DLC session at either end of the connection and eliminate the protocol overhead and polling from occupying the network. DLSw provides for encapsulation in IP at Layer 3, with local acknowledgment of polls. This can reduce network traffic considerably over SNA/SDLC encapsulated and transported over Frame Relay. This also has the effect of virtually eliminating the SDLC time out problem. The difficulty is that it adds a significant burden of overhead to the entire network, as well as potentially expensive DLSw devices. The benefit is that since DLSw eliminates the SDLC protocol overhead and timing problems from the network, any problems associated with the Frame Relay network will not have the catastrophic results that it would have on a purely SNA encapsulation implementation. 14 SNA Over Frame Relay DLSw has the ability to locate SNA, APPN, or NetBios destinations using the MAC address of the destination. SNA traffic can be given priority over other protocols in use in the network, which also helps to avoid time-sensitivity issues. However, because it adds latency due to TCP/IP processing, DLSw should be seriously considered only if the TCP/IP stack is needed to support other (non-sna) applications. DLSw requires extra overhead and puts extra processing pressure on the routers. It also requires an LLC or SDLC session between each end station and the DLSw node, which may limit scalability. Additional Considerations for Implementing SNA Over Frame Relay The protocol conversions and mappings, as described above, and which are used for the successful handling of SNA traffic are critical elements but are not the only consideration. In addition to the question of which fundamental approach is best, there is a separate important consideration. This has to do with the variety of vendor specific capabilities and features built into the switches used in the network, which are not defined by the standards but which are nevertheless used to implement the standards. Some critical switch features that are used to build the network infrastructure for SNA include: The ability to prioritize PVCs by type. In first class switches, each PVC can be assigned a priority based on user-defined criteria, thereby allowing delay-sensitive traffic to be given priority over all other traffic. The ability to assure a high reliability for traffic delivery. This is achieved throughout the entire network. It is accomplished through a robust implementation of the rules governing the use of the Discard Eligibility mechanism, in which the traffic can be marked as not discardable. Technology Guide 15

10 The ability to define Fault Tolerant PVCs. This enables the maintenance of mission-critical operations without disruption of services even in the event of a disastrous network outage. This capability should be provided without requiring the user to implement backup connections for every site. A logical port is defined as a backup for each designated PVC, and can be invoked as part of a Disaster Recovery Plan. Frame Relay Devices There are two basic Frame Relay devices that need to be considered: Frame Relay Access Devices (FRADs), which are the customer premises devices that provide the interface between the FRN and the user s SNA SDLC networks, and the WAN switches that process the frames while in transit in the network. FRADs A FRAD can be a separate device or FRAD functions can be incorporated into a standard router. The FRAD device usually converts the SDLC formats into LLC2 formats and then packages the LLC2/SDLC traffic into frames for transfer. The same FRAD may also provide other interfaces such as to Binary Synchronous (Bisync) and asynchronous devices. As the technology is becoming more mature, FRADs are becoming richer in function and prices are decreasing. Features may include: SDLC-LLC2 conversion to RFC 1490 routed, bridged, or BAN format. Annex G support. RFC 1490 encapsulation. SLIP to TCP/IP conversion using RFC 1490 encapsulation. SNMP management. Automatic dial backup. BSC3270 support. Frame Relay pass-through. Integral DSU s. Single or dual port units. WAN Switches WAN switches should consist of the hardware and software architected to provide the reliability, availability, serviceability, and scalability that is essential to support mission critical applications over Frame Relay. Many of the features are common to any network node. Others, designed specifically for SNA support are: PVC prioritization that allows a customer to assign a priority level on a per PVC basis, which ensures that latency-sensitive traffic can be identified as it enters the switch and is given priority. Fault tolerant PVCs for disaster recovery scenarios, which define a logical port as the backup for each designated PVC, and which allows an operator to dynamically and transparently move all traffic to the backup port. The selection of the right switch for support of SNA over Frame Relay is a critical component in the equation. Regardless of the approach taken, RFC 1490, DLSw, etc., it is the network switch that will allow that extra level of customization to assure that the network not only satisfactorily supports SNA, but actually provides better service than on multipoint leased line circuits. 16 SNA Over Frame Relay Technology Guide 17

11 Quality of Service in Frame Relay Networks The quality of service (QoS) provided by the network subsystem is always an important network design consideration, but is especially so whenever SNA must be supported. SNA is a connection-oriented, finely tuned system that expects a relatively stable and static underlying network with low end-to-end delay. Therefore, SNA is quite sensitive to unplanned changes in network QoS. A major area of concern, therefore, is the mixing of SNA traffic with connectionless-mode traffic (such as LAN bridging and IP) in which resources may be tied up for long file transfers and other competitive traffic. There are two considerations at play: the quality of each individual virtual circuit (e.g., delay, lost frames, etc.), and the impact of the environment (congestion, resource availability, etc.). The best way to maintain QoS, from the service provider s perspective, is to avoid input overloading. One way to assure this is to ensure a sufficient amount of resources by provisioning the network with enough bandwidth to handle any eventuality. This is not always feasible, so attention has to be paid to congestion issues. The major influencing factor on QoS, after spare capacity, is network congestion, which occurs whenever user demand is greater than the available resources. Congestion can certainly be avoided by reducing demand but this is an impractical idea. So, in terms of congestion, there are three specific areas that are of particular importance: enforcement of the committed information rate (CIR), congestion avoidance, and congestion recovery. CIR rate enforcement is performed by examining each data stream to assess whether it can be discarded in the event of congestion. As discussed earlier, the action to be taken depends on whether the Bc or Be limits have been exceeded (and on the manufacturer s implementation). A Graceful Discard feature, as supplied by key switch providers, allows traffic above the Be breakpoint to be forwarded along the network, under the condition that it is the first to be discarded in the event it reaches a congested node with no other option. Congestion management is performed on the basis of queue lengths, and is achieved by actively monitoring the transmission load of each link. In addition to invoking the discarding rules, the presence of congestion is signaled to adjacent nodes by setting the following bits: The Forward Explicit Congestion Notification (FECN) is set in frames being forwarded to the link. The Backward Explicit Congestion Notification (BECN) is set in frames received over the link (and presumably destined for transmission over some other link). The FECN and BECN bits are presented to the attached user devices which are expected to reduce the offered load (but often do not). The FRN can also act unilaterally to reduce congestion by adaptively controlling the amount of traffic discarded at the source node, usually taking into consideration the user s service level agreement (i.e., a user paying for a high Be gets to send more data than one who doesn t). This forms a closed loop control system, allowing proactive control over traffic without depending on the user to act on FECN/BECN notifications. 18 SNA Over Frame Relay Technology Guide 19

12 Additional SNA Support Enhancements In addition to the above, other techniques can also be useful in the support of SNA: Prioritizing SNA traffic over other LAN protocols. Logically assigning fixed bandwidth amounts to SNA, as implemented in Data Link Switching (RFC 1475). Dual PVCs to separate SNA and non-sna traffic flowing over a FRN. Managing SNA/Frame Relay Combinations Manageability is an important attribute of any network, and this is no less true for Frame Relay. The management facilities of all the various network elements must cooperate to form a comprehensive Network Management System (NMS). The three levels of network management (resource managers, system managers, and service managers) are complex enough even when only a single protocol stack is involved. They can become very unwieldy when multiple environments coexist, multiple services are offered, multiple standards are used, and multiple owners are involved. Any FRN consists of three major components that are manageable: the access devices, the internal network switches, and the access and inter-switch links. Some high quality switches do support multiple services and the network management requirements of each (e.g., Frame Relay and ATM). Access Devices and Frame Relay switches typically include SNMP-based management functionality. Thus, the events and activities related to the FRAD can be integrated into any management system that handles SNMP protocols. This is also true of a router that serves as a Frame Relay switch or as a FRAD. A number of features illustrate the types of functionality involved in managing a FRN: 20 SNA Over Frame Relay a) Trunk Administrative Metrics: The Trunk Administrative Cost Metric allows the user to exercise control over the specific path a virtual circuit will take through the network. This can be used to provide path selection control. It can allow the choice of shorter hops or wider bandwidth, etc. In the case of equally weighted trunks, the minimum-hop path is selected. b) Customer Network Management (CNM): It uses the FRF standard MIB in the management application interface between a public service provider and the customer. With CNM, the service provider gives the customer the ability to access information about the performance and characteristics of their network (i.e., the resources in the network that are used by the customer). The CNM, which is based on industry standards (the Frame Relay Forum standard for CNM, FRF.6), provides an easy way for users to feel comfortable with the ongoing operations of their network after migrating from a private network to a shared public network. Measuring and analyzing QoS statistics is an important element of a successful NMS. Specific measurable information about the QoS achieved should be collected and reported to a network management system. Different types of information including configuration data, error statistics, round-trip delays, etc., will need to be collected to help determine the performance of particular components of the network. These QoS metrics are particularly important when contracted service level agreements are used (as is common in IBM SNA shops). Because NetView commands and alerts are transferred using SNA protocol flows, they are handled by a FRN just as any other traffic. The use of Frame Relay transport will not impact the activities of SNA management systems. Technology Guide 21

13 In a public FRN, the underlying network is part of the network supplier s responsibility rather than the end user s, and need not be directly interconnected to SNA management systems. Switch management services, which are used by the network provider, include configuration management (trunks, circuits, and PVCs), security management (password access), performance management (monitor incoming traffic rates and burst sizes, congestion functions), fault management (test sequences, background diagnostics, alarms, etc.), and remote software upgrades. Providing high quality administrative and operational management is a fundamental part of any SNA network. Additional complications may arise when SNA and Frame Relay management must be combined. Managing a combined SNA/Frame Relay network requires the combination of SNA management (typically using IBM s NetView), control over any FRADs through SNMP, and the CNM information provided by the FRN provider (or, in a private Frame Relay network, the use of SNMP at the switch level). The Benefits of Frame Relay The three key strategies for maximizing the benefits of Frame Relay are to: Replace existing private line circuits with an equivalent set of Frame Relay PVCs, thereby reducing monthly costs. Replace existing private line circuits with Frame Relay PVCs and add more paths (such as more direct PVC connections to reduce the hop count) for the same cost. Replace existing private line circuits with higher rate Frame Relay PVCs (such as upgrading 9.6 circuits with 56Kbps ) for increased performance at the same cost. 22 SNA Over Frame Relay ATM and Frame Relay ATM services, which many users believe to be a better solution than Frame Relay for certain applications such as broadcast video and server farm support, are becoming more readily available. Because of this growing acceptance and availability of ATM, many users see an increasingly important need to provide full interoperability between ATM and Frame Relay. This is important for many reasons, including investment protection for existing Frame Relay systems already in place. Full interoperability also provides users with low cost access to high speed networking power needed to support new, bandwidth intensive, business applications. Frame Relay/ATM interoperability is a standards based migration technology that allows enterprise Frame Relay networks to seamlessly scale up to high speed ATM networks. The technology addresses two problems: the need to scale the backbone beyond the 45 Mbps trunks supported by Frame Relay today, and the need for Frame Relay and ATM users to communicate seamlessly. Interoperability, therefore, provides the means for ATM to be the high speed backbone for Frame Relay and to provide seamless connectivity between the two services. This interoperability is defined within the Frame Relay/ATM Service and Network Inter-networking Implementation Agreements as performed by the Inter-networking Function (IWF). This is located, in most cases, on the switch platform at the boundary of the Frame Relay and ATM networks. Primary responsibilities for services provided by the IWF include the mapping of various parameters or functions between Frame Relay and ATM. Frames or cells are formatted and delimited as appropriate, discard eligibility and cell loss priority are mapped, congestion indications are sent or received appropriately, and DLCI to VPI/VCI mapping is performed. Technology Guide 23

14 It maps Frame Relay UNI Q.922 Core based messages onto ATM UNI AA1.5 Class C based messages. The IWF also supports traffic management by converting ATM and Frame Relay conformance parameters, supporting PVC management inter-networking via status indicators, and providing upper layer protocol encapsulation. Because of these capabilities, Frame Relay users can be assured that there is a secure migration strategy for high bandwidth applications that can use the performance capabilities of ATM. Frame Relay A Timely Solution for SNA Networks Frame Relay use has increased rapidly over the last five years. It is a mature, mainstream solution that has been accepted by a growing number of SNA customers. Frame Relay technologies are well-proven, with standards that are stable and well-tested. Interoperability among vendor products has been extensively demonstrated. Public FRNs are widely available, and most router manufacturers have integrated Frame Relay access functionality into their products. The bottom line is that the technical and operational risks involved in choosing to use Frame Relay to support SNA are now very low. For SNA users whose networks are based on dedicated SDLC links, networking costs can be reduced significantly, especially for remote or lightly used circuits. Frame Relay tariffs are dropping and are increasingly attractive. An additional benefit is reduced port counts, because Frame Relay allows multiple virtual circuits to be multiplexed over a single physical access link, reducing the number of 3745 FEP links required on a typical star-configured SDLC network. This combination of cost savings and network simplification has 24 SNA Over Frame Relay driven many large SNA shops to take a close look at Frame Relay. In situations where increased speeds are necessary, or perhaps even just desirable, Frame Relay can be suitable because it offers speeds from 56 Kbps up to 45 Mbps. For many existing terminal networks, where speeds have typically been restricted to 9600bps, a FRN can provide excellent scalability and increased network performance. Changing to these higher speeds can dramatically improve response times for terminal users. Frame Relay also offers reasonable ways to consolidate SNA with other protocols. FRNs include functions to maintain independence among different data streams. Congestion controls minimize the potential for inter-user interference and contention for resources. Inter-networking with other networks such as ATM (via the Frame User-Network Interface) is also supported, and Frame Relay is being considered a part of the plan for the transition to high speed backbone networks. Frame Relay equipment can be used for both public and private networks, allowing hybrid configurations that can integrate seamlessly. Another important feature of Frame Relay is that it is possible to modify the operating parameters (such as bandwidth allocation to a CIR) without disturbing existing users. In fact, new PVCs can be added to new destinations without additional hardware and without turning down the network. This evolution to dynamic configuration control is becoming essential for high reliability and continuous operation. FRNs have also benefited from advances in switching techniques, physical media, protocol design, and management services. They assume increased processing capabilities at the end nodes and much lower error rates on the media. This results not only in simpler components and higher performance but also adds significantly to network reliability, availability, and serviceability. Technology Guide 25

15 Summary SNA has been an important piece of the networking puzzle in large corporations for over two decades, and will remain a factor for the foreseeable future. Use of Frame Relay to support SNA is a major strategic advance and can be justified on economic, technical, and business grounds. This is demonstrated through many testimonials from successful businesses. Reducing the overall cost of communications through the increased sharing of resources and integrating multiple networks are often important goals which can be met by implementing public or private Frame Relay Networks. Frame Relay has proven to be both a viable alternative to leased lines and an upgrade path for X.25 networks. Frame Relay supports increased quality of service, higher speeds, fewer delays, and greater adaptability. It is fully standardized to allow a mix-and-match of vendors. Frame Relay is also a strategic solution since it is well-positioned to work with emerging ATMbased products and services. Frame Relay solutions have proven to be reliable, robust, cost-effective, and flexible when implemented using suitable products and careful selection of vendor products. Appendix A Frame Relay Details The structure of a frame is illustrated in the diagram on the next page. The initial and final FLAG fields are one octet long and serve as delimiters for the frame. The Frame Check Sequence (FCS), the last two octets of the frame, is used to verify the validity of the frame at the receiving station. The Information Field, the ultimate user payload, is not defined in the Frame Relay standards other than as being an integral number of octets (with the default maximum being 262 octets and the suggested maximum frame length being 1600 octets). Different product implementations vary but most are designed to support LAN frame sizes which can be as high as 18,000 octets. A two octet (extendible) Address Field is used to ensure connectivity among any attached stations. F L A G Address Field DLCI (lsb) DLCI (msb) Information Field BIT FECN BECN The Address Field contains the DLCI addressing information (the most and least significant parts are indicated) and various control bits (C/R = Command/ Response bit, DE = Discard Eligibility bit, F/BECN = Forward and Backward Explicit Congestion bits). The C/R bit indicates whether the frame is a command or a response. The control bits were discussed in the section dealing with quality of service. The DLCI has local significance only, and is often different at each end of the link. Ranges of DLCI values have been assigned to different types of connection: call control (0), permanent virtual circuits ( ), multicast ( ), and local management (1023). Address fields greater than two octets are also possible, and can be used to create a form of global addressing. C/R DE F C S F L A G OCTET 26 SNA Over Frame Relay Technology Guide 27

16 Glossary Advanced Program-to-Program Communications (APPC) Implementation of SNA LU 6.2 sessions that permits personal computers in an SNA network to communicate in real time with the mainframe host and other networks. Asynchronous Transfer Mode (ATM) The CCITT standard for cell relay wherein information for multiple types of services (voice, video, data) is conveyed in small, fixed-size cells. ATM is a connection oriented technology used in both LAN and WAN environments. ATM Adaptation Layer (AAL) Each AAL consists of two sublayers: the segmentation and reassembly (SAR) sublayer and the convergence sublayer. AAL is a set of four standard protocols that translate user traffic from higher layers of the protocol stack into a standard size and format contained in the payload of an ATM cell and return it to its original form at the destination node. AAL 1 addresses CBR (constant bit rate) traffic such as digital voice and video and is used for applications that are sensitive to both cell loss and delay and to emulate conventional leased lines. AAL 2 is used with time-sensitive, variable bit rate traffic such as packetized voice. AAL 3/4 handles bursty connection-oriented traffic, such as variable-rate connectionless traffic, like LAN file transfers. It is designed for traffic that can tolerate some delay but not the loss of a cell. AAL 5 accommodates bursty LAN data traffic with less overhead than AAL 3/4. Available Bit Rate (ABR) A class of service in which the ATM network makes its best effort to meet traffic bit rate requirements. Backward Explicit Congestion Notification (BECN) A bit in the frame relay header. The bit is set by a congested network node in any frame which is traveling in the reverse direction of the congestion. (In frame relay, a node can be congested in one direction of frame flow but not in the other.) Cell For ATM, most vendors have agreed that this information package will be developed consisting of 53 bytes or octets. Of these, the first 5 constitute the header; 48 carry the payload. Cell Delay Variation (CDV) ATM performance parameter which specifies the potential variation (+/-) from the expected average transit delay through the network over a given virtual circuit. Cell Error Ratio (CER) ATM performance parameter which specifies the ratio of errored cells to the total cells transmitted over a given virtual circuit. Channel A communication path. Multiple channels can be multiplexed over a single cable in certain environments. The term is also used to describe the specific path between large mainframe computers and attached peripherals. Circuit Emulation A connection over a virtual circuit-based network providing service to the end users that is indistinguishable from a real, point-to-point, fixed-bandwidth circuit. Congestion Excessive network traffic. Congestion control Network management issue for the controlling of traffic flow so switches and endstations are not overwhelmed with information and cells subsequently dropped. 28 SNA Over Frame Relay Glossary of Terms 29

17 Connection Admission Control (CAC) The function of an ATM network which determines the acceptability of a virtual circuit connection request and determines the route through the network for such connections. Constant Bit Rate (CBR) Delay intensive applications such as video and voice, that must be digitized and represented by a continuous bit stream. CBR traffic requires guaranteed levels of service and throughput. Customer Information Control System (CICS) An IBM application subsystem that permits transactions entered at remote terminals to be processed concurrently by user applications. They are often based on mainframe computers. Customer Premises Equipment (CPE) Terminating equipment, such as terminals, phones, routers, and modems, supplied by the phone company, installed at customer sites, and connected to the phone company network. Data Circuit-terminating Equipment (DCE) Equipment that resides at the customer end of a transmission link and provides all necessary termination functions for that link. May be owned by the customer or by the service provider. Data Flow Control Layer Layer 5 of the SNA architectural model. Data Link Connection Identifier (DLCI) A value in frame relay that identifies a logical connection. Data Link Control (DLC) The SNA or OSI layer responsible for transmission of data between two nodes over a physical link. Data Link Control Layer Layer 2 in the SNA or OSI architectural model. Data Service Unit Device on the customer end of a digital circuit that provides framing of sub-rate (under 64 Kbps) customer access channel(s) onto higher rate data circuits. May be combined with a CSU in a single device. Data Terminal Equipment (DTE) The part of a data station that serves as a data source, destination, or both, and that provides for the data communications control function according to protocol. DTE includes computers, protocol translators, and multiplexers. Digital Signal 0 (DS-0) North American Digital Hierarchy signaling standard for transmission at 64 Kbps. Digital Signal 1 (DS-1) North American Digital Hierarchy signaling standard for transmissions at Mbps. Supports 24 simultaneous DS-O signals. Term often used interchangeably with T-1, although DS-1 signals may be exchanged over other transmission systems. Digital Signal 3 (DS-3) North American Digital Hierarchy signaling standard for transmission at Mbps. Supports 28 simultaneous DS-1 signals. Discard Eligible A 1-bit field in a frame relay header that provides a two level priority indicator. Used to bias discard of frames in the event of congestion toward lower priority frames. Similar to the CLP bit in ATM. DLSw Data Link Switching. DLU/S Dependent LU Server. E1 The term for a digital facility used for transmitting data over a telephone network at Mbps. The European equivalent of T1. E3 The highest transmission rate generally available in the European digital infrastructure (34 Mbps). 30 SNA Over Frame Relay Glossary of Terms 31

18 End Node (EN) In APPN, a node that can be source or target, but does not provide any routing or other services to any other node. Flow Control A technique for ensuring that a transmitting entity does not overwhelm a receiving entity. In IBM networks, this technique is called pacing. Forward Explicit Congestion Notification (FECN) A bit in the frame relay header. The bit is set by a congested network node in any frame which is traveling in the same direction as the congestion. (In frame relay, a node can be congested in one direction of frame flow but not in the other). Forwarding The process of sending a frame toward its ultimate destination by an internetworking device. Fractional T-1 A WAN communications service that provides the user with some portion of a T1 circuit which has been divided into 24 separate 64 Kbps channels. Fractional E-1 is in Europe. Frame A logical grouping of information sent as a link- layer unit over a transmission medium. The terms packet, datagram, segment, and message are also used to describe logical information groupings at various layers of the OSI reference model and in various technology circles. Frame Relay High-performance interface for packet-switching networks. Considered more efficient than X.25 which it is expected to replace. Frame relay technology can handle bursty communications that have rapidly changing bandwidth requirements. Frame Relay Forum A voluntary organization composed of Frame Relay vendors, manufacturers, service providers, research organizations, and users. Similar in purpose to the ATM Forum. Gigabits Per Second (Gbps) Billion bits per second. A measure of transmission speed. High Speed Serial Interface (HSSI) Standard for a serial interface at high speeds (64 Kbps and higher up to 52 Mbps) between DTE and DCE equipment over very short distances. Used for the physical connection between a router and a DSU. Integrated Services Digital Network (ISDN) The recommendation published by CCITT for private or public digital telephone networks where binary data, such as graphics and digitized voice and data transmission, pass over the same digital network that carries most telephone transmissions today. Internet Protocol (IP) A Layer 3 (network layer) protocol that contains addressing information and some control information that allows packets to be routed. Documented in RFC 791. Internetworking General term used to refer to the industry that has arisen around the problem of connecting networks together. The term can refer to products, procedures, and technologies. Latency The delay between the time a device receives a frame and the frame is forwarded out of the destination port. Local Management Interface (LMI) ITU-TSS definition for the interface between a user of an ATM network and the network management services of that network. Currently, this interface is for future study. Logical Link Control (LLC) IEEE-defined sub layer of the OSI link layer. LLC handles error control, flow control, and framing. The most prevalent LLC protocol is IEEE 802.2, which includes both connectionless and connection-oriented variants. Logical Link Control, type 2 (LLC2) A connection-oriented OSI logical link control sub layer protocol. 32 SNA Over Frame Relay Glossary of Terms 33

19 Logical Unit (LU) One end of a communication session in an SNA network. LU 6.2 provides peer-topeer communications over an SNA network. Logical Unit Type 6.2. (LU 6.2) The architectural base for APPC communications. An LU 6.2 supports sessions between applications in a distributed data processing environment. Mainframe Mainframe is the name for very large computer systems, typically computer systems which can service from 100 to several thousand users. For very large computer systems the word Host is often used. Management Information Base (MIB) A database of information on managed objects that can be accessed via network management protocols such as SNMP and CMIP. NCP Network Control Program. Network Accessible Unit (NAU) An NAU is the origin or destination of a block of information transmitted through the network. It can be a Logical Unit (LU), a Control Point (CP), a Physical Unit (PU), or a Systems Services Control Point (SSCP). The term NAU was previously used as an abbreviation for Network Addressable Unit, but this has changed with APPN as NAUs are now represented by names rather than by addresses. Network to Network Interface (NNI) Standard for communications between two Frame Relay networks. NMVT Network Management Vector Transport. Payload That portion of a frame or cell that carries user traffic, that is, the frame or cell exclusive of any headers or trailers. Peak Cell Rate (PCR) Parameter defined by the ATM forum for ATM traffic management. Permanent Virtual Circuit (PVC) A defined virtual link with fixed end-points that are set-up by the network manager. A single virtual path may support multiple PVCs. Point-to-Point Protocol (PPP) Successor to SLIP. Provides router-to-router and host-to-network connections over both synchronous and asynchronous circuits. Polling A method of controlling the sequence of transmission by devices on a multipoint line by requiring each device to wait until the controlling processor requests it to transmit. Primary LU (PLU) The LU initiating a session with a Secondary LU (SLU) by sending a BIND. Private Network-to-Network Interface (PNNI) A routing protocol that allows multiple vendors ATM switches to be integrated. It automatically and dynamically distributes routing information, enabling any switch to determine a path within the network. PU Physical Unit. Quality Of Service (QOS) Term for the set of parameters and their values which determine the performance of a given virtual circuit. Reassembly The putting back together of an IP datagram at the destination after it has been fragmented either at the source or at an intermediate node. Response Time The Response Time is measured as the elapsed time between the end of an inquiry or demand on a computer system and the beginning of the response. For example, the length of time between a user typing enter at the end of an inquiry and the display of the first character of the response at the user s terminal is response time. RU Request/Response Unit. 34 SNA Over Frame Relay Glossary of Terms 35

20 SAP Service Access Point. Secondary LU (SLU) An LU receiving a BIND from a primary LU initiating a session between the two LUs. SNA Systems Network Architecture. SNA Network Interconnection (SNI) An architecture for interconnecting separately maintained SNA Networks (Subnetworks). SSCP Systems Services Control Point. Sustainable Cell Rate (SCR) Maximum throughput bursty traffic can achieve within a given virtual circuit without cell loss. Switch In the context of Frame or LAN switching, this refers to a device which filters, forwards, and floods frames based on the frames destination address. The switch learns the addresses associated with each switch port and builds tables based on this information to be used for the switching decision. Some switches are high speed implementations of bridges where switching decisions are made in silicon, usually an Application Specific Integrated Circuit (ASIC). Switched 56 Switched data transmission service at 56 Kbps (as opposed to service on dedicated leased lines). Switched Multimegabit Data Service (SMDS) High-speed, packet switched, connectionless LAN service. Switched Virtual Circuit (SVC) A virtual link, with variable end-points, established through an ATM or Frame Relay network. With an SVC, the user defines the end-points when the call is initiated that are subsequently terminated at the end of the call. With a PVC, the end-points are predefined by the network manager. A single virtual path may support multiple SVCs. Synchronous Data Link Control (SDLC) A protocol for synchronous, code-transparent, serialby-bit data transfer over a link connection. T1 Digital transmission facility operating with a nominal bandwidth of 1.544Mbps. T3 Digital transmission facility operating at 45Mbps bandwidth. TCP/IP Transmission Control Protocol/Internet Protocol. TH Transmission Header. Time Out A Time out occurs, for example, if unit A in the network in a time limited period is expecting an answer from unit B in the network, and this answer does not arrive. A might then assume that B is no longer active in the network and close down the session. Token Ring As defined in IEEE 802.5, a communications method that uses a token to control access to the LAN. The difference between a token bus and a token ring is that a token ring LAN does not use a master controller to control the token. Instead, each computer knows the address of the computer that should receive the token next. When a computer with the token has nothing to transmit, it passes the token to the next computer in line. Traffic Shaping Allows the sending station to specify the priority and throughput of information going into the ATM network and subsequently monitor information progress to meet required service level. Transmission Control Protocol (TCP) A reliable, full duplex, connection-oriented end to end transport protocol running on top of IP. Transmission Control Protocol/Internet Protocol (TCP/IP) The common name for the suite of protocols developed by the U.S. Department of 36 SNA Over Frame Relay Glossary of Terms 37

21 Defense in the 1970s to support the construction of world-wide internetworks. TCP and IP are the two best-known protocols in the suite. TCP corresponds to Layer 4 (the transport layer) of the OSI reference model. It provides reliable transmission of data. IP corresponds to layer 3 (the network layer) of the OSI reference model and provides connectionless datagram service. Transmission Group (TG) In APPN, a Transmission Group is synonymous with a link. In SNA subarea, a Transmission Group may comprise several links in parallel between adjacent nodes, appearing as a single logical link. User Network Interface (UNI) The protocol to define connections between ATM or Frame Relay endstations and the ATM switch including signaling, cell structure, addressing, traffic management, and adaptation layers. Virtual Circuit (VC) A portion of a virtual path or a virtual channel used to establish a virtual connection between two end nodes. Virtual Private Network (VPN) A network service offered by public carriers in which the customer is provided a network that in many ways appears as if it is a private network (customer-unique addressing, network management capabilities, dynamic reconfiguration, etc.) but which, in fact, is provided over the carrier s public network facilities. VTAM Virtual Telecommunications Access Method. Wide Area Network (WAN) A network which encompasses interconnectivity between devices over a wide geographic area. Such networks would require public rights-of-way and operate over long distances. XID Exchange Identification. NOTES 38 SNA Over Frame Relay Notes 39

22 Visit ATG s Web Site to read, download, and print all the Technology Guides in this series. The significant problems we face cannot be solved by the same level of thinking that created them. Albert Einstein