White Paper
Frame Relay and Frame-Based ATM:
A Comparison of Technologies
Larry Greenstein Nuera Communications
VP, Technology, Frame Relay Forum
June 1995
TABLE OF CONTENTS
1. PREFACE...1
2. INTRODUCTION...1
3. INTERWORKING ATM WITH FRAME-BASED DEVICES ...1
3.1 What’s in a Frame? . . . 1
3.2 Frame Relay Service . . . 3
3.3 FUNI Frame-based ATM . . . 3
3.4 DXI Frame-based ATM. . . 5
3.5 Frame Relay/ATM Interworking . . . 5
3.5.1 The Network Interworking Function ...5
3.5.2 The Service Interworking Function...6
4. CONCLUSION ...8
5. REFERENCES ...8
6. GLOSSARY...8
1. Preface
This white paper describes the basic capabilities and functional differences between frame relay, ATM, frame relay/ATM interworking functions[1,2], the ATM Forum’s Data Exchange Interface[3] (DXI), and the ATM Forum’s Frame-based User-to-Network Interface[4] (FUNI).
The DXI and FUNI are ATM access protocols which take advantage of existing low cost frame-based user equipment.
Thus, the DXI and FUNI allow the user equipment to send traffic in frames (as opposed to cells) and require another piece of equipment to perform the function of segmenting the traffic into ATM cells. The segmenting is performed in a data service unit in the case of the DXI, and in the ATM switch in the case of the FUNI. DXI and FUNI software can run on the same hardware that supports frame relay and X.25. On the surface, DXI and FUNI look a lot like frame relay. But when examined closely, there are many differences.
Those who wish to know in depth technical details are referred to the ATM Forum DXI and FUNI, and the Frame Relay Forum FRF.1 UNI[5], FRF.5 Frame Relay/ATM Network Interworking[1], and FRF.8 Frame Relay/ATM Service Interworking[2] documents.
2. Introduction
ATM is growing in activity and interest at an unprecedented pace. While there are not many networks and users in place when compared to the widely available infrastructure of frame relay, most observers agree that this will change over the next five years.
ATM service is based on switching fixed length packets of data which are known as cells. Cell switching is popular for a variety of reasons, one of which is that switch architectures can be optimized to switch cells at much higher speeds than variable length packets. Another is that multiple services requiring a variety of quality of service guarantees can be provided simultaneously. ATM user traffic must be segmented into cells, transmitted, and then reassembled back into its original form. This process is done in a standardized way and the hardware which provides this interface capability is relatively new and typically more expensive than frame relay hardware.
Frame relay service from public carriers is available worldwide. Its reliability and effectiveness have been demonstrated since the first networks were established in 1992. Estimated service revenue in 1994 was $350 million and was forecast to be over $700 million in 1995[6]. Frame relay evolved from X.25 packet switching and uses variable length frames to transport the user traffic across the interface. Frame relay is popular for a variety of reasons. It is very efficient as it has less overhead and wastes less bandwidth as compared to ATM. Also, many types of existing user equipment (e.g., routers) can be upgraded to frame relay without hardware changes. It is a relatively inexpensive way to interconnect multiple LANs when compared to leased lines, yet it provides good performance for LAN applications. And, it is less process intensive than X.25, resulting in higher network throughput with lower delay than X.25.
3. Interworking ATM with Frame-based Devices
The market for low cost frame relay and frame-based user equipment is expected to continue to grow at a strong pace.
Therefore, interworking with ATM is an important issue.
3. 1 What’s in a Frame?
DXI and FUNI allow frame-based access to an ATM network, while frame relay allows frame-based access to a frame relay network. DXI, FUNI and frame relay have similar frame structures. As shown in Figure 1, the DXI header and FUNI header within the frame are identical to each other, but are different from the frame relay header. Note that the DXI/FUNI frame address bits fall into the same position in the header as the frame relay address (which is composed of the upper and lower DLCI).
CN RSVD CLP 1 0 Header structure of RSVD
DXI and F UNI Frame structure of DXI, F UNI and F rame Rel ay
Frame Address Frame Address
FECN BECN DE 1
0 C/R DLCI lower
DLCI upper
BECN = backward explicit congestion notification CLP = cell loss priority
CN = congestion notification C/R = command / response DE = discard eligibility
DLCI = data link connection identifier FCS = frame check sequence
FECN = forward explicit congestion notification RSVD = reserved
SDU = service data unit Header structure of F rame Rel ay
Flag Header User SDU (user traffic) FCS Flag
Figure 1
Comparison of Frames
When the frame is segmented into cells, the DXI frame address and the FUNI frame address both map to the ATM VPI/VCI (virtual path identifier/virtual connection identifier) using identical procedures. Frame relay interworking functions may use these same address mapping procedures, but the frame relay/ATM interworking function is permitted to use other mappings as well.
C e l l Frame
Frame address bits map to VCI/VPI bits
Flag Header User SDU (user traffic) FCS Flag
Payload
Header Header Payload
Payload Header
Figure 2
FUNI Frame Segmented Into Cells in the ATM Switch
The CN bit performs the same function as the frame relay FECN bit. The network sets this bit during periods of network congestion, in the same direction as the traffic affected by the congestion.
The frame relay BECN bit does not have a similar function in the FUNI or DXI (or ATM in general). BECN is a notification back to a frame relay transmitter indicating that traffic it is sending has encountered congestion. BECN allows the network (as opposed to the destination user equipment) to provide congestion indications directly to the sending user equipment.
FECN and CN, however, rely on the destination user equipment upper layer protocol(s) to participate in congestion management by sending an indication back to the offending user equipment within the upper layer protocol(s). As
interface speed increases, the effectiveness of FECN and CN decreases due to the round trip delay involved in notifying the user of the congestion.
The CLP bit performs the same function as the frame relay DE bit. CLP/DE = 1 indicates a cell/frame which is more likely to be discarded in the event a discard is necessary due to congestion. The frame relay C/R bit is passed transparently between frame relay users. The DXI/FUNI does not have a bit in the header which corresponds to the frame relay C/R bit.
3. 2 Frame Relay Service
Frame relay provides a user with multiple independent data links to one or more destinations. Traffic on these data links is statistically multiplexed to provide efficient use of access lines and network resources. Since the multiplexing is at the link layer, end-to-end delay is minimized. Frame relay networks transfer the user traffic within the frame without regard to its contents, thereby providing service which is effectively as transparent as a leased line.
Frame relay service is commonly available at fractional T1/E1 and full T1/E1 rates. Some vendors offer it at rates up to T3 (45 Mbps).
Frames Frames
Frame Relay
UNI Frame Relay Network
User User
User
User
Figure 3
Frame Relay Service 3. 3 FUNI Frame-based ATM
There are differing views of interworking. One way to interwork is to add software to user equipment which contain HDLC (high level data link control) frame-based protocol interfaces, thus providing a new ATM protocol known as FUNI. The FUNI specification was approved by the ATM Forum in 1995 and implementations may become available in the fourth quarter of 1995. The FUNI requires software in the user equipment and a complementary frame-based interface and FUNI software in the ATM switch to which the user equipment connects. Within the ATM switch interface, the frames are segmented into cells and sent into the network. Cells coming from the network are reassembled into frames and sent to the user. Thus, the cost of the segmentation and reassembly hardware is moved from the user equipment to the ATM switch.
There are two key functional differences between FUNI and DXI. One difference is that FUNI provides improved efficiency of access line bandwidth when compared to the cell based access of DXI. This is because when the frame size increases and overhead stays constant, efficiency increases. Cell based access efficiency stays relatively constant with large payloads and can be extremely inefficient when payloads get very small (i.e., less than 48 bytes). Another difference is that FUNI supports fractional T1/E1 rates and DXI does not. Additional important differences are discussed in the following section.
Customer Premises
ATM Network
Frames Cells
Segmentaion and Reassembly
User
FUNIFigure 4
Frame-based UNI (FUNI)
The FUNI is not intended to provide interoperability between ATM users and frame relay users. Other than having a frame structure similar to frame relay and the ability to operate on the same type of hardware as frame relay, the FUNI has little else in common with frame relay. One cannot simply connect frame relay user equipment to an ATM network’s FUNI and expect it to function.
The FUNI consists of more than just a frame-based interface for transporting user traffic. It includes ATM SVC signaling, a simple network management protocol (SNMP) and management information base (MIB), and optional support of operations, administration, and management (OAM) cells/frames. The FUNI MIB is a subset of the DXI MIB.
The above capabilities are important as they are required for the user equipment to obtain cell-based services.
However, not all ATM services are available over the FUNI. The FUNI mandates the support of AAL5 (in the ATM switch), while AAL3/4 is optional. Services requiring the use of other ATM adaptation layers (AALs) are not supported over the FUNI (e.g., circuit emulation) and support of some ATM quality of service classes (e.g., Available Bit Rate) is not possible.
ATM UNI
Cells Frames
ATM FUNI
ATM Network User
User
Cells
ATM User DXI
CSUFigure 5
FUNI Interoperability
When a FUNI user wishes to establish a switched virtual circuit (SVC) to a destination, it does not matter if the destination is another FUNI, a DXI, or an ATM user-to-network interface (UNI)[7]. To establish an SVC, it uses the same signaling procedures as the ATM UNI. And when the FUNI user sends traffic to the ATM network, it does not matter if the traffic terminates at another FUNI or at an ATM UNI. All of these service aspects are transparent.
Equally transparent are the procedures for layer 3 protocol encapsulation over ATM. Routers which support multiple protocols over ATM follow specific procedures defined in RFC-1483[8] and RFC-1577[9] by the Internet Engineering Task Force (IETF). Since the FUNI is an ATM protocol, the multiprotocol encapsulation procedures used at the ATM UNI (and DXI) are used at the FUNI, therefore no new interoperability issues are introduced.
3. 4 DXI Frame-based ATM
Another way of implementing a function similar to the FUNI is to provide the segmentation and reassembly function in an external piece of equipment and place it at the customer premises. In the case of the DXI, this external piece of equipment is integrated with a Channel Service Unit (CSU). Additionally, the user equipment must be configured with the DXI software and an HDLC interface.
Customer Premises
DXI CSU
ATM Network
Frames Cells
Segmentaion
and
Reassembly
User
Figure 6
Data Exchange Interface (DXI)
Other key differences between the DXI and FUNI are: The DXI supports full T1/E1 access rates, but not fractional T1/E1 rates. Cells traverse the user-to-network access line and therefore bandwidth utilization is less efficient than the FUNI. The DXI also has a defined SNMP management protocol and MIB.
3. 5 Frame Relay/ATM Interworking
Beyond providing a means for frame-based user equipment to access ATM networks at the user-to-network interface, an important consideration is how to interwork frame relay networks with ATM networks, and thereby interwork the network users. This leads to two frame relay/ATM interworking scenarios; Network Interworking and Service Interworking. These two interworking functions provide a means by which the two technologies can interoperate.
Simply stated, Network Interworking provides a transport between two frame relay devices (or entities).
Service Interworking enables an ATM user to transparently interwork with a frame relay user, and neither one knows that the far end uses a different technology.
These topics are covered in depth in the implementation agreements FRF.5 and FRF.8 published by the Frame Relay Forum. These implementation agreements define the frame relay/ATM network and service interworking procedures jointly agreed upon by the Frame Relay Forum and the ATM Forum. These implementation agreements currently support only PVC interworking. SVC support is for further study.
3. 5. 1 The Network Interworking Function
The Network Interworking function facilitates the transparent transport of frame relay user traffic and frame relay PVC signaling (sometimes called LMI protocol) traffic over ATM. This is sometimes referred to as tunneling.
This means that multiprotocol encapsulation (and other higher layer procedures) are transported transparently as they would over leased lines. An important application for this interworking function (IWF) is connecting two frame relay networks over an ATM backbone network.
ATM Network
NNI = network-to-network interface IWF = interworking function
User User
Frame Relay Network
Frame Relay Network
Netwo rk IWF
Frame Relay
NNI
User
Netwo rk IWF
Frame Relay
NNI
Figure 7
Example of Frame Relay/ATM Network Interworking
As shown in the figure above, the ATM network is used in place of a transmission facility (leased line) to connect the two frame relay networks. The Network IWF can be external to the networks as shown, but is more likely to be integrated into the ATM network switch or frame relay switch. Each frame relay PVC can be carried over an ATM PVC, or all of the frame relay PVCs can be multiplexed onto a single ATM PVC. This method of connecting frame relay networks may provide economic savings when compared to leased lines. This is especially true when the frame relay NNI is operating at a low percentage utilization.
Network Interworking also includes a scenario in which an ATM host computer emulates frame relay in the service specific convergence sublayer. For more details, refer to the Network Interworking Implementation Agreement FRF.5[1].
3. 5. 2 The Service Interworking Function
The Service IWF does not transport traffic transparently. It functions more like a protocol converter in that it facilitates communication between dissimilar equipment.
SERVICE IWF SERVICE
IWF
Frame
Relay
UNI ATM
UNI
User User
Frame Relay Network
ATM FUNI
User
ATM DXI
User ATM
Network
Figure 8
Example of Frame Relay/ATM Service Interworking
As shown in the figure above, a frame relay user sends traffic on a PVC through the frame relay network to the Service IWF which then maps it to an ATM PVC. The frame relay PVC address-to-ATM PVC address mapping and other options are configured by the network management system associated with the IWF. Again, the Service IWF can be external to the networks as shown, but is more likely to be integrated into the ATM network switch or frame relay switch. Note that in the case of Service Interworking, there is always one ATM PVC per frame relay PVC.
SERVICE IWF SERVICE
IWF
Flag RFC-1490 FCS
Header Frame
Header User SDU Flag
FECN BECN DE 1
0
C/R Frame Relay Frame
ATM Cell DLCI lower
DLCI upper
Mapping / Conversion
CLP HEC
VCI
VCI
VCI VPI
VPI GFC
PT
Cell Payload (first segment of SDU) RFC-1483 Header
GFC = generic flow control VPI = virtual path identifier VCI = virtual connection identifier PT = payload type
CLP = cell loss priority HEC = header error checksum CLP = cell loss priority
Figure 9
Service IWF Header Function Mapping
Frame relay PVC status signaling is converted to ATM OAM cells. Likewise, OAM cells are converted to frame relay status signaling. Therefore, if a failure occurs in one network, the user of the other network will be notified.
Other indications such as congestion indication and discard eligible/cell loss priority are also mapped between networks per PVC.
The Service IWF maps the frame relay DLCI to the ATM VPI/VCI, the FECN bit maps to the PT field in which congestion indication is encoded, and the DE bit maps to the CLP bit.
The frame relay multiprotocol encapsulation procedures (RFC-1490[10]) are not identical to the ATM multiprotocol encapsulation procedures (RFC-1483[8] and RFC-1577[9]). When providing frame relay/ATM Service Interworking for multiprotocol routers, it is necessary for the IWF to convert the multiprotocol protocol data unit headers from frame relay to ATM and vice versa. This header processing can be turned on or off per PVC as some applications do not require it.
4. Conclusion
Applications of frame relay and ATM technologies overlap. Making a choice between the technologies must be driven by business considerations. This includes consideration of:
• availability of equipment and service from multiple suppliers,
• cost of network and user equipment,
• existing installed base of user and network equipment,
• recurring cost of service,
• cost of managing the network (including complexity and required manpower),
• efficiency of bandwidth utilization,
• service classes provided by the network,
• performance (interface speed, delay, throughput),
• interoperability between vendors.
The above considerations are key elements to the selection of network technology. An informed business decision can be made when the above considerations are properly weighed and evaluated.
Frame relay and ATM each have fundamentally unique characteristics. One cannot provide all the features of the other. Therefore, the use of both technologies will continue to grow to keep pace with the applications for which they are best suited.
5. References
[1] Frame Relay/ATM PVC Network Interworking Implementation Agreement (FRF.5), Frame Relay Forum, December 20, 1994
[2] Frame Relay/ATM PVC Service Interworking Implementation Agreement (FRF.8), Frame Relay Forum, April, 1995
[3] Data Exchange Interface Specification Version 1.0, ATM Forum, August 4, 1993 [4] Frame-based User-to-Network Interface Specification Version 1.0, ATM Forum, 1995 [5] User-to-Network Interface Implementation Agreement (FRF.1), Frame Relay Forum, 1992 [6] Vertical Systems Group, May 1995
[7] ATM User-to-Network Interface Specification Version 3.1, ATM Forum, 1994 [8] Multiprotocol Encapsulation over AAL 5 (RFC-1483), IETF, July, 1993 [9] Classical IP and ARP over ATM, (RFC-1577), IETF January, 1994
[10] Multiprotocol Interconnect over Frame Relay, (RFC-1490), IETF July, 1993
6. Glossary
AAL = ATM adaptation layer
AAL3/4 = ATM adaptation layer optimized for connectionless service over ATM AAL5 = ATM adaptation layer optimized for connection oriented service over ATM ABR = Available Bit Rate
ATM = asynchronous transfer mode
BECN = backward explicit congestion notification CLP = cell loss priority
CN = congestion notification C/R = command / response CSU = channel service unit
DE = discard eligibility
DLCI = data link connection identifier DSU = data service unit
DXI = data exchange interface FCS = frame check sequence
FECN = forward explicit congestion notification FUNI = frame-based user-to-network interface GFC = generic flow control
HDLC = high level data link control (a frame-based protocol) HEC = header error checksum
IETF = Internet engineering task force IWF = interworking function
LAN = local area network
LMI = local management interface Mbps = million bits per second MIB = management information base NNI = network-to-network interface
OAM = operations, administration, and management PT = payload type
PVC = permanent virtual connection QOS = quality of service
RFC = request for comment (a document issued by IETF) RSVD = reserved
SDU = service data unit
SNMP = simple network management protocol SVC = switched virtual circuit
TCP/IP = telecommunications protocol/internet protocol UNI = user-to-network interface
VCI = virtual connection identifier VPI = virtual path identifier X.25 = a packet switching protocol
