How To Make A Multilink Ame Relay Work Faster And More Efficiently

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1 1/16/01 - ick Franks / Tiara etworks nc. / [email protected] / Multilink Frame Relay Prior to the advent of the multilink ame relay () protocol, some ame relay users were experiencing broadband application limits imposed by circuits. These users were approaching the -T3/E3 bandwidth and price gap and were without an affordable, cost-effective option to acquire more bandwidth. Consider the example of a headquarters office collaborating with its branch office. sing a network connection, the two offices could run a video conference application while users in each location download information om the nternet, send and receive , or manage a user's forum on the company website. Eventually the bandwidth in the connection runs out and workflow and productivity are seriously impacted. This example is typical of what is happening in enterprise networks today. Broadband applications, including transferring large files, streaming media, intranets, and extranets have become standard business practice. The fact is that more and more business is being conducted over networks. Bandwidth is in high demand as businesses move into real-time operations and adopt the nternet as a mission-critical tool. While the number of applications running over networks has grown, as has the number of users, bandwidth to the WA has not kept pace. Multilink ame relay was developed to address the need for more access bandwidth to support the ever-increasing number of broadband applications. allows ame relay customers to realize costeffective gains in bandwidth using inexpensive circuits without significant change to their network inastructure. Prior to, service providers resorted to the use of inverse multiplexers to satisfy customers needs for access bandwidth between and T3/E3. This, however, required the use of additional hardware at the customer site and at the carrier s central office. What is? is a packet transport protocol based on the adoption of the Frame Relay Forum s End-to-End (FRF.15) and / (FRF.16) mplementation Agreements. t provides physical interface emulation for ame relay devices by aggregating multiple physical circuits to transport data; however, the aggregation of physical circuits appears as one logical data pipe. While the principles of can apply to bit rates such as xds0 or even xt3, x is garnering the major share of interest because of the cost or unavailability of high-speed circuits such as T3 and E3.

2 Bundling, a technique that combines physical interfaces and/or circuits into one or more logical data pipes, enables access to support more total bandwidth than is available on any single physical interface. Bundling provides the benefits of the combined bandwidth of the aggregated physical interfaces, but with substantially less latency. Figure 1 shows the improved bandwidth and reduced latency that can be attained using a logical pipe consisting of bundled circuits transporting agments under the multilink ame relay protocol. Figure 1: Single T1 vs. an Bundle 1 1.5/2 Mbps latency = x 4 bundle 6/8 Mbps latency = x/4 + negligible overhead Greater service resilience is also provided because the bundle continues to function even when individual links within the bundle fail; the bundle automatically throttles down its bandwidth to the aggregate bandwidth of the remaining active links. n the example shown in Figure 1, the failure of one T1 circuit would result in the bundle throttling down to 4.5 Mbps, thereby keeping the ame relay interface up and the data flowing. When the unavailable link comes back up, it is automatically added back to the bundle, restoring the original bandwidth of 6 Mbps. / (FRF.16) Frame relay and operate in two sub-layers within Layer 2 of the OS model. n the higher sublayer, a ame relay ame is transported over a permanent virtual circuit (PVC). n the lower sublayer, the ame is distributed either in its entirety (Figure 2a) or in agments (Figure 2b) across the T1 or E1 links at the Physical Layer (Layer 1). ames or agments are transported over a T1 or E1 link to the DCE at the Central Office (CO) where agments are resequenced and reassembled in the correct order.

3 Figure 2a: Multilink Frame Relay / (FRF.16) Frame Transport Variable-delay circuits Outgoing Frames Data distributed ame-by-ame to T1 or E1 lines. ncoming Frames Frames are resequenced in proper order. Frame Relay etwork The sequence number in the agmentation controls resequencing. Despite use of the agmentation to resequence ames sent across an link, agmentation is not required by the specification. n many instances, algorithms deployed by vendors optimize the transport of whole ames so that any delays that may occur are minimized. For example, agmenting large ames will certainly lead to lower latency and increased throughput. However, smaller ames are transmitted as whole ame relay ames to avoid agmentation/reassembly overhead. Should the option of agmentation be deployed, however, both ends of the link (the and the DCE) must use the specifications for agmentation used in the Frame Relay Forum FRF.12 mplementation Agreement. Fragmentation tends to add a small amount of additional overhead to the. n the example in Figure 2b, the incoming ame relay ame is broken into three agments. (serconfigurable thresholds determine the number and the size of agments.) The first agment contains a copy of the original ame relay. Each agment contains an (used to reassemble the agments in the correct order by the receiving endpoint), a duplicate of the original FR (which preserves the characteristics of the original ame relay ame), and a agment. The contains sequence numbers and beginning and ending bits. The first sets its beginning bit to one and ending bit to zero, and it establishes a sequence number for the related packets. nterim agments set their beginning and ending bits to zero (to indicate an association with the data stream) and increment their sequence numbers. The last sets its beginning bit to zero and its ending bit to one (to indicate that it is the last agment in the data stream), and it also increments its sequence number.

4 Figure 2b: / Fragmentation (FRF.16) ncoming ame relay ame Outgoing agments ag 1 #1 "beginning" bit is set to 1. "ending" bit is set to 0. ag 2 #2 "beginning" and "ending" bits are set to 0. ag 3 #3 "beginning" bit is set to 0. "ending" bit is set to 1. The ser-to-etwork nterface () transports data between the (data terminal equipment) and the central office DCE (connects users to the ame relay network) residing on the edge of the ame relay network. The etwork-to-etwork nterface () connects any two ame relay networks. Figure 3 shows an example of an application using both and interfaces. The devices at remote user sites send ames or encapsulated agments to the DCE at the local Central Office (DCE) over copper T1 or E1 and x lines (/). At the DCE, the ames are correctly resequenced into their original order (if agmentation is being used, agments are reassembled into ame relay packets). The ames are then forwarded om the DCE to a switch in the network that has an to the second ame relay network. The ames are forwarded across the to the second network and then to the access switch providing service to the CPE over or circuits.

5 Figure 3: Multilink Frame Relay /, /, and / Connections DCE 3 4.5/6 Mbps 3 4.5/6 Mbps DCE ode 2 T3/E3 90 Mbps ode 1 or DS0 DCE n terms of how they transport ames, and connections operate in exactly the same way. Differences between and connections occur in the LM (Local Management nterface) layer, which will not be addressed in this document. FRF.16 (/ ) is implemented either between and DCE or two directly connected nodes in two different networks. This is vastly different om FRF.15 (End-to-End ) implementation, which is configured between two devices that can have any number of ame relay switches between them. End-to-End (FRF.15) n cases where service providers do not support multilink ame relay (FRF.16), users requiring more bandwidth than can be provided by a single may opt for the End-to-End implementation. Because existing ame switch equipment does not know how to handle agments, the ames or agments must be encapsulated into ame relay ames at the originating and sent across the lines connected to the same or different service providers. Reassembly (if using agments) and resequencing only occur at the destination. n an End-to-End (FRF.15) network, ames may also be sent in their entirety or in agments. Figure 4 shows how a ame relay ame is agmented for processing in an End-to- End circuit.

6 Figure 4: End-to-End Fragmentation (FRF.15) Multiprotocol encapsulation ncoming ame relay ame Outgoing agments FR ag 1 PVC #1 "Beginning" bit is set to 1. "ending" bit is set to 0. FR ag 2 PVC #2 "Beginning" and "ending" bits are set to 0. FR ag 3 PVC #3 "Beginning" bit is set to 0. "ending" bit is set to 1. The incoming ame relay ame is broken into three agments, each containing a ame relay, an (used to reassemble the agments in the correct order by the receiving end), and the. The first agment may contain the multiprotocol encapsulation, which carries bits that provide traffic congestion notification across the network. These bits will be Ored om all the agments at the time of reassembly to get the final congestion status. The contains sequence numbers and beginning and ending bits. As in the / example above, the first sets its beginning bit to one and ending bit to zero, and it establishes a sequence number for the related packets. nterim agments set their beginning and ending bits to zero (to indicate an association with the data stream) and increment their sequence numbers. The last sets it s beginning bit to zero and its ending bit to one, and it also increments its sequence number. ntermediate FR switches only examine the 2-byte ame relay and then transparently pass the data on to the next network device. FRF 15 is transparent to the network.

7 Figure 5 shows an example of how encapsulation is used for End-to-End transmission. The original ame relay ame is agmented and the subsequent agments are encapsulated into two new ame relay ames in preparation for transmission over the network. Figure 5: End-to-End Encapsulation (FRF.15) ncoming ame relay ame agments Outgoing encapsulated agments in ame relay ames Figure 6, shows an example of End-to-End transmission. n this model, the originating requires more bandwidth than a single T1 can provide and ame switches at point-of-presence (POP) locations cannot provide multilink support. Thus, the must use multiple T1s (provided by the same service provider or two different service providers) to satisfy its bandwidth requirements. Figure 6: End-to-End Transmission Customer POP POP Customer DCE #1 DCE #1 ame relay network DCE #2 DCE #2 ame relay traffic The originating encapsulates ames or agments within ame relay ames and then distributes this new ame traffic among the participating subscribers for transport over the ame relay network to the endpoint. Frames may take varying paths enroute to the final destination and may arrive out of order. The at the endpoint strips the ame relay s and uses the sequencing information stored in the s to determine the correct order for reassembly and resequencing of the data.

8 Summary The price gap between and T3/E3 service is substantial, and service is considerably more available than T3/E3 service. As pricing continues to fall, this price gap is widening. As this trend continues, becomes an even more viable option than T3/E3. The affordable and available characteristics of, combined with the benefits of, give service providers the ability to realize new revenue streams by providing flexible, low-cost network services that match individual customer requirements. also provides techniques that support the dynamic addition and removal of lines to change the total bandwidth available to the subscriber. A secondary result is the added resilience of multiple provisioned interfaces, such that when one interface or circuit fails, the emulated interface continues to support the ame relay service. A key value of this protocol is that in combining multiple physical interfaces or virtual circuits, a network operator can design a ame relay interface supporting more bandwidth than is available for any single physical interface or virtual circuit. And this can be accomplished with minimal change to the existing network inastructure. The introduction of ame relay marked a milestone in the data communications industry. t forever changed the way we think about data networks. The ongoing advancement and refinement of this protocol continues to allow ame relay to remain a prominent and popular data-traffic standard.