Tran:3512 DSU/CSU User`s Manual:Ch7:3512 SDC Oper
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1. Answer DIAL PARAMETERS DDS To Dial Auto Auto From DDS 60 Sec INIT PARAMETERS DDS Alarms On On Idles On On Link Down On On AUTO ANSWER Answer Enable Enable RESTORAL CONFIG Timing Ext P2 Restoral Rate 24 0 24 02. 6 0 MODIFY PORT Pn Intfce V 35 MODIFY NETWORK NC Address 001 002 AUTO ANSWER Answer Enable RESTORAL CONFIG Timing Ext P2 Rest Rate 56 0 3512 SDC Operation 7 11 Configuring SDC Restoral Applications 7 12 3512 SDC Operation A primary line and external restoral line B setup is an A B configuration For details on configuring restoral refer to Chapter 4 Compression on a Restoral Link A digital point to point restoral link using a V FAST modem on Port 2 is shown in Figure 7 4 When the 3512 SDC detects a service interruption on the primary line it can automatically redirect data to Port 2 The devices use an alternate public switched telephone network PSTN line through a modem or a switched 56 device Since the restoral circuit may not support as high a line rate as the primary line you can configure an alternate rate for restoral In this case you might assign reduced bandwidth to less critical applications or drop them altogether to allow critical applications to continue in restoral Port 3 Port 3 256 kbps 256 kbps Unit A 56 kbps Unit B DDS gu 4 Router 3512 SDC f Router Port 2 24 kbps p S 326X FAST 326X FAST b Figure 7 4 Compression Restoral Link Parameters not specified below may be set to factory default values Category Parameter Unit A Unit B MODIFY DSU Data Rate 56 0 56 0 MODIFY NETWORK NC Address 001 002 COMPRESS CONFIG Compress Mode
3. Chapter 7 3512 SDC Operation Contents Dieter TUER TRET 7 2 3512 SDC Configuration and Operation Notes nente 7 2 3512 SIC Appheaton INDIES 5 ceni ede ce pevec ui ed Reis din er Seco NEEE 7 4 jorirw ns c 7 4 Lmk BIZ cusioeose tocco ea C Eoo a Cte Boca ated on Etpe aaa 7 5 AMM EXPaMSUON ives T 7 6 lun pedro eov 7 6 Data Or d P 7 7 Time Division Mul pleximg TDM uiia ice casting vias tetwaserebesuss aerea eh en 7 8 Network Management ipi He pr C abe r 7 8 3512 SIC System Applications Ls cesi eire aru pe CE I Ste vids Etre EAEri INERTE Ipro 7 9 Configuring SDC Primary Applications ierit 7 9 Compression Link Between Two Routers essere 7 9 Compression of Multiplexed Legacy and Router Traffic 7 10 Compression With Central Sit T1 Access eene 7 11 Configuring SDC Restoral Apple ations iecore rtr rte trenes 7 12 Compression on Kestoral Lak paces rience ee eorr ertt rrr geietepail agas 7 12 3512 SDC Operation 7 1 Chapter 7 Introduction This chapter addresses the synchronous data compression SDC feature The chapter provides SDC configuration and operation notes differences between the 3512 and 3512 SDC especially directed to 3512 users who are adding SDC functions Compression notes key concepts and measures of efficiency that you need in order to get maximum compression performance Sampl
4. Rx Receive Link Util parameters indicate the current percentage of Port 3 s line bandwidth being used for user data When there is little data Link Utilization values decrease One would expect this during off peak periods when few users or applications are active but if Link Utilization values are consis tently low during peak traffic periods this indicates that there is unused bandwidth and adjustment of the application environment may be in order The most common cause of low link utilization is a small window size setting in the application protocol The window size is the number of packets a protocol transmits before pausing for an acknowledgment from the receiving device In many environments the default size is 1 which means that the protocol must receive an acknowledgment for each packet before sending the next With this setting the 3512 SDC can provide little if any performance improvement The window size for many protocols may be adjusted to allow multiple packets before acknowledgment The Novell IPX environment is very sensitive to protocol window size The default size is 1 The IPX Burst Mode Network Loadable Module NLM allows the use of a much bigger window size and results in higher throughput NOTE In some situations such as a multi user environment you might prefer to leave the window size small This would cause file transfers or similar functions requiring high bandwidth to be limited by the window size The re
5. col layer or sub layer typically at the physical link or application layer As the measurement point is moved higher in the protocol stack the throughput is lower because more overhead is excluded from the measurement An example of a throughput rate measured at the physical layer is a standard data transparent DSU CSU operating on a 56 kbps DDS line Here throughput is 56 kbps regardless of the actual line utilization The Tx TP and Rx TP parameters indicate the current compressed data transmit and receive throughput measured at the link layer Only HDLC frames are counted as information in the calculation the SDC feature does not distinguish the address length checksum or any other overhead characters from user data All data except flag characters is considered as user data by the compression algorithms You could also measure throughput at the application layer since this excludes all protocol layers overhead and yields the real file transmit rate This would be comparable to using a stopwatch to time the transmission of a user file across a communication link then dividing the file size in bits by the elapsed time in seconds Throughput when measured anywhere above the physical layer is affected by The performance of the application itself The protocol stack overhead window and frame sizes The data s compressibility The communication channel rate Link Utilization The Link Util and
6. duration of the test 3512 SDC Operation 7 3 3512 SDC Application Notes This section provides information that you may find helpful in understanding data compression planning applications and maximizing the 3512 SDC s performance It addresses the following concepts as they concern compression Data throughput Link utilization Anti expansion Flow control Data delay Time division multiplexing TDM Network Management Data Throughput 7 4 3512 SDC Operation Throughput is generally understood to be the rate of information transfer across a communication link The throughput improvement attainable with data compression varies among and within application environments from one moment to the next The characteristics of the data itself strongly affect throughput Some types of data are very compressible others are not compressible at all In a typical user environment the mix of data types varies from moment to moment Overall compressibility in these environments ranges from 2 1 to 4 1 significant improvement when compres sion is added The real key to understanding throughput measurement is understanding exactly what is constitutes information which depends on where within a protocol stack the information is measured Generally information is all data including higher protocol layers overhead passed into the layer where the measurement is taken The information rate can be measured at any proto
7. e system applications descriptions diagrams and parameter settings to address typical requirements most effectively e A sample restoral application a description a diagram and parameter settings to maintain efficient operation during service interruptions Configuring applications requires familiarity with front panel operation refer to Chapter 3 and the menu tree structure 3512 SDC Configuration and Operation Notes The following items apply to the 3512 SDC at all times Single channel operation In this mode Port 3 is the only active port The unit is in a single channel mode when DTE Con x MODIFY main menu MODIFY PORT category is set to BitSteal Normal MPM BitSteal or MPS Bitsteal Channel sharing with compression disabled If channel sharing is configured it must include all three ports Refer to P to CH in Chapter 4 Port 3 DTE interface Port 3 on the 3512 SDC supports only the V 35 interface Initiating restoral DTR controlled restoral initiation is supported only on Port 3 The following items apply when operating in compression mode Point to point Port 3 can operate only in point to point applications However in a compression application between a master and exactly one slave unit multipoint muxed DTE Con options MPM Mux and MPS Mux are allowed X MODIFY main menu MODIFY PORT category 7 2 3512 SDC Operation Data framing Port 3 transmit and receive only synchronous HDLC type data Channe
8. h a network management system the network management feature transmits information either Interruptively during short interruptions in transmission of other data Non interruptively by assigning a very small channel on a 56 0 kbps line about 0 8 kbps to network management information On the 3512 SDC the DTE Con Normal option enables interruptive signalling the Bitsteal and Muxed options enable non interruptive signalling When compression is enabled the unit establishes a Link Access Procedure for Modems LAPM error detection error correction channel on the link Using the Normal interruptive setting conflicts with LAPM causing errors and retransmission of data which degrades performance Avoid such problems by using the Bitsteal or Muxed non interruptive settings 3512 SDC System Applications This section explains how to configure data compression on the 3512 SDC for common application requirements The application examples include Application Figure Compression link between two routers 7 1 Compression of multiplexed legacy and router traffic 7 2 Compression with central site T1 access 7 3 Compression on a restoral link 7 4 Refer to the 55 2 SDC DSU CSU Reference Card which shows control key functions and the menu tree to help you navigate to parameters you want to set Follow these steps to configure your 3512 SDC 1 Complete the procedures in Chapter 1 Introduction and Chapter 2 Verif
9. l sharing Port 3 cannot be part of a channel sharing unit Transmitting network management data To transmit network management data on the digital line set DTE Con to one of the options that provide a non interruptive network management channel BitS Muxed MPM Mux and MPS Mux NOTE The Normal option adversely affects compression performance Link Problem Determination Aid LPDA Port 3 does not support LPDA Timing The unit does not support external DSU timing on Port 3 x MODIFY main menu MODIFY DSU category Timing Ext P3 Timing and Flow Control When P3 Data x MODIFY main menu MODIFY PORT category is set to Sync Ext or Sync Sta set Flow Control COMPRESSION main menu COMPRESS CONFIG category to Off or CTS P3 CTS The available options are Normal and Always On The RTS option is not available x MODIFY main menu MODIFY PORT category P3 CTS and Flow Control When P3 CTS Always On and Flow Control CTS Flow Control takes precedence the unit de asserts CTS when data arrives from the DTE on Port 3 faster than the unit can compress and transmit it Inband Signaling Port 3 does not support RTS DCD DTR and DSR Compression and testing You must disable compression on the remote unit before running a Loop 2 test on Port 3 a Loop 4 test or a front panel initiated RT test Re enable remote compression after completing the test For other tests the 3512 SDC automatically disables compression if necessary for the
10. maining band width available through SDC is available to other users who would otherwise have had a significant performance degradation while the file transfer was in progress 3512 SDC Operation 7 5 Anti Expansion Typical environments with a mix of data types of varying compressibility improve significantly when compression is added However some data such as pre compressed data is not compressible The 3512 SDC s anti expansion feature activates automatically when it detects incompressible data This feature tracks data compression achieved and disables the compression algorithm when its use would cause the data to expand This keeps throughput nearly as high as it would have been without the compression option and adds error detection and error correction benefits Flow Control 7 6 3512 SDC Operation The flow control feature signals a local DTE on the 3512 SDC to temporarily stop transmitting when the SDC s receive buffer is full This can occur for example when the DTE is transferring at the 3512 SDC s maximum of 256 kbps and the 3512 SDC compressing the data at a typical 2 1 to 4 ratio is transmitting it over a 56 kbps link Despite the compression the 56 kbps line cannot accept the data at 256 kbps The 3512 SDC buffers data but when the buffer is full the unit can use flow control to signal the DTE to stop transmitting until the unit is ready for more data The default Flow Control option Tx Clk tur
11. ns off the transmit clock to the DTE when the buffer is full and turns it back on when ready This method works in most environments For DTEs that cannot use clock stopping the CTS option de asserts CTS when the buffer fills This requires a DTE that responds correctly to CTS For DTEs that cannot use either of these methods the Off option turns off flow control With this option the port rate must be reduced to a level that the 3512 SDC can compress and transmit without filling its buffer or else there may be data loss For example if the average data compression is between 2 1 and 3 1 you could set the port rate to 112 kbps on a 56 kbps DDS line However some data loss could occur if a long burst of incompressible data is transferred Data Delay Compared to standard data transparent DSU CSUs data compression features tend to add delay to user data Some of this delay is due to the compression algorithms The compressor must buffer data until it can determine the compressed data representation to transmit across the link More delay is added at the receiving end of an SDC circuit As data is compressed at the transmit end HDLC frame fragments are sent across the link Generally the receiving device must buffer these until an entire frame has arrived before forwarding to the DTE This is because the frame fragments arrival rate cannot be predicted If the receiving device starts to forward a frame to the DTE before the enti
12. nt is shown in Figure 7 2 The 3512 SDCs combine mission critical legacy applications and high throughput LAN applications on common facilities Through the time division multiplexing TDM feature the legacy applications can be guaranteed constant bandwidth and the associated rapid response time the remaining bandwidth is allocated through the compression channel to the LAN to LAN connection Cluster Controller gu Router Port 1 9 6 kbps Unit B OT EEE TA 3512 SDC B Port 3 256 kbps 56 kbps DDS D Y r Router Figure 7 2 Multiplexed Legacy and Router Compressed Data Traffic In this example Port 1 is used for a low speed 9 6 kbps link This link is multiplexed with the router channel which uses the remaining 38 4 kbps compression maximizes throughput between the routers Parameters not specified below may be set to factory default values Category Parameter Unit A Unit B MODIFY DSU Data Rate 56 0 56 0 CHI Rate 9 6 9 6 CH3 Rate 38 4 38 4 MODIFY PORT DTE Con Muxed Muxed MODIFY NETWORK NC Address 001 002 COMPRESS CONFIG Compress Mode Ans Compression With Central Site T1 Access A digital point to point link with central site T1 access using SDC is shown in Figure 7 3 At a site with multiple 56 kbps terminations it is often cost efficient to install a T1 link to group lines from the service provider and split the 56 kbps terminations with an on site T1 multiple
13. re frame has arrived it may complete this step before it receives the remaining fragments from the line These data underruns lead to aborted frames and requires the user application to re transmit the data Buffering at the receive end represents the majority of the added delay in the 3512 SDC The TpDlyMin Throughput Delay Minimization parameter s Rx Receive Clock and Rx Data Delay options provide ways to mitigate it Receive Clock the default forwards partial frames to the DTE as they are received To prevent underruns the receive clock stops temporarily when the unit needs to wait for more data Most synchronous DTEs operate properly with this arrangement For DTEs that do not tolerate receive data clock stopping such as some IBM cluster controllers or where the DTE is providing the receive data clock a T1 mux port the Rx Data Delay option monitors frame sizes and uses a statistical algorithm to predict when the unit can safely begin forwarding fragments without causing an underrun A limitation of this method is that a succession of short frames with an occasional long one interspersed or a succession of frames of widely varying sizes may cause occasional underruns If these occurrences are too disruptive it may be appropriate to use the Off option with this setting the 3512 SDC buffers whole frames before forwarding to the DTE In addition to the features provided for minimizing throughput delay one of the best ways to fu
14. rther minimize delay is to maximize the port rate At the transmit end a high port rate minimizes the time it takes for data to go from the user equipment to the compression function At the receive end a high port rate minimizes the time for buffered data to go from the 3512 SDC to the user equipment 3512 SDC Operation 7 7 Time Division Multiplexing TDM En Some interactive applications particularly critical legacy applications with stringent response time requirements may not function properly with the occasional delays noted thus far Typically these applications have low bandwidth requirements The 3512 SDC s time division multiplexing TDM feature enables you to combine these delay sensitive interactive applications with higher bandwidth applications that can benefit from data compression Set DTE ConzMux to configure TDM Use the Port to Channel and CHn Rate parameters to assign part of the bandwidth to Ports 1 and or 2 for transparent data applications and the bulk of the remaining bandwidth to Port 3 for compressed data applications The transparent channel offers constant throughput and low delay and thus is ideal for the lower bandwidth requirements of interactive applications and the compression channel serves high bandwidth data TDM automatically establishes a network management channel using a small part of the available bandwidth Network Management 7 8 3512 SDC Operation On a 3512 SDC operating wit
15. xer or channel bank With T1 multiplexers that use V 35 serial DCE ports instead of 4 wire DDS type lines you can configure the 3512 SDC for V 35 to V 35 compression as shown on the right side of Figure 7 3 working with a remote SDC unit on a DDS line Here the 3512 SDC s DDS interface is not used instead the local 3512 SDC redirects compressed data through Port 2 using the A B switch feature and a standard restoral cable The T1 multiplexer s DCE port must provide either A DCD signal which allows the 3512 SDC to re direct through Port 2 e ACTS signal set the 3512 SDC s Restoral Type Digital amp RESTORAL main menu DIAL PARAMETERS category If the T1 multiplexer s DCE port supports external timing configure it that way otherwise set the 3512 SDC s Timing Ext P2 Station DIAL PARAMETERS In restoral mode Unit D is always the answering device so its Compress Mode is not used Thus Unit C s Compress Mode must be Originate the default Port 3 256 kbps 56 kbps DDS Port 3 56 kbps DDS se Et 7 L st s 3512 SDC p 0 3512 SDC he q gt z Unit A Unit B Port 2 gs 56 kbps Router D Port 3 256 kbps ul gm 74 _ e O 3512 spc E TE Unit C 56 kbps DDS Unit D Port 2 56 kbps Figure 7 3 Compression Link Using T1 Access to a WAN Parameters not specified below may be set to factory default values Category Parameter Unit A Unit B MODIFY DSU Data Rate 56 0 5
16. ying Line Quality of the 35 2 DSU CSU Quick Start 2 On the MODIFY main menu CONFIGURATIONS category set Factory Yes to set all parameters to the default values 3 Select a configuration and set parameter options appropriately You may set parameters from the front panel or from a network management system Configuring SDC Primary Applications Compression Link Between Two Routers A digital point to point link between two routers is shown in Figure 7 1 In this example compression increases throughput between the two routers Any synchronous HDLC point to point data application is a candidate for SDC for example the Motorola 6500 Series data concentrator offers an ideal application for SDC over DDS links Such a configuration allows applications that require fractional T1 throughput of up to 256 kbps to operate over much less expensive standard 56 kbps DDS leased lines E 7 4 Port Unit A 5 D 3512 SDC Unit B Port L 1 NN Figure 7 1 SDC Compression Link Between Routers 3512 SDC Operation 7 9 Parameters not specified below may be set to factory default values Your configuration may vary Category Parameter Unit A Unit B MODIFY DSU Data Rate 56 0 56 0 MODIFY NETWORK NC Address 001 002 COMPRESS CONFIG Compress Mode Ans 7 10 3512 SDC Operation Compression of Multiplexed Legacy and Router Traffic A digital point to point link in a hybrid SNA and LAN environme
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