AN-664 ISDN Basic Rate Interface Terminal
Contents
1. 31 BRI APPLICATION T R MAINTENANCE alo NETWORK AND slc LAYER MANAGEMENT elo 5 FUNCTIONS 25 LAYER 3 N PLP Q 931 l N T G R 0 LAYER 2 LAYER 2 elt LINK R LAYER DEVICE DEVICE 2 DRIVER DRIVER Ve HDLC 2 CONTROLLER comas 1 7 DEVICE DEVICE 221 PHYSICAL Y Y LAYER 0 CHANNEL vO PES B1 CHANNEL A A Y LAYER A BRI OEE Z 7 7 P S INTERFACE D CHANNEL SOFTWARE Cl A HARDWARE TL H 10726 2 FIGURE 2 2 5 Application Layer Requirements As mentioned above for the sake of this System Design Guide a hypothetical stand alone ISDN voice data terminal TE will be defined that allows voice or data services on either of the two bearer B channels It will support synchro nous and asynchronous X 25 data connections on either bearer channel and asynchronous data on the D signaling channel A Basic Rate Interface Application Controller is required that includes the Call Control procedures of Q 931 and X 31 This controller keeps track of which protocol is on which channel It must also interface to the 1 device drivers for the asynchronous and synchronous terminals as well as the voice related telephony hardware For the X 25 asynchronous data application the TE expects a data application to be running on a PC connected via the R interface a local UART The TE intercepts commands across the R interface and translates them i2. Control Task The DSS1 protocol then sets up the appropriate data service on either the D or a B channel If the network side wants service then the network sets up an ISDN call using the DSS1 call setup procedures In this case the appropriate data control response is sent from the Call Control Task to the Command Translator PAD Task Once the call setup sequence has reached the ACTIVE state then the data phase is entered at which time the Command Translator PAD Task acts like a Packet Assem bler Disassembler PAD and communicates directly with the X 25 PLP Task In this example data is rate adapted from a nominal 1200 baud rate at the UART to either the 16 kbits second D channel rate or the 64 kbits second B channel rate After the data session the call is cleared either by a Hayes command from the PC application or from the far end In either case the X 25 link is released and then the ISDN facility is released using the DSS1 call clearing procedures 3 5 2 Synchronous Data Application For this X 25 synchronous data application example the TE expects to communicate with an X 25 Data Terminal Equip ment DTE running synchronous 56 kbits second data A synchronous channel controller device USART terminates the R interface A custom USART device driver module will be needed to control the USART and to monitor certain data control commands When an X 25 DTE requests service it typically sends a Lay er 2 LAPB SABM comman
3. Func tional Mode CCITT Recommendations Q 921 Q 931 X 25 X 31 and 1 430 Blue Books Geneva 1989 National Semiconductor HPC16400E Data Sheet National Semiconductor HPC16400E User s Manual ISDN Basic Rate Interface Terminal Equipment TE System Design Guide AN 664 LIFE SUPPORT POLICY NATIONAL S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL SEMICONDUCTOR CORPORATION As used herein 1 Life support devices or systems are devices or systems which a are intended for surgical implant into the body or b support or sustain life and whose failure to perform when properly used in accordance with instructions for use provided in the labeling can be reasonably expected to result in a significant injury to the user 2 A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system or to affect its safety or effectiveness National Semiconductor National Semiconductor National Semiconductor Corporation GmbH Japan Ltd 2900 Semiconductor Drive Livry Gargan Str 10 Sumitomo Chemical P O Box 58090 D 82256 F rstenfeldbruck Engineering Center Santa Clara CA 95052 8090 Germany Bldg 7F Tel 1 800 272 9959 Tel 81 41 35 0 1 7 1 Nakase Mihama Ku TWX 910 339 9240 Tel
4. ISDN products or to implement ISDN interfaces into their current products This System Design Guide has attempted to present a compre hensive design solution for those vendors interested in de signing an ISDN BRI product NSC and Telenetworks are committed to providing ISDN solutions to the telecom indus try This Design Guide did not discuss implementation of the two public data protocols V 120 and V 110 It also did not discuss implementation of any private data protocols such as DMI and T Link This was intentional in an effort to some what scope the Design Guide However both NSC and Tel enetworks have developed design support for these proto cols and others NSC has just announced their R Inter face Device the TP3460 which provides many of the hard ware sensitive functions of V 110 Certain V 110 and V 120 software modules are available to also aid a designer in developing a terminal adapter for these protocols For more information on ISDN BRI implementations contact your local National Semiconductor representative or contact Telenetworks at 707 778 6500 APPENDIX A REFERENCES 5ESS ISDN Basic Rate Interface Specification 5E5 Generic Program AT amp T Document 5D5 900 311 December 1987 Chapter III Layer 2 and Management Entity and Chapter IV Layer 3 DMS 100 Basic Rate Interface Specification Northern Tele com Document NIS S208 4 October 1988 Section C Lay er 2 and Management Entity and Section D Layer 3
5. TEI Y X and Y are arbi trary but valid assignment values The X 25 B Channel data termination at Layer 2 requires implementation of the Link Access Procedures of CCITT Recommendation X 25 more commonly referred to as LAPB Like LAPD LAPB also provides in sequence error free frames to the higher layers LAPB can only handle one logical link at Layer 2 2 4 Network Layer Requirements The ISDN DSS1 D Channel signaling termination at Layer requires implementation of the Protocol Control procedures of CCITT Recommendation Q 931 These procedures in clude setting up and tearing down local access connections between the terminal and the Central Office These connec tions can be for voice or data services and can be request ed for the BRI B or D channels For X 25 data calls all packets must be processed by the Layer 3 Packet Layer Process PLP These procedures in clude setting up and tearing down virtual packet switch con nections between end to end X 25 terminals These virtual connections are established in band during the X 25 call setup phase ISDN Basic Rate Interface Terminal Equipment TE Functional Requirements SYNCHRONOUS DATA APPLICATION ASYNCHRONOUS DATA APPLICATION 16 B APPLICATIONS ASYNCHRONOUS SYNCHRONOUS MIT R INTERFACE DRIVER R INTERFACE DRIVER L M TI 0 931 X
6. 400E can address 544 kbytes of memory space however for the ISDN BRI application the entire memory space should be less than 128 kbytes An off the shelf Multitasking Executive MTEX that was de signed specifically for controlling ISDN BRI terminal applica tions has been hosted on the HPC16400E MTEX has the following subsystems Task Scheduler Mail Manager Timer Handler Memory Manager MTEX was designed to provide an environment for commu nication protocol applications with special built in features to handle timers and flow control Processes such as Layer 2 Link Access Procedures and Layer 3 protocols are de signed as tasks running under MTEX Intertask messaging is via the MTEX Mail utilities MTEX also provides memory management utilities for buffer management 3 2 Physical Layer Solution The National Semiconductor 5 Interface Device SID TP3420 terminates the 5 Interface at the physical layer The off the shelf 1 430 BRI Layer 1 software driver module is available to control the SID as defined by CCITT recom mendation 1 430 The SID also has a built in mechanism for switching the bearer channels upon command The SID communicates with the NSC HPC16400E via NSC s MI CROWIRE interface The MICROWIRE interface is a three wire serial interface Sl SO and CLk The NSC COMBO Codec TP3076 can be used for terminat ing the voice path in the TE and converts to analog voice signals The COMBO commun
7. AN 664 ISDN Basic Rate Interface Terminal Equipment System Design Guide Texas INSTRUMENTS Literature Number SNLA018 ISDN Basic Rate Interface Terminal Equipment TE System Design Guide PREFACE This System Design Guide is intended to aid a system de sign engineer in implementing an ISDN Basic Rate Interface BRI Terminal Equipment TE application The Design Guide is divided into the following sections 1 0 ISDN Basic Rate Interface Overview 2 0 ISDN BRI Terminal Equipment Functional Requirements 3 0 ISDN BRI Terminal Equipment Design Solution 4 0 Summary Section 1 is intended to introduce ISDN BRI and to give some justification for implementing such an application Section 2 presents the necessary hardware and software functional subsystems without getting into specific imple mentation details Section 3 presents a design solution for the ISDN BRI application introducing the National Semicon ductor HPC16400E Microcontroller along with the Telenet works ISDN software packages Section 4 summarizes the ideas presented in this System Design Guide References for this System Design Guide are found in Ap pendix A For more information regarding ISDN BRI implementations contact your local National Semiconductor representative or contact Telenetworks at 707 778 6500 1 0 ISDN BASIC RATE INTERFACE OVERVIEW The Integrated Services Digital Network ISDN Basic Rate Interface BRI provides a replac
8. and alone voice data terminal configuration 1995 National Semiconductor Corporation TL H 10726 RRD B30M75 Printed in U S A wia sAs 41 dOej19 U 92H oiseg NASI 799 2 20 azmzZzv com ISDN Basic Rate Interface Terminal Equipment Application ISDN BRI DIGITAL SUBSCRIBER LOOP DSL NETWORK TERMINATION ap U INTERFACE 2 WIRE S INTERFACE 4 WIRE FIGURE 1 TL H 10726 1 2 0 ISDN BRI TERMINAL EQUIPMENT FUNCTIONAL REQUIREMENTS The ISDN BRI Terminal Equipment TE Functional Require ments are layered in accordance with the OSI Layered Model ISDN applications include requirements at the Physi cal Layer the Data Link Layer the Network Layer and the Application Layer Figure 2 reflects these layered functions for each of the following subsystems the Digital Subscriber Signaling System 1 0551 protocol requirements the data protocol termination requirements and the Mainte nance and Management requirements Several types of data protocols exist today e g X 25 V 120 V 110 DMI and T Link This System Design Guide focuses on the requirements necessary to design an ISDN voice data terminal using the X 25 asynchronous and synchronous data protocol for terminal adaptation The defi nition of the layered requirements for the other foreme
9. been re leased 3 6 Maintenance and Management Solution Most of the Management Entity Task s functions are appli cation dependent such as provisioning the ISDN BRI termi nal collecting statistics from the various hardware devices and alarm detection and recovery These functions will re quire custom code development As previously mentioned a software driver is available that activates and optionally deactivates a BRI terminal This driver works in concert with Layer 2 the Management Entity procedures the particular application and of course the NSC SID The TEI Assignment procedures are also included in the Management Entity Task Provisions have been made for both automatic and non automatic assignment proce dures Check and Verify procedures are also included Management functions such as parameter negotiation and higher layer processes are application dependent and re quire custom solutions The HPC16400E onboard UART can be used for the main tenance interface This interface is defined in the physical layer section of this document The maintenance features would only be available when the asynchronous data serv ices are not in progress Additional commands must be add ed to the PC application program to control the mainte nance interface 3 7 Software Development Environment The process of developing a software application around the Telenetworks ISDN software will vary greatly depend
10. d off the shelf Certain functions have been standardized at the application layer but in general custom code design and development is necessary Supple mentary Services are defined in both the voice and data domain however the invocation of these services 18 very application dependent and requires custom solutions Telenetworks does offer a prototype X 31 Q 931 Call trol module that controls voice calls over an ISDN interface as well as asynchronous and synchronous X 25 data calls For the voice application this module controls various te lephony devices by communicating with the Telephony De vice driver modules It also expects to communicate with a Command Translator PAD Task and a USART HDLC soft ware driver module as defined in the following paragraphs 3 5 1 Asynchronous Data Application For this specific X 25 asynchronous data application exam ple the TE expects to communicate with a PC running the Hayes Modem application program The Hayes Modem pro gram communicates out of the PC UART toward a modem The expected modem is replaced by the ISDN TE A Command Translator PAD Task will be needed to interpret the Hayes AT commands for setting up and tearing down the desired bearer service If the PC user wants service then a service request with a called party number arrives at the TE in a Hayes command and is translated into an ISDN Setup command This data control message is sent to the X 31 Q 931 Call
11. d control depending on the device architecture The 1 430 activation and optionally deactivation sequence must be followed to initialize the 5 interface device of an ISDN BRI terminal For a voice application a CODEC is needed to convert ana log voice into digital PCM A keypad and hookswitch along with a display are also required Some software intervention is necessary to activate and deactivate these devices de bounce hardware inputs and control these devices A call progress tone generator is also required to play progress tones e g DIALTONE BUSY and RINGBACK to the ear A set of telephony device driver modules must be available to interface to these hardware devices For data applications hardware and software drivers are needed to control the R interface devices For asynchro nous data applications a UART is required along with a UART Software Driver module to control the interface For synchronous data applications a USART is required along with an HDLC Software Driver module to control the inter face In some applications it is not desirable to hard wire a partic ular bearer channel to the CODEC or to the HDLC port In these cases it may be necessary to provide a switching de vice to dynamically switch a bearer channel to either of the two sources Semiconductor vendors have developed several types of internal digital formats that allow these various devices to communicate Serial decoding is the
12. d to establish the data link The USART HDLC driver module intercepts this SABM and sends a data control service request to the X 31 Q 931 Call Control Task This message prompts the Call Control Task to set up an ISDN bearer channel using the DSS1 call setup procedures Once the call setup sequence has reached the ACTIVE state then the Call Control Task sends a message to the USART HDLC driver module that a bearer channel is available The USART driver then sends the SABM directly to the HPC16400E HDLC driver module The network side can also request service in which case the Packet Handler sets up the ISDN bearer channel using the DSS1 call setup procedures In this case the Call Control Task prompts the USART driver module that a bearer channel is available In either of the above cases DTE to DCE is established and the data phase is entered During the data phase packets are transmitted directly from the USART driver to the HPC16400E dual HDLC driver without any intervention by the TE In this example data is rate adapted from the nomi nal 56 kbits second rate to the ISDN 64 kbits second B channel rate After the data session the network side is responsible for initiating the call clearing process by first disconnecting the X 25 data link with a LAPB DISConnect command The net work side then clears the ISDN facility with the DSS1 call clearing procedures The Call Control Task informs the USART driver module that the bearer channel has
13. ding the LAPD solution The LAPD was prototyped after the CCITT 1988 Blue Book version for Q 921 however it has been tailored to imple ment the Layer 2 Specifications in the 5ESS 5E5 generic software release from AT amp T and the DMS100 BCS 28 ge neric software release from Northern Telecom LAPD logi cal entities are defined based on SAPI and TEI The B Channel is also terminated at Layer 2 with the off the shelf Layer 2 Software Task providing the LAPB solution The LAPB was prototyped after the CCITT 1988 Blue Book version for X 25 Layer 2 3 4 Network Layer Solution The off the shelf Layer 3 931 Task is designed to imple ment the call setup and teardown protocols as defined in the CCITT Blue Book version for Q 931 The Layer 3 Task has also been tailored to implement both the Layer 3 Speci fications in the 5ESS 5E5 generic software release from AT amp T and the DMS100 BCS 28 generic software release from Northern Telecom This protocol terminates the D Channel at Layer 3 Network Link logical entities are defined based on Channel Endpoint Suffix CES Call Reference and Call ID The off the shelf Layer 3 X 25 Packet Layer Processor PLP Task is designed to implement the establishment of X 25 virtual data connections Layer 3 PLP logical entities are defined based on CES and Logical Channel Number 3 5 Application Layer Solution Applications by their very nature are proprietary so very lit tle can be offere
14. ed on the receive side with no chaining available on the transmit side The HPC16400E can be programmed to recognize particu lar Layer 2 addresses combinations This feature is useful if the terminal is to be used in a passive bus ar rangement In such an arrangement messages not intended for a particular terminal on the bus can be filtered by the terminal and discarded without any processor intervention The two HDLC DMA channels are controlled by an off the shelf HPC16400E Dual HDLC DMA software driver module The driver consists of an Interrupt Service Routine and two Service Request Tasks one for each channel The driver is responsible for initializing the synchronous channel hard ware and controlling the transmission and reception of frames It is also required to handle buffer management for messages in both directions Buffers must be allocated for incoming messages and deallocated after the transmission of outbound messages One exception to this rule is that after transmission frame buffers are NOT deallocated by the Driver since they may need to be retransmitted if unac knowledged The Layer 2 Software Module has the respon sibility for deallocating transmitted frame buffers when the appropriate acknowledgement is received This driver con figures both HPC16400E HDLC DMA channels to use the split field feature The D Channel is terminated at Layer 2 with the off the shelf Layer 2 Software Task provi
15. ement for the current sub scriber line interface offering into businesses and homes As shown in Figure 7 the ISDN BRI offering uses the same copper pair as the current conventional phones but pro vides more and better services to the subscriber To har ness these services it is necessary to follow the prescribed layered protocols for developing ISDN BRI Terminal Equip ment TE This System Design Guide functionally defines these protocols and proposes an implementation solution for a particular ISDN BRI terminal design This section de fines what ISDN BRI services are and why they are worth the investment in a new technology The INTEGRATED part of ISDN is apparent in two domains the integration of voice and data on the same interface and the integration of an out of band signaling D channel with the bearer B channels The advantages of this integration are defined in the following paragraphs The SERVICES part of ISDN are the main reason for ISDN s potential growth and acceptance in the future Basic Voice and Basic Data services and even the Big 4 Supplemen tary Services HOLD RETRIEVE CONFERENCE DROP and TRANSFER are not enough to propagate the ISDN cause All of these services are available with the current MICROWIRE is a registered trademark of National Semiconductor Corporation COMBO and MOLE are trademarks of National Semiconductor Corporation National Semiconductor Application Note 664 Telene
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18. icates via the MICROWIRE interface The HPC16400E onboard UART is ideal for the asynchro nous data interface and for the maintenance interface The UART has a high resolution clock for rates up to 19 2 kbaud An off the shelf HPC16400E UART software driver module is available that interfaces to this UART This driver fields messages from tasks and outputs them to the UART as well as receiving input from the UART and sending it to the appropriate task A synchronous communication device is needed to termi nate the synchronous data interface NSC makes a stand alone USART for just this purpose the TP3460 A special USART HDLC software driver module is needed to control the USART and monitor the interface for particular com mands The requirements for this driver are defined below in the synchronous data application section of this document Custom Layer 1 Telephony drivers and routines are needed to control such things as hookswitch keypad input LCD or LED updates local tone generation power ringing and vari ous other application dependent telephony device require ments The HPC16400E has a programmable onboard Serial Time Slot Decoder that provides an interface to the B and D channels of several popular chip to chip interface formats 3 3 Link Layer Solution The HPC16400E has two independent HDLC DMA control ler channels integrated onboard with the Central Processing Unit For ISDN BRI applications one channel runs at 16
19. ile containing a test scenario Under control of the test scenario Task__View sends mail messages to a specified mailbox or mailboxes where they are read by the task s under test Mail messages sent by the task s in response to this input are then displayed by Task__View In this way tasks can be debugged in isolation before integration into the complete target environment The full featured Task__View operates only in a PC environ ment since it uses the DOS file system to handle the test scenario files This is less of a limitation than it might ap pear since many tasks interact only with the MTEX Multi tasking Executive and can be unit tested and debugged in a PC environment running under the PC version of MTEX before integration with other tasks in the target environ ment However some modules in particular 1 drivers can only be tested in the target hardware environment A simpli fied version of Task__View has been ported into the embed ded HPC environment for testing of the I O Drivers Task__ View has also been ported into the Sun workstation devel opment environment 3 9 NSC Development Tools Below is a list of the development tools available for the NSC HPC16400E HPC C Compiler HPC Linker HPC PROM tools HPC Microcontroller On Line Emulator MOLETM NSC TP3500 ISDN Evaluation Target System 4 0 SUMMARY As ISDN BRI becomes more and more prevalent users will require terminal vendors to provide new
20. ing on the application and the target hardware The procedure given below is an example that might apply to the develop ment of an ISDN BRI application Task__View a testing tool developed by Telenetworks is defined below as well as the software tools that are available for the NSC HPC16400E In a typical ISDN BRI application there are two main areas of software development First device drivers must be writ ten for the particular telephony hardware that exists in the target system e g Layer 1 drivers and DMA HDLC driv ers Second a Call Control Application Task must be de signed to control the telephony hardware and interface to the Layer 3 Protocol Control Tasks Typically in such a development process the target hard ware is not available at the time that software development begins This suggests a three step software development process First the Call Control Application Task is written and is unit tested in a PC environment Second when the target hardware becomes available the telephony hardware drivers are written and unit tested in this target environment Finally the entire system software is integrated and tested with the hardware 3 8 Task_View Description To aid in testing ISDN software modules Telenetworks has developed Task__View Task__View is a special purpose debugging task that can be run together with other tasks under the MTEX Multitasking Executive It reads and inter prets a user supplied ASCII f
21. is recommended that the TE only allow asynchronous data service on the D channel to avoid voice service from being blocked In a passive bus arrangement i e multiple TEs on the same 5 interface it is recommend ed that asynchronous data service only be allowed on the D channel Of course this is a decision that must be made and enforced at the application layer 2 6 Maintenance and Management Requirements Every application of this magnitude requires some degree of maintenance and management Some maintenance func tionality is required by the Network while some of the func tionality is strictly application dependent The major mainte nance functions include provisioning the TE statistic collec tion and reporting physical layer alarm reporting and recov ery protocol breakdown recovery and resource depletion handling Other peripheral maintenance functions include loopback mechanisms human interface capability and soft ware PROM revision maintenance Procedures from 1 430 are necessary to activate and option ally deactivate terminals on the 5 Interface Procedures for assignment are defined in CCITT Recommendation Q 921 and in the AT amp T and NTI switch specifications Pa rameter negotiation procedures are also defined in these switch specifications For any BRI implementation exten sions must be made to the Maintenance and Management functions beyond what is defined in specifications It is generally prudent
22. kbits second for the D Channel the other runs at 64 kbits second for data on a B Channel The DMA Control lers receive and transmit entire frames prior to interrupting the processor This allows for a much simpler Layer 2 Driver design than would a FIFO based HDLC controller The HPC16400E has several other features which ease the im plementation of the ISDN BRI application These features are defined in the following paragraphs The HPC16400E can operate in a special split field mode where the Layer 2 Header data is placed in one buffer while the following Layer 3 Information data is placed in another buffer This feature eases the memory management require ments of an application and decreases the need for copying messages from buffer to buffer If the split field feature is not used then HPC16400E DMA chaining features can be fully exploited to simplify the appli cation design On the receive side four buffers can be made available for incoming frames When a frame arrives an in terrupt is generated by the DMA Controller While the proc essor is responding to the interrupt and fielding the current message new frames that arrive will automatically be placed into memory buffers Up to 4 messages can arrive without any processor intervention On the transmit side two messages can be output delimited by a closing opening flag without processor intervention If the split field feature described above is used then two frames can be chain
23. n tioned data protocols is beyond the scope of this Design Guide The X 25 data protocol was chosen because it is the most prevalent data protocol today as well as the best de fined protocol The requirements for transporting X 25 data on an ISDN BRI terminal are defined as part of this paper and are more generally defined in CCITT Recommendation X 31 2 1 System Level Requirements A relatively povverful CPU engine is necessary to process the DSS1 and data communication protocols Small 8 bit microcontrollers are not generally adequate for the tash On the order of 128 of ROM are necessary to implement these CCITT protocol requirements assuming the code is compiled from a high level language such as Careful consideration must be given to controlling an ISDN BRI application There are several asynchronous detailed processes going on that require some level of coordination regarding process scheduling buffer management mes sage passing and timer handling Selection of an adequate design environment in which to nest the various processes is of utmost importance Use of a multitasking executive is recommended 2 2 Physical Layer Requirements To make a terminal a member of the ISDN passive 5 Interface bus a front end 5 interface device is necessary This device must allow for D channel contention and under stand the framing channel Such a device will take varying degrees of software intervention an
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25. nto DSS1 Setup and Teardown commands Service can be requested for either the B or D channel Once a channel is setup then the data phase packet transfer is established with the X 25 PLP When data transfer is complete the PC Application will disconnect with a particular command which will be translat ed into a DSS1 release sequence at which time the channel will be available for another service For the X 25 synchronous data application a synchronous X 25 DTE is expected at the other side of the R interface A USART is required with a special HDLC device driver module to terminate the 56 kbit second data stream The driver initially monitors the link for a link establishment indi cation SABM If the DTE requests service first link estab lishment indication is sent by the DTE an ISDN bearer channel is requested via the DSS1 protocol on the D chan nel If the network requests service first the network will setup the bearer channel using the DSS1 protocol and then establish the data link in band with the DTE After success ful setup of a B channel packets are sent between the DTE and the packet handler At the end of the data session the network side is expected to disconnect the data link in band with a Layer 2 DISC command and then release the call using DSS1 release procedures Voice service is controlled entirely within the TE During the time that synchronous data service is being provided on one of the B channels it
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27. physical layer function that presents the S interface data to the particular devices Care must be taken to ensure that the selected Layer 1 devices all provide the appropriate digital format 2 3 Link Layer Requirements The OSI Link Layer 2 is defined by two sets of procedures High Level Data Link Control HDLC procedures and Link Access Procedures LAP HDLC procedures are typically done in hardware with synchronous channel controllers Link Access Procedures are typically done in software al though some silicon devices do offer limited LAP functionali ty As shown in Figure 2 two HDLC Controllers are required one to terminate the D Channel and one to terminate one of the Bearer Channels for data HDLC software device drivers are needed to control both of these HDLC channels From a software implementation point of view DMA controlled HDLC channels are much more desirable than FIFO based channels because they ease the CPU interaction require ments The D Channel termination at Layer 2 requires implementa tion of the Line Access Procedures of CCITT Recommenda tion Q 921 more commonly referred to as LAPD LAPD pro vides in sequence error free transmission of frames to the higher layers LAPD can handle multiple logical links For the ISDN BRI there are at least three logical links one for signaling functions SAPI 0 TEI X one for mainte nance functions SAPI 63 127 and one for D channel data SAPI 16
28. r services throughout the country and eventually the world using Sig naling System 7 as the inter office backbone The network is also responsible for the appropriate billing mechanisms for the ISDN services User driven applications must be de veloped and supported by the network along with a cost ef fective tariffing structure BRI ISDN will slowly become available to the general public It is currently being deployed in large metropolitan areas targeted for business environments Just when more gener al deployment will occur depends on the rate of deprecia tion of current equipment tariffing rates for the ISDN serv ices and user acceptance of the new ISDN technology End users are just now becoming educated on the virtues of ISDN Ultimately the end users will have to drive OEM ven dors to provide ISDN terminal equipment that brings special services to them These services will have to either cost less than currently available services or be value added and cost effective The remainder of this document provides OEM vendors with an idea of what is involved in designing an ISDN BRI terminal and what building blocks are avail able for implementing such a terminal There are several types of ISDN Terminal Equipment TE TEs can be Voice and or Data PC based or stand alone synchronous and or asynchronous 0B D 1B D or 2B D X 25 V 120 or V 110 or any combination of data protocol support Figure 1 shows a generic st
29. river Custom Software Development by Telenetworks or OEM USART HDLC Driver Application Specific X 31 Q 931 Call Control Task Command Translator PAD Task Extended Management Entity Task Extended Applications for Supplementary Services Telephony Device Drivers ISDN Basic Rate Interface NSC TE System Solution R INTERFACE INTERFACE PC BASED HPC16400E APPLICATION ONBOARD UART 2 ASYNCHRONOUS SYNCHRONOUS ALL TASKS AND DRIVERS OPERATE DATA DATA IN THE MTEX MULTI TASKING ENVIRONMENT HPC164000E UART DRIVER DATA DATA CONTROL PACKETS GD MAINTENANCE GD INTERFACE DATA PAD TASK DATA PACKETS CONTROL X 31 0 931 CN VOICE CONTROL LAYER LAYER 3 MESSAGES 0 931 4 5 LAYER 2 5 LAPB LAPD 1 430 BRI TELEPHONY HPC16400E LAYER 1 DEVICE DUAL HDLC DRIVER DRIVERS DRIVER 7 ESE Ys CONTROLLERS A ESTES B CHANNEL B CHANNEL E CHANNEL EES ARES O SOFWARE MODULE 2 7 1 HARDWARE DEVICE TL H 10726 3 FIGURE 3 3 1 System Level Solution The NSC HPC16400E is a powerful 16 bit CPU engine that is fully capable of processing the DSS1 and B Channel data communication protocols The HPC16400E has two flexible onboard HDLC DMA controller channels that are ideal for terminating the D Channel and data on a B channel It has an onboard UART to terminate the asynchronous data inter face 4 onboard timers and several general purpose 1 0 pins The HPC16
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31. to introduce a UART based mainte nance interface into a design of this complexity This inter face allows for message tracing limited process interaction and optional program download and debug capability all from a PC or terminal attached to the UART A maintenance interface software driver module is needed to manage the screen output and keyboard input functions for the terminal attached to the UART This interface can be shared with the asynchronous data application only being available when the data application is not operating 3 0 ISDN BRI TERMINAL EQUIPMENT DESIGN SOLUTION Figure 3 presents an implementation solution for the ISDN BRI Terminal Equipment TE requirements The solution re volves around the National Semiconductor HPC16400E Mi crocontroller and the Telenetworks ISDN Software pack ages Hardware Solution NSC HPC16400E Powerful 16 Bit Microcontroller 2 Onboard HDLC DMA Controllers 1 Onboard UART 4 Onboard Timers Synchronous Serial Time Slot Decoder General Purpose I O Ports 8 Prioritized Interrupt Levels MICROWIRE Interface e NSC TP3420 S Interface Device SID NSC TP3076 COMBO Codec e USART Software Solution Telenetworks ISDN Software Off the Shelf Packages MTEX Multitasking Executive 1 430 BRI Layer 1 Driver HPC16400E Dual HDLC DMA Driver Layer 2 Task Layer 3 Q 931 Task X 25 Packet Layer Process Task Prototype X 31 Q 931 Call Control Task Basic Management Entity Task HPC16400E UART D
32. tworks February 1990 analog Centrex service offerings For ISDN to be truly suc cessful cost effective end user applications must be devel oped and supported that require the extended services that are only available through ISDN e g Keyset services User to User data and telemetry services on the D channel The DIGITAL part of ISDN provides three digital channels B1 B2 and D between the Customer Premise Equipment CPE terminals and the Central Office Any combination of voice and or data services can be provided on either of the two bearer channels while both signaling and data can be sent over the D channel For example two voice circuits can be implemented over the same wire pair that previously only supported one Data can be sent at up to 64 kbits second rather than the current nominal rate of 2400 baud using a modem End to end digitized information is less susceptible to noise allowing for clearer voice quality and less data retransmissions Several data links running at sub rates less than 64 kbits second can be multiplexed onto a B channel or the D channel using various time division multi plexing techniques not available in the analog domain thus taking full advantage of the channel bandwidth The NETWORK part of ISDN includes marketing and tariff ing issues For ISDN to be successful digital facilities must be available from end to end i e no isolated ISDN islands The network must provide signaling and beare
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