Integrating Euro-ISDN with ATM Technology : Interworking Mechanisms and Services Support L. Mandalos [1], K. Leonidou [2], C. Andreopoulos [3], J. Drakos [4], S. Koubias [5], G. Papadopoulos [6] [1] Research Fellow, [2] [3] [4] Student, [5] Associate Professor, [6] Professor, Director of the Applied Electronics Laboratory Applied Electronics Laboratory, Department of Electrical Engineering & Computer Technology, University of Patras, Patras 26500, Greece e-mail : mandalos@ee.upatras.gr tel. +30 61 997 312 fax. +30 61 997 333 Author responsible for correspondence: Loukas S. Mandalos e-mail : mandalos@ee.upatras.gr tel. +30 61 997 312 fax. +30 61 997 333 KEYWORDS : ATM, ISDN, QoS, INTEROPERABILITY, SIGNALLING EXTENDED ABSTRACT 1. INTRODUCTION Both networks, Euro-ISDN and ATM, use out of band signalling, which means that in order to establish, maintain and tear-down a connection, they use a dedicated logical channel, which is the D channel in the case of the ISDN and AAL 5 cells with VPI=0 and VCI=5. ISDN is a circuit switched network (TDM), designed to provide a set of narrowband services to the end user, using a number of B channels. Each B channel has a 64 Kbps, and in the BRI the user is given 2 B channels (D channel at 16 Kbps), while in the PRI the user has 30 B channels (D channel at 64 Kbps). On the other hand ATM technology provides mainly greater speeds
(e.g. 25 Mbps, 155 Mbps) in a packet switched network, making use of Statistical Time Division Multiplexing STDM). The services to the end user can be CBR, VBR, ABR, or UBR, depending on the availability of network resources and the traffic policing schemes. Contrary to that, the ISDN network provide CBR type services. An IWU has to take into consideration all these aspects in order to provide a connection between an Euro- ISDN host and an ATM host and vice versa. This work focuses on the mechanisms in the IWU during the connection establishment phase, while emphasis is placed in the Quality of Service (QoS) negotiation during Setup. This is due to the fact that the two networks have to accept a common language, in order to interwork. Apart from this, the mapping of the user data is being analyzed, both with PRI and BRI. 2. CONNECTION ESTABLISHMENT 2.1 EURO-ISDN Initiated Setup message is sent by the calling user to the network and by the network to the called user to initiate call/connection establishment. Let s suppose that the ISDN user requests a CBR service i.e. speech at 2x64 kbit/s or alternatively at 64kbit/s. The Bearer Capability Information element would be coded like this (Figure 1): 8 7 6 5 4 3 2 1 Octet 0 0 0 0 0 1 0 0 1 Information Element Identifier 0 0 0 0 0 0 1 1 2 Length 1 0 0 0 0 0 0 0 3 ext CCITT Speech 1 0 0 1 0 0 0 1 4 ext Circuit Mod 2x64 kbit/s 1 0 1 0 0 0 1 1 5 ext Layer 1 G. 711 A-law Figure 1. Bearer Capability Information Element Coding for CBR 2 x 64 Kbps If the network allows Low Layer Compatibility Negotiation the Low layer Compatibility Information Element will be encoded as follows: Information Transfer Capability = Speech (Octet 3) Negotiation Indicator out-band Negotiation = Possible (Octet 3a) Mode = Circuit Mode Information Transfer Rate = 64kbit/sec w let s try to map this into an ATM Setup message so as to be understood by the ATM user who is being called. We will need a function that transforms bits/sec to cells/sec. Lets call it Transform(bits/sec). This function and its resulting cell rates will be presented later in this paper. The mapping onto the ATM parameters in the SETUP message which will be build by the IWU, is the following: i. ATM Adaptation Layer Parameters AAL Type 1 (Octet 5 of UNI v. 4.0 v. 3.1 specification / SETUP) Circuit transport (Octet 6.1) CBR Rate 2x64kbit/s (Octet 7.1)
ii. Broadband Bearer Capability Bearer Class BCOB-A or BCOB-X Traffic Type Constant Bit Rate Configuration Point to Point ATM Transfer Capability CBR with CLR Commitment on CLP=0+1 or absent or CBR for the CLP=0 iii. ATM Traffic Descriptor Forward PCR (Transform(2x64kbits/sec)) Backward PCR (Transform(2x64kbits/sec)) Forward SCR (Transform(2x64kbits/sec)) Backward SCR (Transform(2x64kbits/sec)) MBS 0 Best Effort Indicator Tagging NO iv. End To End Transit Delay It must be less or equal than the ISDN delay. v. Extended Qos Parameters ATM Forum Specific Originating User Acceptable Forward/Backward Peak-to-Peak Cell Delay Variation <= ISDN Delay Cumulative Forward/Backward Peak-to-Peak Cell Delay Variation <= ISDN Delay Acceptable Forward/Backward Cell Loss Ratio <= ISDN loss When the traffic parameters are negotiable either the Alternative ATM Traffic Descriptor or Minimum acceptable ATM Traffic Descriptor information element, but not both, shall be included in the setup message. The Alternative ATM Traffic Descriptor information element contains the Best Effort Indicator only if it is also contained in the ATM traffic Descriptor information element. It can have any combination of Traffic Parameters that is allowed for the ATM Traffic Descriptor information element. If it is determined that the alternative bandwidth requirements are greater than those in the ATM Traffic Descriptor information element, the Alternative ATM Traffic Descriptor information element may be treated as an information element with content error. Forward PCR (Transform(64kbits/sec)) Backward PCR (Transform(64kbits/sec)) Forward SCR (Transform(64kbits/sec)) Backward SCR (Transform(64kbits/sec)) MBS 0 The purpose of the Minimum Acceptable ATM Traffic Descriptor information element is to specify the minimum acceptable traffic parameters in the negotiation of traffic parameters during call/connection setup. The difference with Alternative ATM Traffic Descriptor information element is that: In a setup message a peak cell rate traffic parameter (i.e. Forward PCR) is allowed in the Minimum acceptable ATM Traffic Descriptor only if the corresponding Peak Cell Rate parameter is in the ATM Traffic Descriptor information element of the setup message (e.g. if there is no Forward PCR parameter in the ATM Traffic Descriptor Information element of a setup message then the Forward PCR is not allowed in the Minimum Acceptable ATM Traffic Descriptor information element of the setup message). Forward PCR (Transform(64kbits/sec)) Backward PCR (Transform(64kbits/sec)) Forward SCR (Transform(64kbits/sec)) Backward SCR (Transform(64kbits/sec)) MBS 0
For the case of ISDN user requesting speech at 2x64 kbits/sec and 64kbits alternatively with an ATM user we saw the mapping of the setup message. The following figure (fig. 2) depicts the actions carried out within the IWU in the case that an ISDN user requests connection establishment with a user in the ATM network. ISDN User sends message (through D Channel) IWU Examines message type (if there is a SETUP message) The IWU examines if the called party address belongs to an ATM user connection between ISDN and ATM is requested Sends CALL PROCEEDING message Checks resources and Bearer Capability Information Element for requested Service Can it be generally satisfied by the ATM network? Optionally interchange two max. USER INFORMATION messages (ISDN-IWU) before DISCONNECT Send DISCONNECT message with the appropriate cause number Is negotiation Indicator enabled? Can alternative values be generally satisfied by network? START BUILDING ATM SETUP MESSAGE ATM Adaptation layer Parameters AAL Type 1 Subtype Circuit Transport CBR Rate n x 64 kbits/sec (The multiplier n to be as in the ISDN SETUP message) Broadband Bearer Capability Bearer Class BCOB-A or BCOB-X Traffic Type Constant Bit Rate Configuration Point to Point ATM Transfer Capability CBR cont. Figure 2. Connection Establishment Phase mechanism (Euro-ISDN to ATM) cont.
cont. Is Low Layer Compatibility Information Element with Negotiation Indicator Enabled? In ATM Traffic Descriptor Enable Best Effort Indicator Built Alternative ATM Traffic Descriptor with Traffic Parameters that are in the low layer compatibility Information element in ISDN Setup message: Forward/Backward PCR=Transform(n- x 64kbits/sec) Forward/Backward SCR=Transform(n- x 64kbits/sec) MBS=0 ATM Traffic Descriptor Forward PCR (Transform(2x64kbits/sec)) Backward PCR (Transform(2x64kbits/sec)) Forward SCR (Transform(2x64kbits/sec)) Backward SCR (Transform(2x64kbits/sec)) MBS 0 Best Effort Indicator / Tagging NO End To End Transit Delay It must be less or equal than the ISDN delay. Extended Qos Parameters ATM Forum Specific Originating User Acceptable Forward/Backward Peak-to-Peak Cell Delay Variation <= ISDN Delay Cumulative Forward/Backward Peak-to-Peak Cell Delay Variation <= ISDN Delay Acceptable Forward/Backward Cell Loss Ratio <= ISDN loss END OF ATM SETUP MESSAGE BUILDING IWU sends SETUP message to ATM user If IWU receive CONNECT from ATM sends CONNECT to ISDN, otherwise if IWU receive a RELEASE message issues a DISCONNECT message towards ISDN user Figure 2. Connection Establishment Phase mechanism (Euro-ISDN to ATM)
2.2 ATM Initiated When an ATM user wishes to establish connection with an ISDN user there are no problems concerning the requested Quality of Service parameters, as ISDN is a circuit switched network with constant bit rate connections. The main problem is available transfer rates which in ISDN are limited to 1920kbps. To perform a simple mapping from an ATM SETUP message to an ISDN SETUP message, we need a function to transform cells/sec to bits/sec. The following diagram analyzes the mechanisms in the IWU during SETUP. ATM User sends signalling message (VPI=0/VCI=5, VPI=0/VCI=16 AAL 5) IWU Examines message type (if there is a SETUP message) The IWU examines if the called party address belongs to an EURO-ISDN user connection between ATM and ISDN is requested Sends CALL PROCEEDING message Checks resources and ATM TRAFFIC DESCRIPTOR / BROADBAND BEARER CAPABILITY Information elements for requested Service Can it be generally satisfied by the ISDN network? Send RELEASE message with the appropriate cause number Is there Alternative ATM Traffic Descriptor or Minimum Acceptable ATM Traffic Descriptor? Can alternative values be generally satisfied by network? START BUILDING EURO-ISDN SETUP MESSAGE Check ATM TRAFFIC DESCRIPTOR Information Element cont. Figure 3. Connection Establishment Phase mechanism (ATM to Euro-ISDN) cont.
cont. Is Best Effort Indicator Enabled in ATM Traffic Descriptor Information Element? Map Alternative ATM Traffic Descriptor to Low layer Compatibility Information Element Information Transfer Capability Speech (Octet 3) Negotiation Indicator out-band Negotiation Possible (Octet 3a) Circuit Mode Information Transfer Rate = Transfer (Forward/Backward PCR) Map ATM Traffic Descriptor to Bearer Capability Information Element Information Transfer Capability Speech (Octet 3) Circuit Mode Information Transfer Rate = Transfer (Forward/Backward PCR) END OF EURO-ISDN SETUP MESSAGE BUILDING IWU sends SETUP message to Euro-ISDN user If IWU receive CONNECT from ISDN sends CONNECT to ATM, otherwise if IWU receive a DISCONNECT message issues a RELEASE message towards ATM user Figure 3. Connection Establishment Phase mechanism (ATM to Euro-ISDN) 3. MAPPING USER DATA The user data are transferred within B channels of 64 Kbps each in the ISDN and in the payload of the 53- bytes packets (cells) in the ATM network. In the case that an Euro-ISDN user sends data to an ATM user (after connection establishment has been completed), we have to distinguish whether or not the ISDN user has a Basic Rate Interface (BRI) or a Primary Rate Interface (PRI). This is essential, because the structure of
the two interfaces is completely different, so they have to be treated in a different way. Bellow we present the mapping from ISDN to ATM in both BRI and PRI. The opposite mapping, from ATM to ISDN is analogous. 3.1 BRI mapping to ATM cells In the case of an ISDN user with a BRI interface, the 2 B channels carry 64 Kbps each. We must keep in mind that the ATM cell provides the user with a 48-bytes payload for his data, while the whole cell is 53- bytes. The mapping is shown in next figure. D EURO ISDN BRI B 2 2 B 64 + D 16 B 1 4 bits / 250 µsec 16 bits / 250 µsec 16 bits / 250 µsec ETSI ETS 300 012 Structure 48 bits per frame 4000 frames per sec Every 12 ISDN frames the ATM cell is being filled (12 frames in 3msec) ATM Cell 24 bytes of B 1 (12 x 16 bits) 24 bytes of B 2 (12 x 16 bits) 5-bytes header 48-bytes payload Figure 4. Euro- ISDN BRI to ATM data mapping Each frame of the ETSI ETS 300 012 structure is transmitted every 250µsec containing 2 bytes (16 bits) of each B channel, which equals to 4 bytes of user data. So the IWU collects the user data of 12 frames, summing up 48 bytes, which is the available size in the ATM cell payload. Of course, 12 frames are transmitted in 12 x 250 µsec = 3 msec. This is very small, comparing with the one-way delay of 150msec which is permitted by ITU for voice. The IWU fills the first 24 bytes of the ATM cell payload with data of the first B channel and the rest 24 bytes with the data of the second B channel. A buffer for holding the data of both B channels for 12 frames is necessary. Thus, for every 12 frames of the BRI the IWU sends an ATM cell to the called user. So, 4000 frames per second demand 4000/12 = 334 cells/sec, or 334 x 53 x 8 = 141616 bps. The difference between the 128000 bps of the ISDN and the 141616 bps of the ATM is due to the 5-bytes header of the ATM cell. In the case that only one B channel is active, we have two alternatives: i) The IWU waits for 24 frames to fill the entire cell, so we need 167 cells/sec or 70808 bps (4000/24) and the delay is 24 x 250 µsec = 6 msec, which is negligible. ii) The IWU collects 12 frames fill half the ATM cell, resulting 334 cells/sec or 141616 bps. 3.2 PRI mapping to ATM cells
The case of the PRI is completely different, as the structure of the ETS ETSI 300 011 is organized in 32 slots (0-31), with slots 1-15 and 17-31 containing 8 bits of each one of the 30 B channels. The mapping is much simpler than the previous one. Figure 5 presents the mechanism: slot 0 slot 1 slot 2 slot 15 slot 16 slot 17...... slot 30 slot 31 ETSI ETS 300 011 Structure 30 B channels per frame 8 bit per slot 8000 frames per sec B 1 B 2 B 15 B 16 B 29 B 30 Every ISDN frame the ATM cell is being filled...... 30 bytes of 30 B channels 18 bytes empty ATM Cell 5-bytes header 48-bytes payload Figure 5. Euro- ISDN PRI to ATM data mapping Each ISDN frame contains 30 bytes of user data, which are directly placed in bytes 1-30 of the 48-bytes payload of the ATM cell, leaving the rest 18-bytes empty. This is better to simplify the transmit and the receipt procedures. So the IWU receives 8000 frames per second and transmits 8000 cells towards the ATM user or 8000 x 53 x 8 = 3392000 bps. The delay is equal to one ISDN frame, which is 125 µsec. Regarding, the signalling information the IWU uses a separate buffer to collect the bits of the D channel of the BRI (16 kbps) or the bits of the D channel of the PRI ( slot 16, 64 Kbps). When the receipt of a signalling message is completed, the IWU examines to see what type is it and responds. For instance, if there is a SETUP message, the IWU follows the steps described in paragraph 2, and forms a SETUP message towards the other network. 4. DESCRIBING A MODEL FOR INTERCONNECTION We describe a model of interconnecting two different networks. The idea of having TCP/IP network over ATM network seems to be realistic. This is because in most cases ATM networks are used to provide a high speed backbone for TCP/IP applications. Furthermore in this way we have a clear and robust addressing mode. We use IP addressing instead of using ATM addressing. We have also decide to follow the same concept with the Euro-ISDN network. Figure 6 shows the interconnection between the two different networks.
ATM IWU Application ISDN TCP TCP TCP TCP IP IP IP IP ATM ATM ISDN ISDN Figure 6. An Interworking Scenario REFERENCES [1] G. Kessler, ISDN-Concepts, Facilities and Services, McGraw-Hill, 1990. [2] S. Koubias, L. Mandalos and G. Papadopoulos, An Euro ISDN PRI / ATM INTERWORKING UNIT ARCHITECTURE, Interoperable Communication Networks Journal, special issue on Interoperability of Networks for Interoperable Services, Baltzer Science Publishers, Vol. 1,. 2-4, pp. 445-451, July 1998. [3] L. Mandalos, I. Mountzouris, G. Orphanos, S. Koubias, G. Papadopoulos, Service Interoperability and QoS Negotiation in an Euro ISDN / ATM Interworking Unit, Proceedings of NOC 99, Vol. II, pp. 323-329, Delft, The Netherlands, 22-24 June, 1999. [4] S. Koubias, L. Mandalos, G. Papadopoulos, GENERAL PRINCIPLES FOR INTEROPERABILITY BETWEEN EURO ISDN AND ATM, Proceedings of 8 th IFAC/IFORS/IMACS/IFIP Symposium on Large Scale Systems: Theory and Application (LSS 98), Vol. I, pp. 231-235, Patras, Greece, 15-18 July, 1998. [5] ETSI, ETS 300 012-1, Second Edition, October 1998. [6] ETSI, ETS 300 011-1, Second Edition, March 1998. [7] ETSI, ETS 300 102-1, December 1990. [8] ETSI, ETS 300 102-2, December 1990. [9] ATM Forum, Traffic Management Specification v. 4.0, April 1996. [10] ATM Forum, UNI v.4.0, July 1996. [11] ATM Forum, UNI v. 3.1, September 1994.