ATM BASICS - Chapter 7 doc

113 Chapter 7 How is ATM Changing This chapter discusses the evolution of ATM concepts. A special notice is given to physical interfaces, QoS categories and evolution of methods for ATM and IP interworking. The chapter helps to understand the implications of radical changes that take place at the global market nowadays. Some future trends for ATM evolution are also presented. ATM has evolved over a number of years reflecting the changes in technolo- gies and customer demands. This evolution involved the development of new concepts but it also complemented initial ideas. It is very difficult to briefly present all the changes that ATM observed within last 13 years. However, it is possible to identify major categories in which the changes were the most significant: •Physical interfaces. With the time passing by, a number of new interfaces have been standardized. The key factor was obviously the need for ensuring high scalability and flexibility of ATM solutions. The new inter- faces were introduced along with technological achievements, especially in the domain of optical transmission. Hence, the transmission speed has evolved from 155,52 Mbps initially up to 1Gbps. At the same time, with regards to market needs a number of low speed interface specifications were released. They include for instance: transmission with the speed of 25,6 Mbps over UTP, ATM over Fractional Links, Inverse Multiplexing For ATM, Frame-based ATM Interface. •Signaling and routing protocols. Initially, ATM allowed only for the use of permanent virtual connections (PVCs). With the introduction of UNI 3.0 it became possible to establish switched connections (VCs). This implied the need to define signaling protocols for the use within ATM net- works. Hence, the protocols such as IISP and next PNNI were designed. Finally, to allow for network interworking there was a need for appropriate protocols such as BICI and AINI. •Traffic management. This category covers all the QoS related issues. With the time passing by and changing requirements a few new QoS categories (service classes) have added. Additionally, some TM mechanisms have been revised and updated. •Interworking with other technologies. It has to be noted that has clearly defined rules for interworking with almost any other transmis- sion and networking state-of-the-art technologies. One can enumerate here concepts such as ATM and IP interworking, LAN Emulation, ATM and FR interworking. •ATM applications and services. Although ATM has been designed to serve for data, voice and video applications, initially data trans- mission was the application of highest interest, mostly due to the techno- logical and marketing issues. A few years’ later standards describing both voice (e.g. Circuit Emulation Services) and video (Video on Demand) were completed and released. Today the work in this area is concentrated on the voice transmission with the use of AAL2. Some developments in the three of those areas will be discussed in more details. The strength of ATM is related to its flexibility and scalability. And this is why, that even today, there are still ongoing efforts to make ATM technology ‘friendly’ and open to new and innovative concepts. This process can clearly noticed with regards to ATM and MPLS interworking. ATM Basics 114 7.1 Inverse Multiplexing for ATM As it was stated earlier, ATM has become today an important technology in some access networks, especially where bandwidth and QOS are the critical factors. The variety of interfaces has been actually limited to the E1/DS1 and E3/DS3. The similar situation was relevant to Frame Relay users for frame relay users who wanted bandwidth between T1 and T3. In addition to that a rapid growth of mobile telephony networks stimulated the market need for solutions capable of provisioning bandwidth at the level of several Mbps in a cost efficient way. In order to face these challenge, a new concept called Inverse Multiplexing for ATM has been invented. The idea of IMA actually goes far beyond the definition of the physical interface. The major purpose of IMA is to provide inverse multiplexing of an ATM cell stream over multiple physical links so that the effective capacity observed by the ATM layer is the multiplication of the single link capacity. At the far end of these physical links IMA retrieves the original stream without breaking the sequencing of cells. The IMA model is given in Fig. 7-1. Chapter 7 115 Fig. 7-1, The IMA Concept This specification is offered for private and public interfaces. IMA defines a new sublayer located between the TC sublayer and the ATM layer: the IMA sublayer. This specification also defines some modifications to the TC sub- layer on which the IMA sublayer is implemented. What are the key benefits of implementing IMA? First of all, the ability to ‘construct ’ a physical link of desired capacity out of low speed links. This allows for filling the gap between E1/DS1 and E3/DS3 interfaces. IMA involves inverse multiplexing and de-multiplexing of ATM cells in a cyclical fashion among links grouped to form a higher bandwidth logical link whose rate is approximately the sum of the link rates. This entity is referred to as an IMA group. IMA groups terminate at each end of the IMA virtual link. In the transmit direction, the ATM cell stream. At the transmitting side, incoming cells are distributed on cell by cell basis over a number of links within the IMA group. At the receiving side, the IMA software recon- structs the cell order and passes cells to the ATM layer. IMA uses the con- cept of IMA frames, which are signaled with the help of control cells called ICP (IMA Control Protocol) cells. Needless to say, IMA ahs attracted the attention of many operators. This solution has been widely implemented in ATM devices in recent years 7.2 ATM over ADSL ATM has been widely implemented in the access networks that use ADSL technology. The exact model for the use of ATM in ADSL network architec- tures is described in the ‘ATM over ADSL Recommendation’ issued by the Asymmetric Digital Subscriber Line Forum in 1999. The reference model for ATM over ADSL is given in the Fig. 7-2. According to this model, ATM can be used at the following interfaces: •The V interface that connects the Core Network and Access Node (AN). The Access Node serves as an ATM layer multiplexer/concentrator between the Core Network and the Access Network. In the downstream direction it may perform routing and demultiplexing, while in the upstream direction it may perform multiplexing and concentration and higher layer functions. ATM Basics 116 •The U interface that connects individual interfaces in the remote Broadband Network Termination (B-NT) T to the corresponding interfaces in the Access Node. The B-NT is a functional block that performs the func- tions of terminating the ADSL signal which enters the user’s premises via the twisted pair cable and providing either the T, S, or R interface towards the Terminal Equipment (TE). Chapter 7 117 Fig. 7-2, The reference model for ATM over ADSL For the transport of ATM on modems compliant with the ADSL PHY Recommendations, channels, which are in fact Virtual Paths and/or Virtual Channels, shall be independently set to any bit rate that is an integer mul- tiple of 32 kbps. The upper limit, a maximum aggregate capacity, can be determined at the start-up phase. The bit rates for each channel can be set independently for the upstream and downstream direction. 7.2 Evolution of QoS service classes 7.2.1 GFR Originally all the best-effort traffic, including transmission of IP packets, has been assigned to the UBR QoS category. However, the handling of UBR traffic in ATM networks imposes certain disadvantages for some applica- tions. To overcome these limitations ATM Forum defined the Guaranteed Frame Rate (GFR) QoS category. The GFR service category is intended to support non-real-time applications generating large portions of data and may require better than best effort. It is designed for applications that may require a minimum rate guarantee and can benefit from accessing addition- al bandwidth dynamically available in the network. From this point of view, GFR is similar to the ABR concept but it does not require adherence to a flow control protocol. The service guarantee is based on AAL-5 PDUs, here referred to as frames to and, under congestion conditions, the network attempts to discard complete frames instead of discarding cells without ref- erence to frame boundaries. This approach may increasingly improve the performance observed by some of the ATM applications. The GFR category provides the user with a Minimum Cell Rate (MCR) guarantee under the assumption of a given maximum frame size (MFS) and a given Maximum Burst Size (MBS). The user may always send cells at a rate up to PCR, but the network only promises to carry cells in complete frames at MCR (simi- larly to cells in ABR service category). Traffic in excess of MCR and MBS will be The user can send frames either unmarked or marked by setting up the CLP bit. An unmarked frame is one in which all cells have CLP=0. A marked frame is one in which all cells have CLP = 1. Such a frame has lower priority The CLP must be the same for all cells in a frame. By sending a ATM Basics 118 frame marked, the user indicates to the network that such a frame is of less- er importance than an unmarked frame. As oppose to the ABR model, the GFR service category does not give to the users explicit feedback regarding the current level of network congestion. Note that GFR service category only applies to virtual channel connections, because frame delineation is not generally visible in a virtual path connection. The GFR mechanisms allow users to expect a minimum service rate when the network is congested, while being able to send at a higher rate when additional resources are available. However, this can be achieved without he complex rate-based congestion control deployed in case of the ABR service category. 7.2.2 Differentiated-UBR QoS has become in recent years an important issue not only within ATM networks. It has become increasingly common for technologies that tradi- tionally offered only best effort service (e.g., Ethernet, IP) to incorporate ser- vice differentiation mechanisms. In fact, two different models for provision- ing of QoS at the IP level were designed and tested. The first model is called the IntServ (Integrated Services) assumes identification of single flows and separate reservation of network resources for every flow. The model has been in use several years but its deployment is constrained due to the sig- naling load it can introduce in certain parts of an IP network. The second model is called DiffServ and is based on the opposite approach. DiffServ classifies the traffic at the edge of an IP network, according to the SLA con- ditions and applies adequate treatment to every IP packet transported with- in an IP network. The packets are assigned to a number of specific classes called Behavior Aggregates, which implies the way they are handled within the network. The QoS is provided by means of appropriate queuing tech- niques. DiffServ is growing in popularity as it also complements mecha- nisms provided by MPLS. Therefore, there is a need to transport IP traffic, which is already assigned to classes defined by DiffServ. What is more important, the traffic should observe adequate treatment in an ATM net- work. Chapter 7 119 In order to optimize the support of differentiated services over ATM, it is desirable that the UBR service category be extended with similar capabili- ties. The Addendum to TM 4.1, issued by ATM Forum defines a mechanism by which a UBR connection may be associated with one of a set of network specific Behavior Classes. When this mechanism is used, the adaptation function may assign traffic to a UBR connection, based in part on a mapping of the higher-layer service classification to the Behavior Class with which that UBR connection is associated. In result the ATM network is able to dif- ferentiate the treatment of UBR connections based on the Behavior Class with which each such connection is associated (e.g. as the result of DiffServ classification process), for example by governing access to queuing and scheduling resources. By coordinating the adaptation function at the edge of the ATM network with the behaviors implemented within the network, an operator can enable consistent service differentiation on end- to-end basis, thus ensuring QoS for the transmitted traffic. 7.3 The evolution of methods IP and ATM interworking Transmission of network layer packets, and especially IP packets, was the most important application in early years of ATM. In fact it stimulated the development of the ATM technology in terms of standards and switching devices. The combination of high speed offered by ATM switches with traf- fic engineering capabilities were the key factors attracting ISPs and carri- ers. The large number of fundamental differences between ATM and IP makes the interworking between these two technologies a challenging task. Over last ten years several methods have been proposed and implemented. The evolution path for these methods is given in the Fig. 7.3. ATM Basics 120 The first standard, which preserved the classical model of routing, was introduced by the IETF in 1993 as Classical IP over ATM. As a part of this standard, the LLC Encapsulation technique was proposed to facilitate transmission of IP packets using AAL 5 and ATM. Limitations of the CLIP model, mostly related to the lack of support for QoS, led to the development of another standard prepared this time under auspicious of the ATM Forum. LAN Emulation was much better suited for the use in LAN environment and was designed to enable smooth migration from legacy LANs to ATM- based architecture. For the price of complexity (a few servers needed per each Emulated LAN) and some configuration effort the end users were given the ability to built large and flexible VLANs (Virtual LANs) over ATM infra- structure and benefit from QoS. Chapter 7 121 Fig. 7-3, The evolution path for methods for IP and ATM interworking Unfortunately, both models preserved the classical model of routing, which highly constrained the benefits coming from the use of ATM as the underly- ing technology. Hence, further improvements were proposed, namely Next Hop Resolution Protocol (NHRP) that was defined by the IETF. With the NHRP, directly connected hosts and routers received the ability to resolve IP addresses of the devices located in other subnetworks. In result direct VCCs, often called ‘shortcut’ connections, could be established on end-to-end basis. Finally, the Multi Protocol Over ATM (MPOA) combined LANE, NHRP and the concept of a Virtual Router. However, the complexity of MPOA implementations caused that this solution was installed at a rela- tively small scale. At the same time, when MPOA was being standardized, a number of vendors accomplished their works on proprietary solutions. These solutions instead of defining new protocols and implementing more servers were focused on the hardware aspects of ATM and IP interworking. Four of those solutions laid the basis for the definition of MultiProtocol Label Switching (MPLS). The development of MPLS standards started in 1997 at the IETF and has not been completely accomplished yet. MPLS rep- resents a totally different idea for using ATM equipment as the underlying technology for IP transmission. It is important to note that once ATM equipment is used in MPLS network, it doesn’t need to use any ATM addresses or any ATM signaling. MPLS can benefit from the switching capa- bilities of ATM equipment. ATM Basics 122 [...]... 74 , 76 , 8 0-8 1, 8 3-8 4, 87, 89, 94, 9 6-9 8, 10 0-1 06, 110, 11 5-1 20 ATM Adaptation Layer (AAL), 9-1 0, 3 2-4 7, 50, 62, 70 , 72 , 85, 91, 10 4-1 09, 121 ATM Addressing, 53, 7 9-8 0, 83 ATM End System Addressing (AESA), 80 ATM Forum, 27, 6 0-6 2, 74 , 78 , 80, 84, 9 0-9 1, 10 4-1 05, 118, 12 0-1 21 ATM Inter Network Interface (AINI), 9 0-9 1, 114 Authority and Format Identifier (AFI), 81 Available Bit Rate (ABR), 61, 70 , 77 78,... 52, 57, 101, 105 Virtual Channel Identifier (VCI), 18, 23, 2 5-2 7, 46, 50, 52, 79 , 8 4-8 5, 87, 9 8-9 9 Virtual Connection (VC), 14, 20, 2326, 4 9-5 0, 52, 55, 63, 67, 69, 74 , 85, 97, 102 Virtual Container (VC), 2 4-2 5, 52, 69, 85, 97, 102 Virtual Path (VP), 1 7- 1 8, 2 3-2 5, 51, 73 , 119 Virtual Path Connection (VPC), 5152, 57, 119 Virtual Path Identifier (VPI), 1 7- 1 8, 23, 2 5-2 7, 46, 5 0-5 2, 79 , 8 4-8 5, 87, 9 8-9 9.. .ATM References [1] af-bici-0013.003 B-ICI 2.0 (ATM Forum 12/95) with B-ICI 2.0 Addendum (12/1996) [2] af-cs-0125.000 ATM Inter-Network Interface (AINI) Specification (ATM Forum, 08/1999) [3] af-ilmi-0065.000 ILMI 4.0 (ATM Forum, 09/1996) [4] af-lane-0021.000 LAN Emulation over ATM 1.0 (ATM Forum, 01/95) [5] af-lane-0084.000 LANE v2.0 LUNI Interface (ATM Forum, 07/ 97) [6] af-mpoa-0114.000 Multi-protocol... Rate (PCR), 6 9 -7 1, 7 4 -7 5, 78 , 104, 118 Permanent Virtual Connection (PVC), 31, 5 0-5 4, 71 , 9 8-9 9, 109 Physical layer, 2, 9-1 1, 14, 28, 91 PNNI Augmented Routing (PAR), 89 PNNI Group Leader (PGL), 89 Protocol Reference Model (PRM), 910, 20, 32, 92 Quality of Service (QoS), 1, 3-5 , 7, 25, 41, 50, 53, 5 9-6 4, 6 6 -7 3, 75 , 77 , 85, 88, 93, 96, 102, 11 3-1 15, 11 8-1 21 Resource Management (RM), 7 7- 7 8 Segmentation... Control (CAC), 63, 7 2 -7 3, 85 Constant Bit Rate (CBR), 34, 6 1-6 2, 70 , 7 3 -7 5, 77 , 94, 101, 105 Cell Delay Variation (CDV), 6 5-6 7, 71 Cell Delay Variation Tolerance (CDV), 6 6-6 7, 71 Cell Delineation, 12, 22 Cell Error Ratio (CER), 68 Cell Loss Priority (CLP), 19, 7 5 -7 6, 118 Cell Loss Ratio (CLR), 6 3-6 4 Cell Misinsertion Rate (CMR), 69 Cell Rate Decoupling, 12 Cell Transfer Delay (CTD), 6 4-6 7 Channel ID (CID),... VLAN - Virtual LAN VoATM - Voice over ATM VoIP - Voice over IP VPC - Virtual Path Connection VPI - Virtual Path Identifier VPN - Virtual Private Network 131 ATM Basics 132 Index Address Resolution Protocol (ARP), 9 8-9 9 Administrative Authority (AA), 8283 Application Program Interface (API), 71 Access Node (AN), 1, 3, 13, 16, 1 9-2 1, 2 5-2 8, 3 0-3 3, 37, 39, 41, 4 3-4 4, 4 6-4 7, 49, 5 2-5 3, 55, 6 0-6 4, 66, 6 8-6 9,... Forum, 07/ 00) [11] af-tm-0121.000 Traffic Management Specification v 4.1 (ATM Forum, 03/99) [12] af-tm-0149.000 Addendum to TM 4.1:Differentiated UBR (ATM Forum, 07/ 00) [13] af-vtoa-0 078 .000 Circuit Emulation Service 2.0 (ATM Forum 01/19 97) [14] af-vtoa-0085.000 rum (DBCES) Dynamic Bandwith Utilization in 64 KBPS Time Slot Trunking Over ATM - Using CES (ATM Forum, 08/19 97) [15] af-vtoa-0113.000 ATM Trunking... AF - ATM Forum AINI - ATM Inter-Network Interface AN - Access Node ASIC - Application Specific Integrated Circuits ATM - Asynchronous Transfer Mode B-ISDN - Broadband Integrated Services Digital Network BICI - Broadband Inter-Carrier Interface BISUP - B-ISDN User Part B-NT - Broadband network Termination CAC - Connection Admission Control CBR - Constant Bit Rate CCS - Common Channel Signaling CDV -. .. Narrowband Services (ATM Forum, 02/1999) 123 ATM Basics [16] af-ra-0104.000 PNNI Augmented Routing (PAR) 1.0 (ATM Forum, 01/1999) [ 17] af-ra-0105.000 Addressing: User Guide Version 1.0 (ATM Forum, 01/1999) [18] af-ra-0106.000 Addressing: Reference Guide Feb, (ATM Forum, 02/1999) [19] ITU-T G.804 ATM cell mapping into Plesiochronous Digital Hierarchy (PDH) [20] ITU-T I.3 57 B-ISDN Semi-Permanent Connection... Variation CDVT - Cell Delay Variation Tolerance CER - Cell Error Ratio CE - Circuit Emulation CES - Circuit Emulation Services CID - Channel ID CLIP - Classical IP over ATM CLP - Cell Loss Priority CLR - Cell Loss Ratio CMIP - Common Management Information Protocol CMR - Cell Misinsertion Rate CP - Common Part CPE - Customer Premises Equipment 1 27 ATM Basics CPI - Common Part Indication CRC - Cyclic Redundancy . over ATM 1.0 (ATM Forum, 01/95) [5] af-lane-0084.000 LANE v2.0 LUNI Interface (ATM Forum, 07/ 97) [6] af-mpoa-0114.000 Multi-protocol Over ATM Specification, Version 1.1 (ATM Forum, 05/99) [7] af-pnni-0055.001. Line AESA - ATM Endpoint System Address AF - ATM Forum AINI - ATM Inter-Network Interface AN - Access Node ASIC - Application Specific Integrated Circuits ATM - Asynchronous Transfer Mode B-ISDN - Broadband. v 4.1 (ATM Forum, 03/99) [12] af-tm-0149.000 Addendum to TM 4.1:Differentiated UBR (ATM Forum, 07/ 00) [13] af-vtoa-0 078 .000 Circuit Emulation Service 2.0 (ATM Forum 01/19 97) [14] af-vtoa-0085.000