ATM BASICS - Chapter 6 pptx

93 Chapter 6 Where is ATM Used Today This chapter presents the most commonly used ATM applications. It identifies market segments as well as operators using ATM infrastructure. The chapter provides basic information referring to the transmission of IP packet over ATM networks. It also covers some of the ATM and Frame Relay interworking aspects. Finally, the reader is given the short introduction to different solutions for transmission of voice services using ATM infrastructure Over last ten years a number of new technologies have become commercial- ly available. The developments in transmission technologies such as the ability to transmit data at different wavelengths (DWDM) combined with the introduction of techniques such as Gigabit and 10Gbit Ethernet changed the market situation. Additionally, the advent of MPLS and advances in QoS based routing in IP network highly influenced the position of ATM. In the public network segment, ATM is important for services delivery, service aggregation and transport. It is a well-defined worldwide standard that has been widely deployed because it works and it is not only for data. This technology is embedded into carrier edge and backbone infrastructures to support enterprise-class applications with high reliability. In fact reliability is the key ATM advantage when compared to all–IP based solutions. Despite the fact that some ISPs and data-only carriers treat ATM as a data transport solution, ATM is actively used to provision voice and TDM services, and handles a significant amount of frame relay and IP services traffic. It is also the primary technology used for DSL traffic aggregation and is used to feed traffic into other service backbones like PON (Passive Optical Network). On the other hand ATM has clearly received a more favorable reception in service provider networks, where its first mission was to provide a scalable core for fast-growing frame relay networks. Large carriers are looking for solutions that let them continue to deploy proven and legacy services generating most revenue, e.g. voice, frame relay and private line. In fact, the capital markets are driving them to this conclusion. At the same time, there are ongoing efforts to prepare the networks for an eventual transition to an IP core. In addition it is important to note that ATM makes a lot of sense for multiservice delivery in access networks, since it has the ability to deliver voice via its CBR service, as well as data via VBR and UBR. ATM remains the most cost-effective method for transporting real-time traffic at OC-12 rates and below. This is proven by the number of wireline and wireless voice carriers who use ATM to transport their voice traffic. Concluding, it is possible to identify a few applications that currently domi- nate as far as the usage of ATM is discussed. The first and probably the one that would decline in next few years is the transmission of IP packet over ATM (IP over TM). The second is ATM and Frame Relay interworking used at large scale by carriers offering Frame Relay services. Last but no least, the growing popularity Voice over ATM solutions would be also covered. There are also many other ATM applications but their market penetration is today relatively small and either they have already found strong com- petitors or they would be replaced soon by solutions mostly based on IP platforms. They include mainly applications using SVC services such as: multi- media, Video Dial Tone and Conferencing Service, Interworking with existing LANs (LANE), Campus/Corporate Enterprise Networks (MPOA). ATM Basics 94 6.1 IP over ATM The initial success of ATM lied largely in its ability to transport legacy data traffic, mostly IP, over its network infrastructure. However due to number of different factors ATM has had a decidedly mixed history in the ISP world. Looking back to the early 1990s, ISPs’ networks consisted of routers inter- connected by leased lines. However, with the growth rate of the Internet ISP network operators were forced to migrate to higher-speed technologies by the mid-1990s. At that time, ATM was available at the higher speed of 155 Mbps, and soon after at 622 Mbps. High-capacity ATM switches were also significantly less expensive than high-capacity IP routers. Consequently, ATM became the backbone technology of choice for most of the world’s large ISPs. Today ATM has lost quite a lot of its attractiveness on behalf of IP and MPLS-based solutions. ATM technology as a way to implement network backbones, is not all that important in the ISP world any more. A few ISPs do use ATM to do traffic engineering (laying down specific paths for traffic aggregates to avoid overloaded network links). A few others use ATM because it can be a cost-effective layer-2 interconnect or because ATM is the only high speed service offering they can get in some locations. But the fact that the underlying technology is ATM layer is only currently relevant to those ISPs that are performing traffic engineering, and with the rollout of MPLS, ATM will become less required. Deeper in the core ATM offers little benefit. MPLS could completely replace ATM in core networking applications, providing that the MPLS players do a better job of building LSRs that are box-for-box evolutions of ATM switches. Here it is important to note that ATM switches can be upgraded to MPLS routers. In fact building MPLS networks using ATM infrastructure is widely concerned as one of the most important methods for quick and efficient MPLS deployment. What is also important, the ATM and MPLS can coexist in the same switching device. This case is referred to as ‘Ships In the Night’. Chapter 6 95 Finally, ATM is an important technology in some access networks, especially in the access portion of the network, where bandwidth and QOS are crit- ical. This particularly refers to DSL. Yet the fact that it is ATM is generally irrelevant—all that is needed is a way to provide virtual circuits to sepa- rate customer connections in order to ensure QoS. Concluding, the ISPs gradually change the position of ATM in their networks. They consider ATM as the technology suitable for the access part of their networks rather the technology for the core network, where ATM is being rapidly replaced with MPLS. ATM Basics 96 Fig. 6-1, LLC/SNAP Encapsulation Over a decade a number of different methods for IP and ATM integration have proposed, standardized and implemented. Some of them are still in use but most of them appeared to be unsuccessful (e.g. LANE, MPOA). Network layer packets can be transported over ATM network in two different ways. Firstly, it is possible to transmit different protocols over the same VC. The protocols are distinguished with the help of additional headers. This method is called LLC/ SNAP encapsulation and it was defined by the IETF (initially in RFC 1483). Secondly, each protocol is carried over a sepa- rate virtual channel. This method is called VC-base multiplexing. The LLC header is placed in front of the PDU that is carried in the payload field of the CS of AAL5. The three-octet LLC header contains information that identifies the protocol of the PDU. If a routed ISO protocol is encapsulated, the PDU follows the header directly and the protocol is identified in the protocol data, using a Network Layer Protocol Identifier (NLPID). When a routed non-ISO protocol is encapsulated, the LLC header is followed by the Subnetwork Attachement Point (SNAP) header. The presence of the SNAP header is indicated in the LLC header. The SNAP header contains two fields (the structure of the SNAP header is shown in Fig. 5.7): the 3-byte Organizationally Unique Identifier (OUI) and the 2-byte Protocol Identifier (PID). A combination of OUI and PID values indicates the particular bridged or routed protocol, such as IPv4 for instance. The simplest and the most popular method for IP and ATM interworking is Classical IP over ATM (CLIP), an IETF standard for internetworking IP and ATM, firstly described in IETF RFC 1577. Currently CLIP is defined in RFC 2225. CLIP was the first method of internetworking IP and ATM, and was developed at a time when ATM technology was immature. CLIP deploys an ‘overlay model’, which means, that CLIP effectively ignores ATM’s proper- ties, treating it as a transmission technology or as simply underlying ‘wires’ used to carry IP packets. CLIP preserves the classical model of IP routing, that is, the end-to-end IP routing architecture stays the same. Simply stat- ed, this means that traffic wanting to go from one IP subnet to another IP Chapter 6 97 subnet has to go through an IP router with all the consequences. CLIP oper- ates in much the same way as IP over Ethernet. In the case of CLIP, it is necessary to map destination IP addresses to destination ATM addresses in case of SVC connections, or in a PVC environment, to map IP addresses directly to VPI/VCI values. In an Ethernet network the task of mapping IP addresses to hardware addresses is assigned to ARP (Address Resolution Protocol), which extensively makes the use of Ethernet broadcasting capa- bilities. In CLIP model a modified version of ARP, called ATMARP, is used to accomplish address mapping. The CLIP architecture varies depending on the type of virtual connections. In SVC environment (shown in the Fig. 6-2) an ATMARP server must used as the mechanism for hosts to resolve destination IP addresses to destination ATM addresses. An ATMARP server provides its services within the Logical IP Subnet (LIS). The LIS is used to refer to an IP subnet deployed over an ATM network as part of CLIP. Any transfer of IP packets directed to a host outside the LIS must involve an IP router. This is a must even though the two hosts can be the members of the same ATM network. In CLIP model a host that wishes to send data to another host must register its network and ATM addresses in the ATM ARP server. Then it can issue a query to the server to obtain the destination ATM address for a given IP address. Once the server resolves and IP address into ATM address, a host can establish a direct SVC to the destination host. ATM Basics 98 CLIP model in a PVC environment, which is more popular nowadays, assumes that the operation of mapping between destination IP addresses and VPI/VCI values can be done without the use of any server. However, ATM ARP messages are used to check the destination ATM addresses for a host on its open virtual connections. Additionally, manual dimensioning of PVCs can be treated as the rudimentary mechanism for traffic engineering especially for networks of well-defined and stable architecture. This approach is used, for instance, by GSM/GPRS operators within their IP backbone networks provided ATM is the underlying technology. Chapter 6 99 Fig. 6-2, Classical IP over ATM in SVC environment 6.2. Voice over ATM The major source of revenue for most operators around the world is tele- phony services. Voice services generate over $200 billion in U.S. revenues and it is likely to remain so for the coming years – while the main source of growth will be IP-based services. This means that networks have to be opti- mized for carrying packet-oriented traffic, but at the same time they must deliver a reliable and high quality voice service. Voice is considered by most to be a commodity, and ATM has played a role in lowering the cost of service for some carriers. Although most recent innovations in voice services are based on IP and MPLS, and end-to-end voice over IP architectures promise far greater cost reductions than result from simply using ATM for bulk transport. The ATM is continuously considered as the mature and reliable technology. This point of view is especially typical to large incumbent telecom operators who were involved in the ATM standardization process. They are therefore generally in favor of this technology, since it offers a safe migration path for them. 6.2.1 Circuit Emulation Services Circuit Emulation (CE) represents today a stable and reliable standard, which has been widely implemented by ATM equipment suppliers. The CE function enables existing TDM circuits to be mapped over ATM. CE thus give the chance to migrate an existing TDM network to ATM whilst pre- serving the previous investment in TDM equipment. Circuit Emulation products are available for all major circuits, including the American T1 and European E1 standards. When using CE the ATM network simply provides a transparent transport mechanism for structured TDM circuits. Voice is encoded into these links as in a normal TDM network (e.g. PSTN) using PCM, ADPCM, or other encoding & compression mechanisms. The network will ensure that the delivered circuit is reconstructed exactly as received. Circuit Emulation uses the AAL1 mechanism to segment the incoming E1 or T1 traffic into ATM cells with the necessary timing information to ensure that the circuit can be correctly reassembled at the destination. The Fig 6-3 shows the typical CE application. ATM Basics 100 In the figure an incoming E1 circuit is plugged into the ATM switch. The ATM switch then performs the CE function and transmit cell using the CBR VCC. The interworking between TDM and ATM standards may be achieved in a number of ways. It may be the case that the ATM switch has a special CE module, which is a plug in board with E1 or E1 interfaces and Application Specific Integrated Circuits (ASICs) to perform the CE function. Furthermore, the ATM switch may have an E1 or T1 interface cards and the CE function may be a software function running on the main switch proces- sor. It is also feasible to for the PBX (or similar TDM equipment) to have an ATM interface board. In this case the voice samples are sent out within ATM cells from the PBX towards the ATM switch. Chapter 6 101 Fig. 6-3, Architecture for CES CE can be deployed in two different modes: structured and unstructured. In unstructured CE mode the network does not attempt to recognise the internal circuit structure. Rather it simply transmits the entire circuit across the network. At the destination the original stream of bits is reconstructed. This method doesn’t allow for accessing particular TDM voice channels repre- sented by time slots within ATM network. Hence, the unstructured CE car- ries the entire circuit across the ATM network with no recognition of the internal framing structure. This unstructured service can emulate for instance a 2 Mbps data leased line. In structured CE mode the ATM network recognises the internal structure of the circuit and is able to recover this structure at the receiving end. Circuit structure refers to the timeslots. E1 for example, contains 32-times- lot frames, whereas T1 contains 24 timeslots. An example of structured CE is given in the Fig. 6-2. In this example single timeslots are extracted from the source E1 and mapped to an ATM cell. This means that particular timeslots may be mapped to different virtual circuits and consecutively to different destinations. Several timeslots from a source circuit may be mapped to one virtual circuit. It can be easily seen in the example that all time slots no 1 from each frame is mapped to an ATM cell. Therefore, depending on the TDM circuit type certain latency is inevitably introduced. In the worst case 47 time slots representing a voice connection must be col- lected before the cell can be created. This can highly influence QoS observed by the applications. In order to overcome this limitation a partial fill mechanism can be used, which means that cell recognised as a long distance or international call can be heavily padded. Another option can be to map several timeslots of different voice connections into one cell and therefore one VC. ATM Basics 102 [...]... CPCS, as well as FR-SSCS The IWF function is responsible for all mapping and encapsulation functions that are necessary to ensure that FR-TE does not have to be upgraded to ATM The scenario 2 assumes the use of ATM CPE equipped with FR-SSCS, which communicate with FR-TE by means of the IWF (like in Scenario 1) Fig 6- 7 , Frame Relay and ATM PVC Network Interworking (Scenario 1) 109 ATM Basics The second... Switched trunking using ATM VCCs provides high bandwidth efficiency within the ATM network The idea of switching at the level of AAL 2 circuits, which involves swapping of CID, is given in the Fig 6- 3 It is important to note that any node capable of switching of AAL 2 circuits must have the basic functionality of an ATM switch Fig 6- 5 , AAL2 switched trunking model 1 06 Chapter 6 If the network can route... and ATM network interworking, •BulFrame Relay and ATM service interworking 108 Chapter 6 In first method, Frame Relay is transparently transported over high speed ATM network Frame Relay users are not aware of the ATM present in the core The Fig 6- 7 presents the configuration called Scenario 1 that described in FRF.5 document In this scenario, which is also referred to as Frame Relay Transport over ATM, ... Frame Relay and ATM The model for this method is given in the Fig 6- 8 This method of interworking is used whenever a Frame Relay customer transmits and receives data from an ATM user The architecture is based on the idea that none of the customers needs to perform functions related to external technology Fig 6- 8 , Frame Relay and ATM Service Interworking (Translation Mode) 110 Chapter 6 All the translation.. .Chapter 6 Fig 6- 4 , Structured mode of CES CES’s advantages are the simplicity of implementation; the ATM network is used to provide virtual replacements for physical links in an existing network CES provides an ideal stepping-stone from legacy TDM networks to full ATM- enabled broadband solutions The simplest form of CES exhibits a few... accepted to be approximately 20 nodes), it is very costly to develop a workable architecture without some form of voice switching and routing Fig 6- 6 , The potential scenario for deployment of AAL 2 switched connections 107 ATM Basics To conclude, a major benefit of ATM trunking using AAL2 for narrowband services is bandwidth savings This objective can be achieved thanks to the use of voice compression techniques... Interworking, the ATM CPE has no knowledge that the distant device is connected to a Frame Relay network Therefore, the use of FR-SSCS is precluded ATM and Frame Relay can be executed in two modes: in Transparent Mode and in Translation Mode The latter method allows for solving the problem of incompatibilities between encapsulation methods used in ATM (RFC 1483) and in Frame Relay (RFC 1490) 111 ATM Basics 112... ATM interfaces 104 Chapter 6 The CES concept has been implemented in a number of solutions designed with the intention to attract incumbent network operators such as PTTs who wish to migrate their networks towards and offer new services They can benefit from the savings imposed by the use of multiservice platforms while offering wide range of legacy services which still generate most profit 6. 2.2 ATM. .. equivalents per port speed) was roughly the same for ATM and Frame Relay due to higher port speed mix for ATM Revenue generated in that year was about 4 times bigger for FR comparing to ATM Even today the revenue generated by FR is higher Hence, there is still the need for solutions for interworking between ATM and Frame Relay There exist a number of methods for ATM and Frame Relays interworking In fact the... Firstly, with CES it is not possible to provide any statistical multiplexing The ATM network does not differentiate between idle and active timeslots of TDM, this means that idle traffic/time-slots are carried some network capacity is wasted Secondly, everyone using CES must realize that simple CES voice trans- 103 ATM Basics port consumes about 10% more bandwidth (due to the ‘cell tax’) than it would . 93 Chapter 6 Where is ATM Used Today This chapter presents the most commonly used ATM applications. It identifies market segments as well as operators using ATM infrastructure. The chapter. Fig. 6- 3 . It is important to note that any node capable of switching of AAL 2 circuits must have the basic functionality of an ATM switch. Fig. 6- 5 , AAL2 switched trunking model Chapter 6 107 If. resulting data streams structured in AAL 2 connections are then merged into a single sequence of cells Chapter 6 105 ATM Basics 1 06 so that they can be transmitted over a single ATM virtual circuit.