49 Chapter 3 Virtual Circuits The actual service offered by means of ATM technology is connection-ori- ented. Although connections in ATM are implemented internally with pack- et switching, the end user is provided with virtual circuits. From this chap- ter the reader may learn different types of virtual circuits that are in use in contemporary ATM networks. The chapter presents the general concept of permanent and switched connections as well discusses methods for setting them up, maintaining and releasing them. As it was presented earlier, the basic B-ISDN services require a compromise between pure packet switching and pure circuit switching. The actual ser- vice offered within an ATM network is connection oriented, but internally it is implemented with packet switching, not circuit switching. The term ‘con- nection’ is used to name the logical circuit that is set-up between end-users having physical links to ATM network. A virtual connection is a concatena- tion of logical links between neighboring ATM switches. It is not defined by means of a time slot or a physical pair of wires but it is defined by table entries inside ATM switches. The term ‘virtual’ is an equivalent word for logical as far as ATM circuits are discussed. Today, a number of connection types are offered: permanent virtual connections, switched virtual connec- tions, and soft-permanent virtual connections. 3.1. Permanent Virtual Connections Historically, at the early stage of ATM deployment only permanent virtual connections (PVCs) were available. The simplest method for the establish- ing of a virtual connection in ATM is the exact configuration of parameters such as VPI, VCI, QoS category, AAL type and others. In large networks this function is implemented within a centralized management system (e.g. Network Management System). PVCs are requested by the customers man- ually (e.g. by contacting the carrier directly with a fax) and in most cases remain in place for several days or months. Permanent connections are equivalent to leased lines available in PSTN infrastructure. Sometimes they are even called virtual leased lines (VLL). When a failure occurs in a net- work and the service is unavailable there is no need to reestablish the con- figured PVCs. Once the problem in a network is solved all configured PVC are in place. Permanent connections are removed manually or with the help of the management center. PVCs require some manual configuration to set up the records in a switch map via the use of the management system. This is shown in the Fig. 3-1. This type of the virtual connection does not require dynamic call control (involving signaling processes), but the bandwidth management is still necessary. The release of a PVC again involves manual operation with the help of the management system. The greatest advantage of a PVC is that neither ATM addresses nor ATM signaling is needed for the setup of a PVC. Edge devices, typically located at the customer premises, could be built in a simpler and cheaper way. However, there are also some disadvantages. The bandwidth is guaranteed all the time despite the periodical lack of traffic generated by the user (i.e. at weekends). If a topology of ATM network changes, PVC must be manual- ly reconfigured. In result PVCs are not flexible from the operator’s point of view as long as rapid changes take place in the ATM network. Needless to say, ATM PVCs are usually far more expensive than other types of virtual circuits. A number of ATM applications can be deployed using PVC concept. The most popular applications are Classical IP over ATM (CLIP), Cell Relay Service (CRS), Frame Relay Service (FRS). PVCs as Private Line DS1/DS3 services can be also supported using Circuit Emulation Service. ATM Basics 50 There are two types of Permanent Connection that may be established in ATM switches: Virtual Path Connections (VPCs) and Virtual Channel Connections (VCCs). These connections normally allow cells to flow in both directions and are established by the operator. No signaling is required between the connection user and the ATM switch. A VPC is a connection where routing is performed based on the value of the VPI of each cell. This type of the PVC is composed out of a number of Virtual Path Links (VPLs), which are connected together by cross-connections. VPLs are identified by a unique VPI value. This is a local identifier and may be different for each VPL in the VPC. At each end of a VPC there are VPC end points. A VPC can contain a number of VCCs. These VPC end points are normally associated with the customer equipment. Often the term Virtual Path (VP) is used instead of Virtual Path Connection (VPC) since both terms have the same meaning. Chapter 3 51 Fig. 3-1, Permanent Virtual Connections A VCC is a connection where routing is performed based on the value of the VPI and VCI of each cell. A VCC consists of a number of Virtual Channel Links (VCLs), which are connected together through a number of switches. VCLs are identified by a unique VCI. This is a local identifier and may be different for each VCL in the VCC. The VCC connects together two VC end points. These VC end points are normally associated with the customer equipment. VCLs require VPC end-points to be defined for them. These VPC end-points are needed because VCs need to be assigned to, or belong to a VPC. Often the term Virtual Channel (VC) is used instead of Virtual Channel Connection (VCC) as both terms have the same meaning. Some of the constraints that limit PVC flexibility can be overcome with some modifications simplifying the setup process. With the ordinary PVC a customer will pay for the whole period of time when PVC is set up since the resources in the network are reserved as long as the connection is open. However, the operator may offer a modified version of a PVC service called Scheduled PVCs, which will be active only within agreed hours. The opera- tor can schedule the activation of all types of permanent cross-connection at an appointed time in the future. The operator can create a completely new cross-connection, that is to be activated at an appointed time, or schedule an already established cross-connection to be activated or deleted at the appointed time. How can a customer benefit from using a SPVC service? For instance, one can imagine a customer who wishes to needs ATM connectivi- ty only within office hours on working days (i.e. from Monday to Friday). In result the customer will be charged relatively less than for an ordinary PVC and the operator will be able to offer unused network capacity to other cus- tomers. Generally, PVC cannot be seen as a preferred method for large scale networks because the manual configuration is highly constrained in the case when several thousands or even millions of subscribers have to be managed simultaneously. ATM Basics 52 3.2. Switched Virtual Connections Some services use ATM infrastructure occasionally, so there is no need to reserve for them network resources on permanent basis. So while PVC ser- vices were introduced into the network first, the ultimate goal was to sup- port switched virtual connections (SVCs). While PVC service can be com- pared to a leased line, a SVC service is similar to a telephony call. This is why SVCs are also referred to as signaled virtual connections. In order to dynamically setup a connection on user demand, an ATM address and a sig- naling process must be used. SVCs are established for the period of trans- mission of data between ATM end devices. The primary objective is to increase the efficiency of network resources and consecutively lower the cost since users can be charged for the duration of the connections time. SVCs can be established between parties that are equipped with ATM addresses and can communicate with signaling protocols. In reality, before the con- nection is established a number of complex signaling operations must be executed. Resources in ATM network in SVC environment are reserved dynamically so in order to setup a connection with guaranteed bandwidth and QoS, the optimal route has to be discovered and signaled. Therefore, in addition to a signaling process a routing component must be also used. SVC services provide more opportunities and apply to more applications. In case the network is unable to satisfy the user, the call is rejected. Needless to say, the deployment of SVC service in a network requires that advanced traffic management functions must be executed to ensure the bandwidth and QoS for customers. The same signaling protocol stack is used to setup, maintain and release connections. If a serious network failure occurs, SVC must be reestablished by users, so again the behavior of an SVC is similar to a phone call. From the user perspective SVC services have two important disadvantages. First of all, they involve ATM addressing and complex signaling, which may increase the cost of user equipment when compared to PVC only capable devices. Secondly, due to the dynamics of ATM networks in certain moments requested service may appear to be unavailable due to current traffic load conditions. Chapter 3 53 Switched connections can be used by applications such as: multimedia transmission (integrated voice and video), Video Dial Tone and Conferencing Service, Interworking with LANs and IP (CLIP, LANE, MPOA), Frame Relay. 3.3. Soft Permanent Virtual Connections The opposite capabilities of permanent and switched connections led to the development of the concept that combines these two approaches. Soft Permanent Virtual Connections (SPVCs) can be seen from the perspective of the end user as a PVC. The end user can skip from signaling process and the resources in the nearest switch are reserved permanently. As long as the ATM Basics 54 Fig 3-2 Switched Virtual Connections user does not generate any traffic, no virtual connection exists within the ATM network. If the nearest switch detects the actual transmission from the user, it triggers the process similar to the setup of an SVC. This process is executed only within the network and the destination is not involved. Once the user terminates his transmission, resources in the network are released and the connection within the network is removed. The approach represented by SPVCs combines the simplicity of permanent connections and high flexibility of switched connections. Chapter 3 55 Fig. 3-3, Soft Permanent Virtual Connections 3.4. Point-to-Multipoint Connections There are some applications that require multicasting capabilities from the transmission network. Hence, multipoint connections are also a feature of ATM networks. They are used for instance in all ATM LAN interworking techniques. They are the most important feature of broadcast networks such as those providing video on demand. The concept of multicasting in ATM is completely different from models present in Ethernet and IP. Instead of using multicast addresses, ATM uses the special class of virtual circuits, which are called point-to-multipoint connections. These connections origi- nate in a node that plays the role of a root. All the end devices at which the point-to-multipoint is terminates are called leaves. The support of point-to- multipoint connections requires that any ATM switch at which at the con- nection has got more than two legs must replicate cells belonging to this con- nection. Hence, ATM switches are considered as replicating points. Note that multicast connections in ATM are unidirectional. The data is transmit- ted from a root towards leaves. ATM Basics 56 Fig. 3.4, Point-to-Multipoint connection model The process of setting up a point-to-multipoint connection involves setting up a point-to-point connection in the first place. It must be indicated to the network that this connection is to be multipoint. Once the initial point-to- point is set up additional destinations (leaves) can be added. There are two alternative mechanisms that can be used here: •Sending a request using special message to the root (the originator of the original point-to-point). •With signaling version 4.0 or higher issuing a Leaf Initiated Join (LIJ) request to the network. In the case of LIJ, the root does not necessarily know of the existence of the new leaf. This is problematic: how does the potential leaf identify the con- nection to which it wants to be added? The answer is by the use of a Globally Unique Connection Identifier (GUI) and a server that can allocate GUIs to the required cell stream. Services available with the switch management software typically allow the operator to configure a VPC or VCC point-to-multipoint cross-connection across the switch. Chapter 3 57 ATM Basics 58 . connections in ATM are unidirectional. The data is transmit- ted from a root towards leaves. ATM Basics 56 Fig. 3. 4, Point-to-Multipoint connection model The process of setting up a point-to-multipoint. permanent connections and high flexibility of switched connections. Chapter 3 55 Fig. 3- 3 , Soft Permanent Virtual Connections 3. 4. Point-to-Multipoint Connections There are some applications that require. 49 Chapter 3 Virtual Circuits The actual service offered by means of ATM technology is connection-ori- ented. Although connections in ATM are implemented internally with pack- et switching,