code 128 font for excel 2010 4: ATM and ISDN in Objective-C

Printing PDF417 in Objective-C 4: ATM and ISDN

4: ATM and ISDN
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This chapter covers two widely adopted WAN technologies: Asynchronous Transfer Mode (ATM) and Integrated Services Digital Network (ISDN) ATM covers the need for incredibly high-speed data, voice, and video transfers, whereas ISDN covers the need for moderatespeed data and voice connections in areas where no other low-cost possibilities (such as Digital Subscriber Line [DSL] or cable) exist, as well as providing demand-dial connections that are significantly faster than Plain Old Telephone Service (POTS)
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Asynchronous Transfer Mode (ATM) is a relatively new technology (compared to Frame Relay, anyway) that is becoming the technology of choice for high-speed multiservice connections ATM came about because of a growing need for a technology that could support many different types of traffic over varying distances, across administrative and national boundaries, at a wide variety of speeds, and with full quality of service (QOS) support As a result, ATM is one of the most complex and versatile protocols in use today Luckily, most network administrators and engineers don't need to understand all of the complexity behind ATM Therefore, this chapter simply covers the basics of ATM and how it works in most environments
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ATM is a technology that doesn't map very well to the OSI model because it was built using the broadband ISDN (BISDN) model However, it maps primarily to the datalink layer, with some specifications that could be placed in the physical and network layers Central to ATM is the concept of multiservice transmissions The requirement that ATM be able to adequately handle many types of transmissions has led it to become quite a bit different from the other protocols that we have discussed However, it is also quite similar to Frame Relay, including the concept of virtual circuits (VCs) Virtual Circuits Like frame relay, ATM uses virtual circuits both permanent virtual circuits (PVCs) and switched virtual circuits (SVCs) to make and address connections to other ATM devices
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Unlike Frame Relay, however, ATM defines one additional type of virtual circuit: the soft permanent virtual circuit (SPVC) An SPVC is basically a PVC that is initiated on demand in the ATM switches From each endpoint's perspective, the SPVC appears to be a standard PVC; but to the ATM switches in the cloud, the SPVC differs in one significant way With a PVC, a VC is created statically throughout the cloud and is always up With an SPVC, however, the connection is static only from the endpoint (DTE) to the first ATM switch (DCE) From DCE to DCE within the cloud, the connection can be built and rebuilt on demand The connection is still static once built, unless a link problem causes the VC within the cloud to come down In this case, without the manual intervention required on a PVC, the SPVC automatically attempts to reestablish the VC by using another route With a PVC, the provider needs to manually rebuild the PVC in the cloud This feature of SPVCs leads to higher reliability of the link, with no additional configuration required on the DTE end In addition, while SVCs in Frame Relay are not very well defined, the call establishment and termination features that an SVC requires are pretty well defined in ATM Consequently, you can actually get SVC service from many providers for your ATM WAN connections While SVC service results in dynamically available connections based on current needs and a possible reduction in costs, it is considerably more difficult to configure In addition, SVCs are usually tariffed, or priced based on connect time, class of service, and bandwidth used, which can actually lead to an increase in cost if connections stay active for a long time For these reasons, we will still concentrate on PVCs ATM adds a few more pieces to the VC puzzle as well, assaulting you with an "acronym soup" of different classifications of VCs In addition to the differentiation between SVCs, PVCs, and SPVCs, you now have to differentiate between User to Network Interface (UNI) and Network to Network Interface (NNI) connections On top of this, UNI and NNI connections are further differentiated between public and private connections (Sometimes, you have to wonder if the unstated goal for designing a new technology is to confuse as many people as humanly possible) Let's tackle the UNI versus NNI differentiation first User to Network Interface (UNI) is the connection from a user device (DTE) to a network device (DCE) In a simple ATM scenario in which your company is connected to the provider using a router, and you are connected to the router through an Ethernet, the connection from the router to the provider would be a UNI connection The connection from you to the router would not be a UNI connection, because to be an ATM UNI connection, it must first be an ATM connection Network to Network Interface (NNI) is the connection from ATM switch (DCE) to ATM switch (DCE) within the cloud You will not need to deal with this type of connection in most cases, unless you work for a provider or have your own large-scale ATM backbone in-house As for the public versus private designation, this simply defines whether the connection is over a public infrastructure (such as a provider's cabling) or a private infrastructure (such as your own ATM backbone) The main difference is in the cabling and distance requirements for the two; typically, determining whether your link is public or private is not very difficult Figure 4-1 illustrates the UNI/NNI and public/private designations
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Figure 4-1: ATM VC differentiation ATM Addressing ATM addressing is an interesting topic because ATM defines addresses on two separate levels First, ATM addresses its VCs, similar to frame relay DLCIs Second, ATM addresses the ATM device itself, similar to Ethernet's Media Access Control (MAC) address We will discuss the VC addressing first ATM addresses VCs with virtual path identifier (VPI) and virtual circuit identifier (VCI) pairs The VPI sets parameters like the bandwidth for the connection and QOS options VCIs have the same QOS requirements as the VPI and fit within the bandwidth allocation of the VPI, and are then run inside the VPI to actually make the connection The basic premise of VPI/VCI pairings is that it is easier for the provider to manage than DLCIs For example, rather than having 15 DLCIs for your company at odd intervals such as DLCI 534, 182, 97, 381, and so on, and having to look at a chart to figure it all out, the provider can have one VPI for your company with 15 VCIs inside it All the engineer needs to know is which VPI you have VPI/VCI pairings are also more scalable than DLCIs; for example, rather than having 10 bits to uniquely identify the connection, you have 24 bits (8 VPI plus 16 VCI) to identify a UNI VC In addition, VPI/VCI pairings allows most of the call setup overhead to be completed on the VPI, with the VCI simply accepting the VPI's parameters Figure 4-2 shows an example of VPI/VCI pairings
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