Chuyển đổi lý thuyết P5

The ATM Switch ModelThe B-ISDN envisioned by ITU-T is expected to support a heterogeneous set of narrowband and broadband services by sharing as much as possible the functionalities provided by a unique underlying transport layer based on the ATM characteristics. As already discussed in Section 1.2.1, two distinctive features characterize an ATM network: (i) the user information is transferred through the network in small fixed-size packets, called cells1, each 53 bytes long, divided into a payload (48 bytes) for the user information and a header (5 bytes) for control data; (ii) the transfer mode of user information is connection-oriented, that...
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Chuyển đổi lý thuyết P5 Switching Theory: Architecture and Performance in Broadband ATM Networks Achille Pattavina Copyright © 1998 John Wiley & Sons Ltd ISBNs: 0-471-96338-0 (Hardback); 0-470-84191-5 (Electronic)Chapter 5 The ATM Switch ModelThe B-ISDN envisioned by ITU-T is expected to support a heterogeneous set of narrowbandand broadband services by sharing as much as possible the functionalities provided by a uniqueunderlying transport layer based on the ATM characteristics. As already discussed inSection 1.2.1, two distinctive features characterize an ATM network: (i) the user informationis transferred through the network in small fixed-size packets, called cells1, each 53 bytes long,divided into a payload (48 bytes) for the user information and a header (5 bytes) for control data;(ii) the transfer mode of user information is connection-oriented, that is cells are transferred ontovirtual links previously set up and identified by a label carried in the cell header. Thereforefrom the standpoint of the switching functions performed by a network node, two differentsets of actions can be identified: operations accomplished at virtual call set up time and func-tions performed at cell transmission time. At call set-up time a network node receives from its upstream node or user-network inter-face (UNI) a request to set up a virtual call to a given end-user with certain trafficcharacteristics. The node performs a connection acceptance control procedure, not investi-gated here, and if the call is accepted the call request is forwarded to the downstream node orUNI of the destination end-user. What is important here is to focus on the actions executedwithin the node in preparation of the next transfer of ATM cells on the virtual connection justset up. The identifier of the virtual connection entering the switching node carried by the callrequest packet is used as a new entry in the routing table to be used during the data phase forthe new virtual connection. The node updates the table by associating to that entry identifier anew exit identifier for the virtual connection as well as the address of the physical output linkwhere the outgoing connection is being set up. At cell transmission time the node receives on each input link a flow of ATM cells eachcarrying its own virtual connection identifier. A table look-up is performed so as to replace in1. The terms cell and packet will be used interchangeably in this section and in the following ones to indicate the fixed-size ATM packet.158 The ATM Switch Modelthe cell header the old identifier with the new identifier and to switch the cell to the switchoutput link whose address is also given by the table. Both virtual channels (VC) and virtual paths (VP) are defined as virtual connections betweenadjacent routing entities in an ATM network. A logical connection between two end-usersconsists of a series of n + 1 virtual connections, if n switching nodes are crossed; a virtual pathis a bundle of virtual channels. Since a virtual connection is labelled by means of a hierarchicalkey VPI/VCI (virtual path identified/virtual channel identifier) in the ATM cell header (seeSection 1.5.3), a switching fabric can operate either a full VC switching or just a VP switching.The former case corresponds to a full ATM switch, while the latter case refers to a simplifiedswitching node with reduced processing where the minimum entity to be switched is a virtualpath. Therefore a VP/VC switch reassigns a new VPI/VCI to each virtual cell to be switched,whereas only the VPI is reassigned in a VP switch, as shown in the example of Figure 5.1. VC switching VCI VCI VCI VCI 1 2 6 1 VPI VCI 1 VCI 1 7 VPI VCI 2 1 VPI VCI 6 1 VCI 2 VCI 2 VPI VPI VCI 4 3 2 VCI 4 VP switching ATM VP/VC switchig Figure 5.1. VP and VC switching A general model of an ATM switch is defined in Section 5.1 on which the specific archi-tectures described in the following sections will be mapped. A taxonomy of ATM switches isthen outlined in Section 5.2 based on the identification of the key parameters and propertiesof an ATM switch.The Switch Model 1595.1. The Switch ModelResearch in ATM switching has been developed worldwide for several years showing the feasi-bility of ATM switching fabrics both for small-to-medium size nodes with, say, up to a fewhundreds of inlets and for large size nodes with thousands of inlets. However, a unique taxon-omy of ATM switching architectures is very hard to find, since different keys used in differentorders can be used to classify ATM switches. Very briefly, we can say that most of the ATMswitch proposals rely on the adoption for the interconnection network (IN), which is the s ...