emains the main focus and applied by the user to ensure that the source data flow conforms to the following sections outline the contents of the thirteen draft the agreement. Enforcement discards cells which are in excess of recommendations relevant to ATM, highlighting the key points and the negotiated levels. providing detail on the primary features (CCITT, 1990). The content is, of course, provisional and subject to change as the process of 41.3.5 Key Advantages of ATM standardisation continues: many of the paragraphs are annotated 'for further study'. The basic multiplexing and switching characteristics described The structure of the standards is shown in Figure 41.9. Under a above give rise to some of the most important advantages of ATM: top-level document giving an overview of B-ISDN, there are five groups of documents covering: 1. Flexibility. 2. Inherent adaptation between information flow rates and trans- 1. ATM functions. 2. Broadband services. port resources. Network throttling enforcement Source Discards Buffer cells Optional Mandatory Network edge Figure 41.8 Flow conditioning functions Standards for ATM 41/9 1.121 Broadband aspects of ISDN 1.113 Vocabulary Service of terms Network 1.150 1.211 ATM functional B-ISDN service 1.610 characteristics aspects Principles of B-ISDN access 1.361 1.362 1.311 1.321 1.327 General Protocol B-ISDN layer functional network reference Functional specification description aspects model architecture 1.432 1.363 1.413 B-UNI B-ISDN Physical layer specification specification Figure 41.9 CCITT I-Series standards for B-ISDN 3. Network aspects. each virtual path can carry a number of virtual channels distin- 4. User-network interface. guished by the 'VCI' field. (Figure 41.5.) 5. Operations and maintenance. 2. Defines a virtual channel connection (VCC) as a concatenation of virtual channel links that extends between two points where 41.4.1 I.113: Vocabulary of terms for broadband the adaptation layer is accessed. VCCs may be switched or aspects of ISDN. semi-permanent, provi