1、 STD-SMPTE RP 137-ENGL 1777 8357403 0002837 708 SMPTE RECOMMENDED PRACTICE Tributary Interconnection for Digital Control Interface RP 139-1 997 Revision of RP 139-1992 Page 1 of 9 pages 1.2 Definitions 1 General For the purposes of this practice, the following defini- tions shall apply: 1.1 Scope Th
2、is practice describes the mechanism for the trans- fer of control messages between tributaries used within a general-purpose communications channel of an interface system which transports data and digital control signals between equipment utilized in the pro- duction, post-production, and/or transmi
3、ssion of visual and aural information. It is intended that the mechanism described in this practice be utilized when transferring control messages between tributaries used as a pari of an overall system. The tributaries may be located either within a local network or on separate local networks which
4、 are interconnected by means of gateways and an interconnection bus. It is further intended that this mechanism, when used as part of an overall system, shall allow the intercon- nection of programmable and nonprogrammable equipment as required to configure an operational system with defined functio
5、ns, and will allow rapid reconfiguration of a system to provide more than one defined function utilizing a given group of equipment. 1.1.1 The message transfer mechanism makes use of virtual circuits, linkage directories, and system service messages (defined below). 1.1.2 The primary intent of this
6、practice is to define the mechanism enabling the transfer of messages between tributaries for the purpose of controlling equipment by external means. 1.2.1 virtual machine: A logical device consist- ing of a single device or a combination of devices that respond in essence or effect as a generic typ
7、e of equipment; e.g., VTR, video switcher, telecine, etc. 1.2.2 virtual circuit: A transparent, unidirec- tional, logical communications connection be- tween virtual machines. The communications path, in reality, passes through other levels and is propagated over a physical medium. 2 Normative refer
8、ences The following standards contain provisions which, through reference in this text, constitute provisions of this practice. At the time of publication, the editions indicated were valid. All standards are subject to revision, and parties to agreements based on this practice are encouraged to inv
9、estigate the possibility of applying the most recent edition of the standards indicated below. CCITT Recommendation X.21, Interface Between Data Terminal Equipment (DTE) and Data Circuit- Terminating Equipment (DCE) for Synchronous Operation on Public Data Networks CCITT Recommendation X.24, List of
10、 Definitions for Interchange Circuits Between Data Terminal Equip- ment (DTE) and Data Circuit-Terminating Equipment (DCE) on Public Data Networks CCITT Recommendation X.25, Interface Between Data Terminal Equipment (DTE) and Data Circuit- Copyright 8 1997 by the SOCIETY OF MOTION PICTURE AND TELEVI
11、SION ENGINEERS 595 W. Hartsdale Ave., White Plains, NY 10607 (91 4) 761 -1 1 O0 Approved August 12, 1997 STD-SMPTE RP 139-ENGL 1997 8357403 0002838 844 R P 1 39-1 997 Terminating Equipment (DCE) for Terminals Operat- ing in the Packet Mode and Connected to Public Data Networks by Dedicated Circuit C
12、CITT Recommendation X.27, Electrical Charac- teristics for Balanced Double-Current Interchange Circuits for General Use with Integrated Circuit Equip- ment in the Field of Data Communications IS0 4903:1989, Information Technology - Data Communication - 15-Pole DTE/DCE Interface Connector and Contact
13、 Number Assignments ISOAEC 3309:1993, Information Technology - Tele- communications and Information Exchange Between Systems - High-Level Data Link Control (HDLC) Procedures - Frame Structure IS0 43351 993, Information Technology - Telecom- munications and Information Exchange Between Systems - High
14、-Level Data Link Control (HDLC) Elements of Procedures 3 Interconnection within a local network 3.1 Message transfer The mechanism for message transfer between tribu- taries is based broadly on the principles of communi- cations layering and makes use of virtual circuits. This allows for the establi
15、shing of, and breaking down of, multiple links between the tributaries. System service messages perform this function. A linkage directory is established within the bus controller for each working session. The directory is considered to be a system service feature and provides for the establishment
16、of multiple virtual circuits through the network. 3.2 Linkage directory The linkage directory shall establish a relationship between virtual machines: .e., a virtual circuit. Estab- lishment of the linkage directory shall be completed as the initial task in each working session. The linkage director
17、y resident within the system service level of the bus controller binds message sources and desti- nations. Linkage information may originate in any application level, and shall effect directory construction within the system service level of the bus controller. Linkage messages are reserved messages
18、 within the system service subset of all message dialects; they establish and disconnect virtual circuits within the network. The bus controller, on receipt of a transmission request from the supervisory level of any tributary, will identify the destination tributary by reference to the linkage dire
19、ctory; acting as an intermediary, it will forward the message as directed. 3.3 Multiplexing within tributaries Tributaries, in general, have a single supervisory level address, and a single physical connection end point to the bus. Alternative multiplexing mechanisms, as described below, enable mult
20、iple virtual circuits to pass through any single connection end point. 3.3.1 Mu It i p I e , I o g i ca I I y independent virtu al machines, each with a unique supervisory level address, may be attached to the communica- tions channel through a common connection end point. Multiplexing is then perfo
21、rmed by multiple polling of the addressing entity residing within the supervisory level (see figure 1). NOTE - It may be noted that any individual tributary address may achieve a higher priority - and hence an improved response time at the expense of that of the remaining tributaries - by being allo
22、cated more than one poll within each poll sequence. 3.3.2 Alternatively, a single supervisory level address may be multiplexed to multiple logi- cally independent virtual machines with selec- tion being performed by a logical switch residing within the entity of the destination tributary system serv
23、ice level (see figure 2). The required virtual machine is selected from those associated with the single Supervisory level address, by means of a system service virtual-machine-select message (see 4.4). This is transmitted from the bus controller under the direction of the linkage directory held wit
24、hin its system service level, to the destination tributary system service level, immediately prior to the transmission of any control message, or sequence of control messages, destined for that specific virtual machine. The selected routing will remain in existence until receipt, by the system servi
25、ce level, of a new virtual- machine-select message thereby minimizing the message traffic on the communications channel. Page 2 of 9 pages STD-SMPTE RP 137-ENGL 2777 = 8357Li01 0002839 780 VIRTUAL MACHINE 2 SELECTED 7 I LOGICAL SWITCH SELECTION BY MEANS OF A VIRTUAL MACHINE SELECT MESSAGE SYSTEM SER
26、VICE LEVEL ELECTRICAL I MECHANICAL LEVEL ELECTRICAL l MECHANICAL LEVEL - SYSTEM SERVICE LEVEL C.E.P. SYSTEM SERVICE LEVEL (1 C.E.P SYSTEM SERVICE LEVEL I ADDRESS ADDRESS SVPERVISORY LEVEL EACH ACTIVE AODRESS 15 BY THE BUS CONTROLLER IN DEFINED SEOUENCE POLLED SEPARATELY RP 139-1 997 SUPERVISORY LEVE
27、L I ADDRESS I i- Figure 1 - Multiplexing within supervisory level 3.3.2.1 The reverse route of each virtual circuit, when required, will be selected similarly by the logical switch resident within the entity of the system service level of the multiplexed tributary. This selection is performed on rec
28、eipt of a con- trol or response message from any one of the virtual machines attached to the system service level of the tributary. The system service level will then instruct its supervisory level to transmit the appropriate virtual-machine-select message to the supervisory, and hence the system se
29、rv- ice, level of the bus controller. 3.3.2.2 System service level group assign and deassign commands shall be used to assem- ble/disassemble groups of virtual machines within the system service level, from those asso- ciated with a single supervisory level address, for simultaneous control purposes
30、. Figure 2 - Multiplexing within system service level Virtual circuits employing virtual group identifiers shall be recorded as additional entries within the bus con- troller linkage table. 3.3.3 It should be noted that a bus overhead exists in each method of virtual circuit multiplex- ing. Where th
31、e multiplex is to take place within the supervisory level (3.3.1), the overhead will take the form of additional polls in each cycle. System service level multiplexing (3.3.2) introduces an additional control message (the virtual-machine- select message) prior to each virtual machine mes- sage, or s
32、eries of virtual machine messages, destined for an alternative virtual machine. The choice of multiplexing mechanism, where used, rests with the system designer in recognition of spe- cific design considerations. Page 3 of 9 pages STD-SMPTE RP L3-ENGL 1997 = 8357401 0002820 4T2 m RP 139-1997 3.4 For
33、bidden configurations Some virtual circuit configurations may be forbidden due to the function of the particular tributary; .e., the functions of the tributaries are incompatible. Checking mechanisms should be employed to ensure that ille- gal virtual circuits cannot be established. Most of the chec
34、king would be performed in the system service level according to predefined rules within the particu- lar network. Some rules could be readily derived from the type of tributary (built in) while others may be imposed by the user or system designer. 4 System service messages System service messages a
35、re messages contained in the system service subset of all message dialects and shall be used to command the performance of system functions. These functions include but are not limited to: 4.1 Segmentation and reassembly These processes enable the transfer of messages which exceed the maximum superv
36、isory level message block length (see figure 3a). The parsing mechanism for segmentation and blocking is described by the state diagram given in figure 4. 4.1.1 A data segment shall take the following form (see figure 3b): - 1st byte: Keyword SEGMENT. - 2nd byte: Number of segments remaining; last s
37、egment is O; segment count shall be sent in se- quentially descending order. - Remaining bytes: Segment data. No further message shall follow a data segment message within a single supervisory level block. 4.2 Blocking and deblocking These processes enable the concatenation of mes- sages within a si
38、ngle supervisory level message block. 4.2.1 A data block shall take the following form (see figure 3c): - 1st byte: Keyword BLOCK. - 2nd byte: Byte count (N), where N is the number of bytes in the block data. - Remaining bytes: Block data. 4.2.2 The supervisory level shall transfer the byte count to
39、 the system service level. 4.3 Establishment of virtual circuits This process is effected through the management of the linkage directory contained within the bus control- ler. 4.4 Selection of a virtual machine This process enables the selection of a virtual ma- chine from those previously assigned
40、 to a tributary. 4.5 Tributary reset This command returns the tributary to its power-up default state. 4.6 Group assigddeassign These commands establishbreak down system serv- ice level groups of virtual machines for joint control purposes. 4.7 Virtual group assigddeassign These commands establishhr
41、eak down supervisory level groups of tributaries for joint control purposes. 5 Interconnection of local networks 5.1 Interconnection bus Interconnection of individual local networks shall be by means of an interconnection bus (see figure 5). Linking of the local network to the interconnection bus sh
42、all be by means of a GATEWAY. e ISO/IEC 3309 and 4335 (HDLC), in accordance with CCIlT Recommendation X.25 - LAPB, shall be used for the data link layer protocol between the gateway and the interconnection bus coupler; the physical interface shall employ the connector and pin assign- ment as specifi
43、ed by CCIlT Recommendation X.21 (IS0 4903); using balanced signalling as specified by CCITT Recommendation X.27; and with interface sig- nals as specified by CCITT Recommendation X.24. $ Page 4 of 9 pages STD-SMPTE RP L39-ENGL 1997 8357403 OD02821 339 I OC ln I I OC in1 I CONTROL MssAGE DATA 1256 BY
44、TES MAX In MAXmn7 1 o. SUPERVISORY LEVEL BLOCK b. FORMAT OF DATA SEGMENTS c. FORMAT OF DATA BLOCKS R P 139-1 997 Figure 3 - Data segments and blocks Page 5 ai 9 pages STD-SMPTE RP L39-ENGL L997 8357403 0002822 275 m RP 139-1997 EVENTS El -BLOCK KEYWORD E2 - LAST BYTE OF SUPERVISORY LEVEL MESSAGE E3
45、- KEYWORD NOT SYSTEM SERVICE MESSAGE I BLOCK: SEGMENT E4 -SUBSEQUENT CEQMENT KEYWORD E5 - US1 BlE OF BLOCK DATA Ee - NITIAL SEGMENT KEYWORD CONDITIONS SL- SEGMENT COUNT O SC - FINAL SEGMENT SEGMENT COUNT - O ACTIONS Al -PASS DATA BLOCK TRANSPARENTLY FOR HIGHER LEVEL PARSING A2 -PASS DATA TRANSPARENT
46、LY NO PARSINQ REWIRED ON MSG LEVEL A3 - PASS CONCATENATED SEQMENTS TRANSPARENTLY A4 -STORE INCOMING SEQMENT Figure 4 - Segmentationhlocking state diagram 5.2 Gateway The gateway is a logical device whose task is to transfer messages between a local network and an external interconnection bus coupler
47、. The gateway provides for the interchange of messages between multiple local networks. The gateway will maintain a linkage directory in its system service level. The linkage table will allow the gateway to be seen by the bus controller as a set of virtual tributaries linked by virtual circuits. The
48、 gateway will provide for all protocol conversions required to convert from the interface bus supervisory and electrical/mechanicaI level standards as specified in SMPTE RP 113 and ANSVSMPTE 207M, respec- tively, to the HDLC data link and X.21 physical link laye rs . The gateway will provide decodin
49、g of group ad- dresses provided for in the supervisory level V Figure 5 - Local network interconnection (SMPTE RP 113) and will forward messages addressed to these groups over the interconnection bus as discrete individual select addresses. Where more than one external tributary is addressed by a group message, the individual messages to all such tributaries shall be dispatched sequentially as individual messages from the gateway. Translation takes place in the system service level of the gateway. The func- tional structure of the gateway is shown in figure 6. 6 Guideline
