1、Ii w 0 c3 STDmEIA TIA/EIA/IS-787-ENGL 1999 111 3234b00 Ob24221 9TT H TIAIEIA INTERIM STANDARD Common ATM Satellite Interface Interoperability Specification (CASI) TIA/EIA/IS-787 JULY 1999 TELECOMMUNICATIONS INDUSTRY ASSOCIATION Elretronic Indurtri- Alliance - STD=EIA TIAIEIA/IS-787-ENGL 1999 3234b00
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8、nt has been approved by the Telecommunications Industry Association. Distribution of this TIA/EIA Interim Standard for comment shall not continue beyond 36 months from the date of publication. It is expected that following this 36 month period, this TIAEIA Interim Standard, revised as necessary, wil
9、l be submitted to the American National Standards Institute for approval as an American National Standard. Suggestions for revision should be directed to: Standards this feature is useless in the satellite and wireless bursty error environment, with the net result that the cell headers of a large nu
10、mber celis can get corrupted, leading to cell losses or misinsertions. The CASI addresses this issue by providing FEC Coding based Forward Error Correction and Interleaving that combats bit errors characteristic of satellite and wireless links. 3. Unlike fiber links, satellite and wireless links hav
11、e bit error rates that tend to fluctuate over time depending on atmospheric conditions (from to The CASI addresses this issue by dynamically and adaptively changing the amount of Forward Error Correction based on measured link conditions. 4. Unlike terrestrial fiber links, the amount of available ba
12、ndwidth in satellite and wireless networks may be a limiting factor. For bandwidth efficiency, The CASI utilizes a number of techniques to maximize the amount of useable bandwidth over satellite and wireless links including dynamic coding, ATM header compression and ATM data compression. 1.2 Functio
13、nal Reference Model Figure 1 depicts the Functional Reference Model for the Common Air Interface Protocol. Satellite Modern Catell%e Modem 1.2.1.1 Figure 1. Functional Reference Model for the Common Air Interface Protocol In overview, the CASI provides a new transmission protocol layer for transmitt
14、ing ATM via satellite. This layer includes the Satellite-ATM (Sat-ATM) layer. The Sat-ATM layer accepts ATM cells from the standard ATM layer at a managed rate, re-formats these ATM cells in accordance with the Sat-ATM layer specification, and groups Sat-ATM layer ATM cells into multi-cell frames fo
15、r transmission over the satellite link. In turn, frames received over the satellite link are disassembled into their constituent Sat-ATM layer cells that are then reformatted into standard ATM cells and passed to the conventional ATM layer. The CASI is defined as the entire process of transferring A
16、TM cells from the terrestrial conventional ATM layer to the Sat-ATM layer, reformatting conventional ATM cells to the Sat-ATM format, grouping cells into frames, applying error-correction schemes to the frame, and delivering the frames to the satellite physical layer for serial transmission to the s
17、atellite modem. Note that the specifics of both the ATM and satellite physical layer are not part of the CASI The CASI defines only a logical interface, the physical manifestation of which is not described in this document. The physical implementation of the CASI is left unspecified as it could take
18、 any of several 2 STDOEIA TIA/EIA/IS-787-ENGL L999 3234b00 Ob24227 318 TIA/EIA/IS-787 forms. a non-exhaustive list of which includes: implementing the CASI as an individual piece of equipment: a software module within an ATM switch; or additional processing functions on a satellite modem. This docum
19、ent specifies the CASI logical interface with sufficient detail for any implementation of the CASI to be inter-operable with any other implementation to the extent that the transit of conventional ATM cells through the interworking CASI pair is essentially transparent. As an example implementation o
20、nly, note that conventional ATM cells could enter one CASI via a standards-based ATM interface. e.g. DS-3. and leave the interworking CASI via the cell bus of an ATM switch. The ATM and satellite physical interfaces of the CASI (or device into which CASI is embedded) are assumed to follow the approp
21、riate standard. and rate management is assumed to be done outside the CASI 1.3 Relationship to the B-ISDN Reference Model Figure 2 depicts the B-ISDN protocol reference model as shown in ITU 1.321. It is composed of a user plane, a control plane and a management plane. Above the physical layer, the
22、ATM layer provides call transfer for all services and the AAL provides service-dependent functions to the layer above the AAL. The layer above the AAL in the control plane provides call control and connection control. The management plane provides network supervision functions. The same layered mode
23、l used for generic B-ISDN with exceptiondadditions clearly identified can characterize the CASI. 1.3.1.1 Figure 2. B-ISDN Protocol Reference Model 1.3.2 Description of the Planes As depicted in Figure 2, the CASI can be characterized at the highest level as only concerned with aspects of the User pl
24、ane and the Layer Management plane from the ATM layer and down. Unless otherwise specified, all other planes would have the same function and description as for the generic B-ISDN protocol model. 3 STD-EIA TIA/EIA/IS-787-ENGL 1999 = 3234b00 Ob24228 254 TIA/EIA/IS-787 1.3.3 User Plane The layered str
25、ucture of the user plane provides for transfer of user information, along with associated controls such as: flow control; error detection: error recovery; error control coding techniques; interleaving: compression; etc. In the case of the CASI, only the ATM Layer and Physical Layer portions of this
26、plane are of relevance. 1.3.4 Layer Management Plane Layer management performs the functions related to resources and parameters residing in its associated protocol layer. Functions characterized as Operation and Maintenance (OAM) information flows specific to the layer concerned are handled by laye
27、r management, such as: Bit Error Rate (BER) monitoring; detection of loss of cell delineation: detection of error control coding violations; detection of uncorrectable errors; link indications; connectivity verification and negotiation and control of protocol options. In the case of the CASI, only t
28、he ATM Layer and Physical Layer portions of this plane are of relevance. 1.3.5 Functions of the individual layers of the B-ISDN and CASI Protocol Reference Model 1.3.5.1 B-ISDN User Plane Figure 3 depicts the functions of the B-ISDN User plane in relation to the protocol reference model as shown in
29、iTU 1.321 and Figure 2 of this contribution. Here is where we begin to see in more detail the specific areas of the generic protocol model that the CASI addresses. It is in the shaded areas of Figure 3 were modifications or additions to existing functions will be defined for the Common ATM Satellite
30、 Interface Protocol (CASI). I Hiehcr laver functions I Hizhcrinym I !csl AAL I Convemcria Bit timing Physid mcdium IPM/ I Error conrrol code generariodverificarion interleaving Compression CS Convcrgcncc rublaycr PM Physicd durn SAX Sccmcnwuon and ruwrnbly sublaycr TC TrYimission convcrzcncc 4 STDOE
31、IA TIA/EIA/IS-787-ENGL 1999 M 3234b00 Ob24229 190 TIA/EIA/IS-787 1.3.5.2 Figure 3. Functions of the B-ISDN User Plane in relation to the protocol reference model 1.3.5.3 Physical Layer Functions As shown in Figure 3, the Physical Layer consists of two sublayers. The Transmission Convergence (TC) sub
32、layer performs all functions required to transform a flow of cells into a flow of bits which can be transmitted and received over a physical medium. TC sublayer functions may include such things as: cell- rate de-coupling ; HEC sequence generatiodverification; cell delineation; transmission frame ad
33、aptation: transmission frame generatiodrecovery; error control coding, interleaving; compression; etc. The Physical Medium (PM) sublayer includes only physical medium dependent functions such as: actual bit timing; bit alignment; line coding; modulation and clock recovery. A flow of valid cells must
34、 cross the boundary between the ATM Layer and the Physical layer. The Physical Layer must provide a clock to the ATM Layer. This clock is derived from the line rate of the Physical Layer (e.g., a bit clock). The TC sublayer of the Surellire Physical Layer is specified as part of the CASI for operati
35、on over the physical medium of a geosynchronous, transponding (non-processing) satellite link. The PM sublayer of the Physical Layer will not be part of the CASI specification. This sublayer encompasses the actual satellite modem functions andor any physical medium such as a wire or fiber used to in
36、terconnect the CASI function to a satellite modem (in the case where the CASI function were implemented in a switch or a stand-alone box separate from the modem). The only requirement placed on the PM sublayer by the CASI function is that it provide timing to the TC sublayer. Note: The Physical Laye
37、r must provide a clock to the ATM Layer. This clock is derived from the line rate of the Physical Layer (e.g., a bit clock). 13.5.4 ATM Layer Functions The ATM Layer functions include: Generic flow control; Cell header generatiodextraction; Cell VPWCI translation; Cell multiplex and demultiplex; etc
38、. In the generic B-ISDN protocol model, the ATM Layer is unique and independent of the underlying Physical Layer. This may not necessarily be the case for the CASI. Techniques and functions necessary to support ATM transmission over the error and delay environment of a satellite require an ATM layer
39、 which is specifically designed to interoperate with the CASIS particular TC sublayer. 2. ATM Interface The terrestrial ATM physical layer protocol processor handles terrestrial receive cell synchronization, and ATM HEC processing functions. The actual ATM physical interface will be compliant with a
40、ccepted ATM standards. User ATM cells from the terrestrial ATM layer are received by the Sat- ATM cell reception block. The HEC byte of the ATM cell header is discarded, and idle or unassigned cells used for cell rate adaption are filtered out and discarded. The Sat-ATM layer cell stream now contain
41、s 52-byte cells, with no idle cells. Every “frame” time, a fixed sized frame is created that contains a frame header and data from cells or packets. Note that some cells and packets may get split across frames. This frame is then sent through the FEC encoder which appends a number of checkbytes to t
42、he frame. This frame is then sent to the interleaver, which reorders the bytes among I frames before transmission over the satellite interface. Satellite transmission is provided by the SATELITE interface physical layer. Data is transmitted over the satellite link at a rate that is managed by the SA
43、TELITE interface physical layer. 5 3. SATELLITE Interface Frames received over the satellite link are transferred from the satellite physical layer and received at the satellite interface. Each frame is sent through the de-interleaver, which de-interleaves I received frames. Frames exit from the de-
44、interleaver and are then sent through the FEC decoder, which corrects any bit errors in the frame. The actual satellite physical interface implementation will be compliant with accepted serial interface standards. Next, the correct frame is disassembled and cells and packets are extracted. Cells are
45、 then given to the ATM physical layer protocol processor which adds the HEC byte to the ATM header and then transmits the cells over the ATM line interface at the terrestrial line rate; generating idle cells as and when needed. 4. Frame Structure? The fundamental unit of transmission over the satell
46、ite link is a CASI fram must support certain functions and requires the following elements: 0 counter for frame number 0 0 Reed-Solomon checkbytes. counter for number of ATM cells in frame indicator for size of partial packets in frame Th CASI fram structure The CASI will utilize a fixed frame with
47、a configurable frame length. The CASI frame is N octets long, with 64 c=N O) of Reed-Solomon coding check bytes in the end. 6 STDoEIA TIA/EIA/IS-787-ENGL 1999 W 3234b00 Oh24233 849 m Frame Number O 1 2 3 4 5 6 TIAIEIAIIS-787 Coding Field Use -suggested value oft far-end CASI should use for its trans
48、mit frame Reed-Solomon coding. -special value (coding field = O) used by acquisition algorithm. -least significant bit used by compression routine. -bits 1-7 reserved for future use by other functions -reserved for future use -reserved for future use . -value oft that will be used starting in next f
49、rame numbered O. -value oft that will be used starting in next frame numbered O. -value oft that will be used starting in next frame The frame header has four fields - Count0 fiumber of fixed-size ATM cells in frame payload. Includes the last partial cell, if any. Does not include the first partial cell, if any. 7 Size0 Size of first partial cell or packet in payload divided by 4. numbered O. -value oft that will be used starting in next frame FrameNum The frame number. Each frame is sequentially numbered O, 1, ., 7, O, 1, 2, 3, . Coding Used by adaptive Reed-Solomon coding algori
