Multiplex communications – Pathfinding or routing – Switching a message which includes an address header
Reexamination Certificate
1999-08-11
2003-11-11
Jung, Min (Department: 2663)
Multiplex communications
Pathfinding or routing
Switching a message which includes an address header
C370S509000
Reexamination Certificate
active
06647012
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an interface apparatus and frequency-fluctuation test method in a network in which data to be transmitted is assembled into a fixed-length cell and then sent. More particularly, the invention relates to an interface apparatus and frequency-fluctuation test method through which a test for fluctuation in frequency can be performed by a simple arrangement.
B-ISDN (broadband-ISDN) switching technology, which is based upon the asynchronous transfer mode (ATM), is being put to practical use as a means for realizing broadband communication.
FIGS. 6A
to
6
D illustrate B-ISDN system configurations, in which only ATM cells that travel from right to left are shown. In actuality, however, ATM cells travel in both directions.
FIG. 6A
shows a configuration in which ATM terminals
12
,
13
are connected to an ATM switch
11
and communication between the ATM terminals is performed by ATM cells via the ATM switch
11
. In these figures, UNI signifies a user network interface.
FIG. 6B
shows a configuration in which various user terminals
14
,
15
are connected to ATM terminals
12
,
13
, respectively. The ATM terminals
12
,
13
convert data from the user terminals to ATM cells and then send the ATM cells to the side of the ATM switch
11
, and convert ATM cells received from the ATM switch
11
to data for the user terminals and then send the data to the user terminals.
FIG. 6C
shows a configuration provided with interworking function units (IWFU)
18
,
19
that provide a function for interworking with other networks (e.g., frame relay networks)
16
,
17
, respectively.
FIG. 6D
shows a configuration in which the ATM switch
11
internally accommodates an interface converter
11
a
and performs an internal conversion between the data of another network and ATM cells.
A B-ISDN of the above-described type affords a service through which user data is sent at a constant rate. This is a CBR (Constant Bit Rate) service. With the CBR service, the timing of a user clock on the receiving side (i.e., the receiving user clock) must be made to coincide with the timing of the user clock on the sending side. If the user clock on the sending side is synchronized to the clock of the network, the timings of the user clocks on the sending and receiving sides can be made to agree by generating the receiving user clock from the network clock on the receiving side.
However, there are instances where the timing of the user clock [1.544 Mbps at DS1 (Digital Signal Level 1) and 44.736 Mbps at DS3 (Digital Signal Level 3), which are standardized in the G.700 series of DS recommendations of the ITU] on the sending side is not in sync with the timing of the network clock on the network side. For example, there are cases where the user terminals
14
,
15
in the configuration of
FIG. 6B
send user data using internal clocks of their own, and cases where the network clocks of the networks in
FIGS. 6C and 6D
differ from the network clock of the ATM network. Even if the nominal value of the frequency of the sending user clock is known and the receiving side generates a receiving user clock having the same nominal value by frequency dividing the network clock (an example of which is 155.56 MHz), a timing error develops between the sending user clock and the receiving user clock, a consequence of which is that a faithful CBR service cannot be provided.
The SRTS (Synchronous Residual Time Stamp) method has been proposed as a method of synchronizing the receiving user clock to the sending user clock. The SRTS method incorporates timing information of the sending user clock in an ATM cell on the sending side, extracts this timing information of the sending user clock from on the receiving side and synchronizes the receiving user clock to the sending user clock based upon the timing information extracted. In order to send the timing information of the user clock, use is made of AAL-1 (ATM Adaptation Layer-1) standardized as an ATM cell by DS recommendation I.363, etc., of the ITU.
FIG. 7
is a diagram useful in describing the format of an AAL Type-1 (AAL-1) ATM cell, and
FIG. 8
is a diagram useful in describing the format of a 1-byte SAR-PDU header. In the AAL-1 ATM cell, a 48-byte information field is composed of an SAR-PDU payload having a length of 47 bytes and an SAR-PDU (PDU is the abbreviation of Protocol Data Unit) having a length of one byte. The 47-byte SAR-PDU payload is used to transfer user data. The 1-byte SAR-PDU header is composed of a 4-bit SN (Sequence Number) field and a 4-bit SNP (Sequence Number Protection) field.
The SN field is subdivided into two subfields, namely CSI (Convergence Sublayer Identifier) and SC (Sequence Count) subfields, and so is the SNP field, namely CRC (Cyclic Redundancy Check) and EPB (Even Parity Bit) subfields. The SC subfield counts cells cyclically from 1 to 8 (i.e., 1, 2, . . . , 8, 1, 2, . . . , 8, 1, . . . ) and makes it possible to monitor the sequence of the cells. Error detection and correction of the sequence number (SN) is performed by the CRC and EPB subfields. The CRC is a value based upon a polynomial (G(X)=X
3
+X+1) with respect to the sequence number. The EPB is an even-numbered parity bit in the SAR-PDU header. The CSI bit is the CS (Convergence Sublayer) function of the AAL-1 ATM cell and is used to send and reproduce the timing information of the user clock in a manner described below.
In accordance with the SRTS method, the timing information of the user clock is composed of 4-bit information (RTS
4
, RTS
3
, RTS
2
, RTS
1
) referred to as an RTS (Residual Time Stamp). This RTS information is transferred by the CSI bit, which is the CS function of the AAL-1.
FIG. 9
is a diagram for describing the format of the RTS information. The RTS information format is a multiframe format for eight ATM cells. Since the user data is transferred by the SAR-PDU payload, the number of bits of user data in eight ATM cells is 3008 bits (8 cells×47 bytes×8 bits).
The CSI bit has an 8-bit structure (CSI
0
to CSI
7
) corresponding to SC (Sequence Count) values of 0 to 7, respectively. Four-bit RTS information is sent by CSI bits (CSI
1
, CSI
3
, CSI
5
, CSI
7
) of ATM cells whose SC values are 1, 3, 5, 7. In other words, RTS
4
is transferred by an ATM cell whose SC value is 1, RTS
3
by an ATM cell whose SC value is 3, RTS
2
by an ATM cell whose SC value is 5, and RTS
1
by an ATM cell whose SC value is 7.
FIG. 10
is a diagram useful in describing the RTS-information generating cycle. In accordance with the CBR service, sending user data D
TU
is data having a constant speed, and a clock synchronized to this data is a sending user clock C
TU
in FIG.
10
. In an ATM cell, the information of the sending user data D
TU
is transmitted by the SAR-PDU payload, and the RTS information, which is the timing information of the sending user clock C
TU
, is transmitted by the CSI bit. Consequently, the RTS-information generating cycle T
TS
is equal to T
TU
×3008, where f
TU
represents the frequency of the sending user clock and T
TU
(=1/f
TU
) represents the time equivalent to one bit of the user data. Let the clock for generating the RTS data be a transmission RTS sampling timing clock C
TS
. In such case the RTS information is generated at the rising edge of the clock C
TS
, and the transmission RTS sampling timing clock C
TS
is obtained by frequency dividing the sending user clock to 1/3008.
In the SRTS method, the network clock frequency f
N
(an example of which is 155.56 MHz) synchronized to the line timing on the network side is frequency divided to generate a network frequency-divided clock C
NX
(frequency f
NX
=f
N
/2
X
, where 1/2
X
represents the frequency dividing ratio). The frequency dividing ratio 1/2
X
is decided in such a manner that the ratio of the network frequency-divided clock frequency F
NX
to the nominal value f
NOM
of the user clock frequency will fall within the range 1≦(f
NX
/f
NOM
)<2.
Next, the network frequency-di
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