CA1300768C - Isnd system having susbcriber line multiplexer with means for establishing different data links through d-channels on subscriber line and high rate transmission line - Google Patents

Abstract

Abstract of the Disclosure:
In an ISDN system comprising a plurality of DSLs each having a plurality of DTEs and an ISDN switch having a TEI list, a multiplexer is used for enabling each DTE to access the ISDN switch through a single primary rate DTL in the time division technique. The multiplexer comprises subscriber interfaces connected to DSLs, repectively. The subscriber interface has first TEIs assigned to DTEs on the corresponding DSL and has a first data link with each DTE on the basic interface D
channel using a first LAPD frame signal with the fisrt TEIs. The subscriber interface has a second TEI
assigned thereto and registered in the TEI list in the ISDN switch. The subscriber interface derives information field from the first LAPD signal and forms a second LADP frame signal having the second TEI and the derived information, The multiplexer statisticaly multiplexes a plurality of second LAPD signals from subscriber interfaces into the D channel on the DTL to form a second data link between each subscriber interface and the ISDN switch so that load of the ISDN
switch is reduced. Addition of a switch selection code to the second TEI enables to switch the multiplexer to another ISDN by use of a cross-connector on the DTL.
Use of line selecting signal in DTE signal also enables the multiplexer to transmit the DTE signals onto a different private line.

Description

; `
~0~7~3 ISDN SYSTEM HAVING SUBSCRIBER LINE MULTIPLEXER
WITH MEANS FOR ESTABLISHING DIFFERENT DATA
LINKS THROUGH D-CHANNELS ON SUBSCRIBER LINE
AND HIGH RATE TRANSMISSION LINE

Background of the_Invention ~; l. Field of the Invention The present invention relates to an integrated services di~ital n~twork ~ISDN) system accoxding to S recommendations o the CCITT ~International Telegraph and Telephone Consultative Communication) and, in particular, to subscriber line multiplexing for multiplexing signals, particularly, control signals on a plurality of subscriber lines on a high rate digital 10 transmission line extending to an ISDN digital switch.

2. Description of the Prior Art In the ISDN system, a plurality of subscriber lines extend from an exchanging office to user areas.
Each subscriber line terminates to a network termination 15 (NT) unit. A user bus is connected to the NT unit and extended in the user area. One or more user's ~' ~

` '' ' ~3(1~07~8 ;

equipments or terminal equipments tTES) such as a digital telephone set or sets (which will only be called telephone sets hereinafter), a facsimile set or sets, a personal computer set or sets, and the like.
The CCITT recommendations I series prescxibes protocol of user-network connection or interface in connection with seven classified layers which are numbered by 1 through 7. For example, layer 1 is directed to physical connecting conditions such as 10 transmission rate, pin number of connectors and others, layer 2 being for data link, layer 3 for call control from a call setup to call release completion.
According to the I series, communication information and control si~nal are transmitted through 15 dierent channels which are multiplexed in a time divi~ion fashion. The communication information channel is called a B channel and the control information channel i~ re~erred to as a D channel. It is possible to transmit communication information through the D
20 channel.
As a basic one of user-network interface structures, a basic interface is determined which comprises two B channels of a transmission rate of 64 kbps which are separately called Bl and B2 channels and 25 a single D channel of a transmission rate of 16 kbps.
Another user-network interface structure is called a primary rate interface which comprises 23 ~

channels of 64 kbps and a single D channel of 64 kbps.

~` h ~ 3007~

As will be noted from the above description, the single D channel is commonly used by a plurality of TEs connected in the user bus.
In connection with the layer 2 for data link, a 5 plurality of logical connections or links are made on the D channel by use of LAPD (Link Access Procedure on D
channel). Various control information in layer 2 and layer 3 can be transmitted through ~he links. ~ frame format of the LAPD is in conformity with the HDLC
10 (High-level Data Link Control Procedure) and comprises a start flag, an address field, a control field, an information field, a rame check sequence and a stop fla~ The frame format will later be described in detail with reference to the drawings.
lS The address field is rewritable to identify each one logical link. The con-trol field is used for type identlficatlon o~ the IIAPD frame itself. There are defined a number of types for the LAPD frame~ For example, I (Information) frame is used for information 20 transfer in a multiframe mode and UI (Unnumbered Information) frame is used for information transfer in an unacknowledged information transfer mode.
Transfer of call control information in the layer 3 is performed by use of the I frame or the UI
25 frame. The call control information is inserted in the information field in each of the I and UI frames as a layer 3 message. A format of the layer 3 message comprises a common information portion and an individual .

' -` ~3~768 information portion. The common information portion comprises a protocol discriminator for identifying the layer 3 message, a call reference for referring to a call, and a message type. There are a number of types 5 for the layer 3 message, for example, SETUP for identifying a call request, REL COMP for identifying notice of completion of channel disconnection and release completion of a call reference, and others.
A large number of subscriber lines are extended 10 rom an exchange office having an ISDN digital switch to various users. When a user such as a company has a plurality of subscriber lines, it is advantageous to multiplex those subscriber lines into a single transmission line through which signals on the plurality 15 o~ subscriber lines are transmitted as a TDM (time division multiplexed) signal, That i9, ~ channel signals and D channel signals on a plurality of subscriber lines are time-division multiplexed and are transmitted through the signal transmission line.
Providing that each subscriber line is dependent on the basic interface having two B channels of 64 kbps and a single D channel of 16 kbps and that a primary rate digital transmission line (1544 kbps, 23B + D
channels) is used for the single transmission line, only 25 four D channels (64/16 = 4) can be multiplexed.
In ordex to resolve the problem, the Bell Communications Research, Inc. proposed a "Statistical Multiplexing" in Bell Communications Research Technical 31D~768 Advisory, TA-TSY-000397, May 2, 1987 (Reference 1).
According to the Statistical Multiplexing, 16 D channel signals on subscriber lines of the basic interface are statistically multiplexed into a D channel on the 5 primary rate transmission line. In order to identify each subscriber line and to identify multiplex, the identification information is tagged to the D channel information on each subscriber line to form a new D
channel signal which is transmitted to the transmission 10 line in the statistical multiplexing. The new D channel signal will later be described in detail in connection with the drawings.
According to the statistical multiplexing, an increased number of D channels can be multiplexed in 15 comparison with the conventional time division multiplexing. However, a data link is still set up between a TE on one subscriber line and an ISDN switch or exchanger ~o that it i5 impossible to reduce load of the ISDN switch.
In multiplexing o a plurality of sub~criber lines, it is impossible to handle those lines which belong to diferent exchange offices.
In another ISDN system, a private line or a privately used line is used. It i8 also impossible in 25 the multiplexing to handle the private line and subscriher lines which belong to an exchange office together.

~30~ 8 Summary of the Invention:
Therefore, it is an object of the present invention to provide an ISDN system having a subscriber line multiplexer wherein load of an ISDN switch can be 5 reduced.
It is another object of the present invention to provide an ISDN system having a subscriber line multiplexer which can process subscriber lines which belong to different exchange officesO
It is still another object of the present invention to provide an ISDN system having a subscriber line multiplexer which enables subscriber lines to connect to a private line.
The present invention is applicable to an ISDN
15 ~Integrated Services Digital Network) system comprising a plurality o~ ~ubscriher lines associated with a plurality o~ terminal equipments, a subscriber line multiplexer connected to the plurality of subscriber lines, each of the subscriber llnes transmitting a basic 20 TDM (Time Division Multiplex) signal between the plurality of terminal equipments and the subscriber line multiplexer at a basic transmission rate, the texminal equipments having different terminal equipment TEIs (Terminal Endpoint Identifiers) assigned thereto as 25 basic TEIs, respectively, the basic TDM signal comprising a plurality of basic information channels for transmitting information and a basic control channel for transmitting a basic control signal, the basic control ~ 130076~3 signal compri~ing a specific one of the basic TEIs and control information from a specific one of the terminal equipments, a first ISDN digital switch having a TEI
list for registering TEIs, and a high rate transmission ; 5 line coupling the subscriber line multiplexer and the first ISDN digital switch for transmitting a secondary TDM signal therebetween at a primary transmission rate ; higher than the basic transmission rate, the primary TDM
signal comprising a plurality of primary information 10 channels and a primary control channel, the subscriber line multiplexer statiF~tically multiplexing the basic control channel signals on the plurality of subscriber lines inko the primary con~rol channel to thereby transmit the control in~ormation to the first ISDN
15 digital swikah. According to the present ivnention, the subscriber .line multiplexer comprises a plurality of subsariber intcrface ~neans coupled to the plurality of subscriber lines, respect.ively and internal bus means coupled to the high rate transmission line and commonly ; 20 connected to the plurality of subscriber interface means, Each of the subscriber interfaces comprises ~: channel separating means coupled with each one of the subscriber lines for separating the basic control channel signal from the basic TDM signal to produce a 25 separated signal, basic TEI list for memorizing the basic TEIs, basic control means responsive to the separated signal for deriving the control information as a derived control information, the basic control means . .

l" ~

` ~ ~3~0~8 : 8 establishing a basic data link between the specific terminal e~uipment and the basic control means through the basic control channel in cooperation with the basic TEI list, primary control means having a subscriber TEI
5 assigned to the each one of the subscriber line : interface means as a primary TEI which is registered in the TEI list as a primary TEI list of the first IS~N
di~ital switch, the primary control means responsive to the derived control information for making a primary 10 control signal which comprises the primary TEI and the derived control information without the specific basic TEI, and transmitting means or transmitting the primary control signal into the primary control channel through the internal bus in the statistical multiplexing fashion 15 so that ~ pr.imary data link is established between the primary control means and the first ISDN digital switch through the primary control channel in cooperation with the primar~ TEI list.
The subscriber line multiplexer may further 20 comprise transmission line interface means coupled to the high rate transmission line for detecting the secondary TDM signal on the high rate transmission line to produce a timing signal, the transmission line interface means being connected to the internal bus 25 means for coupling the internal bus means to the high ; rate transmission line, and a timing signal line connecting the transmission line interface means to the plurality of subscriber interface means for delivering .

-- ~300~768 g the timing signal to the plurality of subscriber interface means, the transmitting means detecting the primary control channel based on the timing signal.
Aacording to an aspect of the pesent invention, 5 the ISDN system ma~ ~urther comprise a second ISDN
digital switch and cross connecting means fox selectively coupling the high rate transmission line to one of the first and second ISDN digital switches. Each of the subscriber interface means further compxises 10 setting means for setting a switch selection code for selectin~ one of the fixst and second ISDN digital switches. The switch selection code is contained in the pximary control signal together with the primary TEI.
The cross connecting means comprises detecting means for 15 detect~ng the swi~ch selection code in the primary control ~ignal to deliver the secondary control signal to that one of the first and second ISDN digital switches which is identified by the switch ~election code.
According to another aspeot of the present invention, the ISDN system may urther comprise an additional high rate transmission line coupled to the subscriber line multiplexer~ Each of the terminal equipments being provided with means for generating line 25 selection signal for selecting one of the high rate transmission line and the additional high rate transmission line. The selection signal is contained in the basic TDM signal to be delivered to the subscriber -"~ 13~076~3 multiplexer. Each of the subscriber line interface means further comprises additional transmitting means connected to the primary control means for transmitting the primary control signal onto the additional high rate 5 transmission line, and selection signal detector means ; coupled to the each one of the subscriber lines for detecting the line selection signal to produce an enabling signal and an unabling signal, the enabling signal being delivered to one of the transmitting means 10 and the additional transmitting means to transmit the primary control signal to one of the high rate transmlssion line and the additional high rate transmission line which i9 identified by the line selection signal, while the unabling signal being 15 delivered to the other of the transmitting means and the additional transmitting means.
B~ie~ Description o the Drawings-Fig. 1 is a block diagr~m view of a known ISDN
system;
Fig. 2 is a block diagram view of another ISDN
system wherein signals on a plurality of subscriber lines are multiplexed in the time division fashion so as to ~e transmitted to an ISDN digital switch through a single line;
Fig. 3 shows various frame formats of D channel signal used in the system of Fig. 2;
Fig. 4 shows frame forma~s of the D channel signal used in the present invention;

.

~3~(:37~3 Fig. 5 is a block diagram view of an ISDN system using a subscriber line multiplexer according to an embodiment of the present invention;
Fig~ 6 is a block diagram view of a subscriber 5 interface used in Fig. 5;
Fig. 7 is a block diagram view of an ISDN system according to a second embodiment of the present invention wherein a pluralit~ of subscriber lines are cross-connected to different ISDN digital switch;
Fig. 8 is a front view of a subscriber interface used in the subscriber line multiplexer according to the second embodiment;
Fig. 9 is a circuit diagram view of a cross connector in Fig. 7;
Fig. 10 is a block diagram view of a main portion of a third embodiment;
Fig. ll is a block diagram view o a line selector in F.tg. 10;
Fig. 12 is a perspeckive view of the line 20 selector of Fig. 11;
Fig. 13 is a block diagram view of a portion of a fourth embodiment; and Fig. 14 is a block diagram view o~ a subscriber :~ interface in Fig. 13.
Descri~tion.of.~referxed.Embodiments:
Prior to description of embodiments, a known ISDN will be described for the purpose of better : understanding of the present invention.
~ .

~L300768 Referring to Fig. 1, a plurality of digital subscriber lines (DSLs) 20 are connected to at least one ISDN digital switch or exchanger 21. In the figure, two ISDN digital switches 21 are shown to be connected 5 through an in~er-office line. Each of DSLs 20 is terminated at an individual network termination (NT) unit 22. The NT unit 22 is connected to a subscriber bus 23 to which a plurality of terminal equipments (TEs) 24, for example, digital telephone sets, facsimile sets, 10 personal computers, and the like are connected in a parallel form. Each subscriber bus 23 is terminated to a terminal resistor 25 at an extended end.
In the system, communication can be performed between one or more TEs 24 in one subscriber bus 23 and 15 one or more 'r~s 24 on another subscriber bus through the ISDN digital switch or switches 21.
The shown ISDN system i5 arranged dependent on the CCXTT recommendations described in the preamble.
For example, the basic interface is used between the bus 20 23 and the NT 22. That is, two B channel signals and a single D channel signal are transmitted on bus 23 and subscriber line 20 between NTs and the ISDN digital switch 21 through NT 22. According to the D channel protocol for layers 2 and 3, the LAPD frame is used for 25 the D channel.
Referring to Fig. 3, an LAPD frame 31 comprises a start flag field (FRAG) of one octet length, an address field (ADDRESS) of two octet length, a control :

-` ~300768 field (CONTROL) of one octet length, an information field (INFORMAION) of a variable length ~260 octets at maximum), a frame check sequence (FRAME CHECK SEQVENCE) ; of one or two octet length, and a stop flag field (FRAG) 5 of one octet length.
Various frame types such as I, UI, and others are determined for the LAPD frame and each type is identified in the control field as described in the preamble The address field comprises EA0 of one bit, C/R
of one bit, SAPI of 6 bits, EAl of one bit and TEI of 7 bits similar to a frame portion as shown at 33 in Fig.

3. EA is an address extension bit and EA0 means continuation to the next octet while EAl means a last 15 octet in the field. C/R is ~or indication of which the ram~ is command or response. SAPI is a service access point identiier and 'rEI is a terminal endpoint identifier. SAPI can take a value from 0 to 63 and several ~unctions are determined for several values, for 20 example, SAPI~0 means that the rame is for call control signal while the remaining values are not yet determined. TEI can take a value from 0 to 127 and TEI
values from 64 to 126 are assigned to TEs 24 in each user and registered in a TEI list (not shown) in the 25 ISDN digital Rwitch 21. Values from 0 to 63 are left for free use by users and the remaining value 127 is determined to identify a broadcasting mode.

.

`
, Oa76~1 The TEI value i~ not fixed ~o a specific TE but is assigned to each TE when connected to the subscriber line in order to insure portability o TE~.
TEI assignment procedure is also determined in 5 the I series of the CCIT~ recommendations and is performed between the TE and the ISDN digital switch by use of the UI frame of the LAPD with SAPI=63 and TEI=127 being set in the address field when the TE is connected ; to the subscliber line connected to the ISDN digital 10 switch. Detail of the assignment procedure is not described herein but will be understood by reference to the I series.
In order to reduce the number of subscriber : lines 20 extending from the digital switch 21 to a user 15 such as a company~ a plurality of subscriber lines are multiplexed to a high rate transmission line such as a pri.mary rate transmission line, as described in the preamble.
Reerring to Fig. 2, a remote digital terminal : 20 (RDT) 26 is connected to a primary rate transmission line 27 extending rom the ISDN digital switch 21. A
plurality of subscriber lines 20 are connected to RDT
26, and each subscriber line is provided with NT 22, a user bus 23, and a plurality o~ TEs 24 connected to the ; 25 user bus 23. B channel siynals and D channel signals on ~ the subscriber lines 20 are multiplexed in the time ; division fashion and are transmitted through the primary rate transmission line 27 to the ISDN digital switch.

i3~)7~i8 Bell proposed the "Stati~tical Multiplexing" in Reference 1 as described in the preamble to enable to multiplex an increased number of D channel signals.
In order to insure the statistical multiplexing, 5 Bell uses a new LAPD frame format.
Referring to Fig. 3, the new LAPD frame format 32 is characterized by inser~ion of the LAPD frame 31 on the subscriber channel into an information fleld of the new L~PD frame 32, although the start and s~op flags 10 tFLAG) in frame 31 are removed. Accordingly, the new LAPD ~rame format is appreciated as a multiple LAPD
frame format. Furtharr in the new LAPD frame, SAPI in the addre3s ~ield is ~ixed to be a value 32 which is determined to identify the new frame after 15 multipl~aation, as shown at 33 in ~ig. 3. One of values from 1 to 16 is assigned to TEI in the same address field as D5~ numbers, as shown below the addres field format 33 in Fig. 3.
VI is inserted in a control field (CONTROL) of 20 the new LAPD format so that the new frame is the UI
frame.
According to the statistical multiplexing manner using the new or multiple LAPD frame, the IS~N digital switch 21 must process two control information sets for 25 two TEIs in one D channel signal. This means that load of the ISDN digital switch is not decreased but . increased.

i`` ~3~0768 The present invention aims to reduce load of the ISDN digital switch in use of the statistical multiplexing of D channel signals on a plurality of subscriber lines. A fundamental concept of the present 5 invention will be described below.
Referring to Fig. 4, the LAPD frame 31 of the ~
channel signal on the subscriber line is shown together with the address field (ADDRESS) 34 and has bePn ~ described in connection with Fig. 3. Another LAPD frame ; 10 35 i8 also shown together with its address field 36 and has a format similar to the LAPD frame 31.
Accoxding to the present invention, the LAPD
frame 31 on the ba~ic interace is converted into the other LAPD frame 35 which is then transmitted onto a 15 high rate or primary rate transmission line in the statistical multiplexing ~a~hion~ Accordingly, the LAPD
frame 31 will be called a basic L~P frame and its various fields will be referred to as individual field names with a term of "basic" being prefixed to the 20 names. On the other hand~ the other LAPD ~rame 35 will be called a primary LAPD frame and its individual field names will be called individual field names with a term "primary" being prefixed to the names.
In the basic address field 34 in the basic LAPD
25 frame, SAPI ls set to be 0 for the call control and TEI
is set as a value (64-126) assigned to a TE which originates the D channel signal. TEI inserted in the basic address field 34 will be called a basic TEI and is ; t ~ ~30~768 not registered in the TEI list in the ISDN digital switch but is registered in a basic TEI list provided in a subscriber line multiplexer which will later be described in connection with Fig. 5.
In the multiplexer, the primary LAPD rame 35 is formed. The basic information field in the frame 31 is copied into a primary information field (INFORMATION) in ~he primary frame 35. A primary control field in the primary LAPD frame 35 can take a control field 10 information such as UI or I necessary between the multiplexer and the ISDN digital switch. Therefore, in some case~, the primary control field is same as the basic control field, but in other cases the primary control ~ield is different from the basic control field.
lS TEI in the primary address ield 36 is not similar to the basia TEI but i9 a value assigned to the subscriber and registered to the primary TEI list in the ~SDN
digital switch. The TEI in the primary frame 35 will be referred to as a primary TEI.
In layer 3 for call control, the hasic TEI is written into layer 3 message a~ a part of the call reference in order to enable to distinguish the TE
originating the call reques~.
The call reference area in the layer 3 message 25 has an increased portion of one octet in the primary rate interface in comparison with the basic interface.
The basic TEI is written into the increased portion in the primary LAPD frame.

130(~768 1~

A plurality of primary LAPD frame signals formed for the subscriber lines are transmitted to the ISDN
digital switch through a primary rate transmission line after statistically multiplexed.
. 5 According to the procedure of the present invention, the basic information from a TE can be transferred to the ISDN di~ital switch reliably but the ISDN digital switch need not process two control information sets for two TEIs. That is, the subscriber 10 lines are seen from the IS~N digital switch as if they are terminal equipments in the conventional system. The ; multiplexer distinguishes TEs on a subscriber line by the TEIs. Accordingly, the ISDN digital switch is freed from an excessive load.
Reerring to Fi~. 5, the shown ISDN system is ~imilar to that in Fig~ 2 and the similar portions are represented by the same reference numerals~
In the sy~tem of the embodiment, a subscriber line multiplexer 37 is corresponding to the RDT 26 in 20 Fig. 2 but is different from the RDT 26. The subscriber line multiplxer 37 is or performing the conversion from the basic LAPD frame to the primary LAPD frame and also performing the time division multiplexing of the primary LAPD frame signals in the statistical multiplexing 25 fashion. The subscriber line multiplexer 37 comprises a plurality of subscriber interfaces 38 connected to a plurality of subscriber lines 20, a transmission line interface 39 connected to the rate transmission line 27, ~.

``~ (1076~3 an internal bus 40 connecting the plurality of subscriber interfaces 38 to the transmission line interface 3g commonly, and a timing signal line 41 for supplying a timing signal from the transmission line S interface 39 to subscriber interfaces 38. The subscriber interface 38 actually performs the conversion from the basic LAPD frame to the primary LAPD frame and transmits the primary LAPD frame signal to the internal hus 40 for performing the statistical multiplexing with 10 like primary LAPD frame signals from the other subscriber interfaces 38 to produce the multiplexed signal, The transmission line interface 39 is fox detecting a ~rc~me on the TDM signal on the primary rate transmission line 27 to produce the timing signal and lS for transmitting the multiplexed signal on the internal bu~ 40 to the tran9miqsion line 27 after adding a frame bit or a 6ynchronization bit.
In the shown embodiment, NTs are not used and TEs 24 are directly connected to the subscriber lines 20 20. That is, the subscriber interEaces 38 function as the network terminations. The ISDN digltal switch 21 is : shown to have the TEI list as a primary TEI list 42 which is for registering the primary TEI as described in connection with Fig. 4.
Now, the TDM signal on each of the subscriber lines 20 is called a basic TDM signal and the B channel and the D channel in the basic TDM signal are called a basic B channel and a basic D channel, respectively. On -` ~3~076~

the other hand, the TDM signal on the transmission line 27 is called a primary TDM signal and the B channel and the D channel in the primary TDM signal are called a primary ~ channel and a primary D channel, respectively.
Referring to Fig. 6, each of the subscriber interface 38 comprises a B/D channel multiplexer/demultiplexer (MUX/DMUX) 44, a basic LAPD
controller 45, a basic T~I list 46, a central proc~ssing unit (CPU) 47, a primary LAPD controller 48, a B channel 10 transmitter/receiver (T/R) circuit 49, and D channel transmitter/receiver (T/R) circuit 50.
A basic TDM signal (2B ~ D for the basic interface) is supplied to the B/D channel mtlltiplexer/demultiplexer 44 rom the subscriber line lS 20. The B/D channel multiple~er/demultiplexer 44 is for separatin~ the basic D channel signal or the basic control signal ~xom the basic TDM signal as a separated signal having the basic L~PD frame 31 in Fig. 4. The BjD channel multiplexer/demultiplexer 44 serves as a 20 channel separator. The separated signal is applied to the basic LAPD controller 45 while the basic B channel signal is subjected to the rate adaptation and then supplied to the B channel transmitter/receiver circuit 49 as the primary B channel signal.
The basic LAPD controller 45 refers to the basic ~` TEI list 46 and removes the basic start and stop flag portions, tha basic address field, the basic control field, and the basic frame check sequence from the ~30~)76~

derived signal (31 in Fig. 4) under control of the central processing unit 47 when the TEI in the basic address field t34 in Fig. 4) is registered in the basic TEI list 46. The remaining basic information field in 5 the basic L~PD ~rame (31) of the derived signal is supplied to the central processing unit 47 and applied to the primary LAPD controller 48 therefrom. The primary LAPD controller 48 holds the primary TEI which is assigned to the subscriber inter~ace 38 itself and 10 registered in the primary TEI list in the ISDN digital switch 42. The primary LAPD controller 48 receives the basic information field and forms the primary LAPD frame (35 in Fig~ ~) signal or the primary control signal.
There~ore, the basic LAPD controller 45, the basic TEI
15 list 46, the central proce5sing unit 47, and the primary LAPD controller 4a function to convert th~ basic control signal into the primary control signal.
~ ssignment of the basic TEI and registration of the basic TEI in the basic TEI list 46 can be performed 20 between each one of TEs 24 and the corresponding subscriber interface 38 according to a procedure similar to the TEI as~ignment procedure determined by the I
series. The primary TEI is assigned and registered in primary TEI list 42 accoxding to the similar assignment 25 procedure performed between each one of the subscriber interfaces 38 and the ISDN switch 21.
The primary control signal is applied to the D
channel transmitter/receiver circuit 50 and is , transmitted into an unused time slot in the primary D
channel on the internal bus 40 at a timing according to the timing signal applied thereto through the timing signal line 41. Thus, a plurality of primary control 5 signals made in the plurality of subscriber interface 38 are statistically multiplexed in the primary D channel.
While, the primary B channel signal is transmitted in a primary B channel on the internal bus 40 from the B channel transmitter/receiver circuit 49 in lO the time division fashion so that basic B channel signals in the plurality of the subscriber interfaces 38 are multiplexed. Thus, the primary TDM signal carries the primary B channel signals and the primary control signals to the ISDN digital switch 21 through the 15 primary rate transmission line 27~
In receivlng the primary TDM signal originated by the IS~N digital switch 21, the D channel transmitter/receiver circuit 50 receives the primary D
channel signal whi.ch i8 supplied to the primary LAPD
:20 controller 48. When the primary L~PD controller 48 detects the own primary TEI assigned to the subscriber : interface 38 itself in one of ~he primary control signals in the primary D channels, the primary LAPD
controller 48 derives the primary information field in 25 the primary control signal ~35 in Fig. 4). The derived primary information field is supplied to the basic LAPD
controller 45 through the central processing unit 47.
The basic LAPD controller 45 cooperates with the central - ~3~0768 processing unit 47 and the basic TEI list 46 and forms the basic control signal of the basic L~PD frame (31 in Fi~. 4) in which the derived primary information field is written into the basic information field and the 5 basic address field, respectively. Thus, the basic LAPD
controller 45r the basic TEI list 46~ the central processing unit 47, and the primary LAPD 48 function to convert the primary control signal to the basic control signal. The converted basic con~rol signal is supplied 10 to the B/D channel multiplexer/demultiplexer 44.
The B channel transmitter/receiver circuit 49 receives the primary ~ channel signal in a specific one o primary B channels on the primary TDM signal indicaked by the central processing unit 47. The 15 primary B channel signal is sub~ected to the rate adaptation and is also supplied ko the B/~ channel multiple~er/demultiplexer 44 as a basic B channel signal. The B/D channel multiplexer/demultiplexer 44 multiplexes the basic control signal supplied from the 20 basic LAPD controller 45 and the basic B channel signal supplied from the B channel transmitter/receiver circuit 49 to form the basic TDM signal which is transmitted to the subscriber line connected to the subscriber interface 38. One of TEs 24 on the subscriber line 20 25 receives the basic control signal according to the ba,~ic TEI in the signal.
In the embodiment, a basic data link is established by the basic LAPD signal between one of TE

- ~30()~61~
.

24 on the su~scriber line 20 and the subscriber interface 38 connected thereto. While, a primary data link i also established by the primary LAPD signal between the subscriber interface 38 and the ISDN digital 5 switch 21. Accordingly, the ISDN digital switch 21 only processes the primary control signal so that the ISDN
switch 21 has a reduced load. The subscriber interface 38 serves as a switch for TEs 24 on the corresponding subscriber line 20 for the D channel signal.
Referring to Fig. 7, ISDN system shown therein comprises a first and a second ISDN digital switches 21 and 51, a subscriber line multiplexer 37 similar to that in Figs. S ancl 6, and a cross connector 52 for cross connecting the basic rate transmission line 27 to the 15 first and the second ISDN digital switch. One of the subsaribcr lines belongs to not the first ISDN digital swikch 21 but the second I9DN digltal switch 51.
There~or~, the one sub~criber line is depicted by 20' and the corresponding subscriber interface is 2~ represented by 38'. The remaining subscriber lines 20 helong to the first digital switch 21.
Re~erring to Fig. 8, each of the subscriber interfaces 38 and 38' is provided with a setting switch 53 for manually setting a switch selection code 25 identifying one of the first and second ISDN digital switches 21 and 51. In the present embodiment, the first and second ISDN digital switches 21 and 51 are numbered to be 1 and 2, respectively. When the setting ,:

switch 53 is set to No. 1, the switch selection code is 1 and is held in ~he primary LAPD controller (48 in Fig.
6) in the corresponding subscriber interface 38. On the other hand, the setting swi~ch 53 is set to be 2 in the 5 subscribex interface 38' and the selection code of 2 is held in the primary LAPD controller 48 in the subscriber interface 38'. The switch selection code is written into the primary address field in the primary LAPD frame signal together with the primary TEI when the primary 10 LAPD controller 48 ~orms the primary LAPD frame signal 35 in Fig. 4.
The cross connector 52 comprises a detecting means for detecting the switch selection code in the primary control signal in the primary D channel on the 15 transmission line 27. When the detecting means detects the selection signal, the cross connector 52 delivers the primary control signal to one of the first and second ISDN digital ~witches 21 and 51 which is identified by the detected switch selection code.
Referring to Fig. 9, the cross connector 52 comprises a shift register 54 for receiving the primary TDM signal on the transmission line 27 which signal is shifted therein one by one bit. The shift register 54 is connected to a flag comparator 55 at a predetermined 25 stage and is commonly connected to a first and a second code comparator 56 and 57 at a different stage of the register 54 so that the switch selection code is checked `- ~30~)'768 by the first and second code comparators 56 and 57 when the start flag is checked by the flag comparator 55.
When the flag is detected by the flag comparator 55, a detected signal is delivered to a flipflop 58, and 5 AND gates 59 and 60. Thenf the flipflop 58 is set and the AND gate 60 is opened while the AND gate 59 is closed.
The first code comparator 56 has a reference value 1 and compares the switch selection code in the 10 primary TEI field with the value 1. When the selection code is 1, the code comparator 56 produces a first code detectin~ signal which is then supplied to a flipflop 61 through an AND gate 62. Then, the flipflop 61 i9 set and another AND gate 63 is open. Thus, the primary 15 control ~ignal in the shift register 54 is delivered to the first ISDN digital switch 21 through the AND gate 63.
When the selection code i5 2, the second code comparator 57 produces a second code detecting signal : 20 which sets a flipflop 64 through an AND gate 65.
Accordingly, an AND gate 66 is open and the primary control signal is delivered from the shift register 54 to the ~econd ISDN digital switch through the AND gate 66.
The first and second code comparators 56 and 57 serve as the detecting means in the cross connector 52 as described above.

-0~8 Thereafter, when the flag is detected by the flag comparator 55, the comparator 55 again produces the detected signal which resets the flipflop 58. Then, the AND gate 59 is open and the AND gate 60 is closed. The 5 detected signal is applied to a delay circuit 67 through the AND ga~e 59 and resets that one of the flipflops 61 and 64 which is in ~he set condition after being delayed by 8 bits at the delay circuit 67. Then, the corresponding one of the ~ND gates 63 and 66 is closed 10 so that the primary control signal is stopped after completion of delivering the primary control signal from the shift register 54.
In the manner as described above, the primary control signal is cro5s connected to one of ~he first 15 and the ~econd ISDN digital switches 21 and 51.
Returning to Fig. 8, each of the subscriber interaces 38 and 38' can be provided with a manual TEI
setting switch 68. As a result, the primary TEI is manually set by the manual setting switch 68. When the 20 primary TEI i8 set by the switch 68, the primary TEI
assignment and registration procedure is performed according to the I series of the CCITT as described above between the subscriber interface 38 or 38' and the first or the second ISDN digital switch 21 or 51. The 25 TEI assigned by the first or the second digital switch 21 or 51 is different from the manually set primary TEI, a lamp 69 on the subscriber in~erface 38 or 38' is illuminated. Then, the manual setting switch 68 is ,~
~, v ., .

" ~30(~76l~

again operated to set a fresh primary TEI. After repetition of the operation, the lamp 69 is extinguished when the manually set TEI is consistent with a TEI
assigned by the first or the second ISDN digital switch 5 21 o~ 51.
Referring to Fig. 10, an ISDN system according to another embodiment comprises a first basic rate transmission line 271 connected to a first ISDN digital switch and a second basic rate transmission line 272 10 connected to a second ISDN switch.
The first and second basic rate transmission lines 271 and 272 are connected to a first and a second suhscriber line multiplexer 371 and 372, respectively.
Each of the first and second subscriber line : 15 multiplexers 371 and 372 are similar to each other and comprises an arrangement similar to the subscriber line multiplex~L- 37 a~ shown in Figs. 5 and 6. Therefore, the firsk and second multiplexers 371 and 372 are shown to have blocks represented by the same reference 20 numerals as in Fig. 5 and description thereto is omitted for the purpose o simplification of the description.
Each of the subscribers lines 20 is coupled to the first and the second subscriber line multiplexers 371 and 372 through a line selector 70. The line '. 25 selector 70 is for selectively connecting the corresponding subscriber line 20 to the first and the second subscriber line multiplexers 371 and 372.

13(~0768 .

Referring to Fig. 11, the line selector 70 comprises a selection signal input switch 71.
Referring to Fig. 12, the selection signal input switch 71 comprises two key switches which are numbered 5 1 and 2. No. 1 key switch is pushed down, a selection signal is set a binary "1" for selecting the first transmission line 271. On the other hand, No. 2 key switch is pushed down, a selection signal is set a binary of "0" for selecting the second transmission line 10 272.
The subscriber line 20 is connected to a selection signal multiplexer 72 through a first port 73 so as to multiplex the basic TDM signal on the subscriber line 20 an~ the selection signal from the 15 selection signal input switch 71.
According to the I series of the CCITT, a frame o~ an elec~ric signal on the basic inter~ace comprises ~8 bits and includes an unused bits called "S" bits in addition to the ~ channel bits, the D channel bits, and 20 other used bits.
The selection signal is inserted into the "S"
bits by the selection signal multiplexer 72 to produce a multiplexed signal.
The multiplexed signal is applied to a selection 25 signal separator 74. The selection signal is separated from the multiplexed signal at the selection signal separator 74 and is supplied to a selection control circuit 75. The remaining signal that is the basic TDM

~30~)761~

signal is applied to a switch 76 which is controlled by the selection control circuit 75.
The switch 76 has a second and a third port 77 and 78 so that the ba~ic TDM signal from the selection 5 signal separator 74 is selectively connected to the second and the ~hird ports 77 and 78 by switching operation of the switch 76. The second port 77 and the . third port 78 are connected to the first and the second subcriber line multiplexers 371 and 372.
When the.No. 1 key switch is pushed down, the switch 76 is connected to the second port 77 and the basic TDM signal is applied to the corresponding sub~criber inte.rface 38 in the first subscriber line multiplex~r 371. Then, the basic TDM signal is 15 converted into the primary ~DM signal by the ~irst subscri.ber line mi.ltiplexer 371 in the similar fashion in the embodiment o~ Figs. 5 and 6 and is transmitted to the irst ISDN digital switch through the transmission line 271.
On the contrary, when No. 2 key switch is pushed down, the basic TDM signal is applied to the corresponding subscriber interface 38 in the second : subscriber line multiplexer 372 through the third port 78. Accordingly, the basic TDM signal will be converted ; 25 to the primary TDM signal and is transmitted to the second ISDN digital switch through the second transmission line 272~

, ~ ~

. , .
, ' ' -` ~ 3~

In the actual use, when one or more of the subscriber lines 20 belong to the first ISDN digital switchr those line selectors 70 connected thereto are adjusted to the selection signal of a binary bit "l" by 5 pushing the No. l key. The remaining line selectors are set to generate the selection signal of binary bit "0"
by pushing down No. 2 key switch.
When all of the subscriber lines 20 belong to the first ISDN digital switch and when the second lO transmi6sion line 272 is a private line connected to an external subscriber unit, all of the line selectors 70 may usually be adjusted to the selection signal of a binary bit "1" 50 that the basic TDM signals on all of the subscriber lines 20 are supplied to the first 15 subscriber line multiplexer 371. When it is desired to connect a speciic one of the subsariber with the private line 272, a specific line selector 70 connected to the specific subscriber line is set ~o produce the selection signal of a binary bit of "0" by pushing down 20 No. 2 key switch. Then, the specific line is coupled with the private line 272 through the second subscriber line multiplexer 372.
~ eferring to Fig. 13, an ISDN system according to another embodiment has a first and a second 25 transmission line 271 and 272 similar to the embodiment of Fig. 10. The first transmission line 271 is connected to a first ISDN digital switch, while the second transmission line 272 is connected to a second : . ... ..

~3~768 ISDN digital switch or is a private line for connecting to a specific external subscriber.
A single subscriber line multiplexer 37' comprises a first and a second transmission interface 5 391 and 392 which are arranged similar to each other and similar to the transmission line interface 39 in Fig. 6.
The subscriber line multiplexer 37' further compris~s a plurality of subscriber interfaces 38" which are connected to the first transmission line interface 10 391 through a first internal bus 401 and a first timing signal line 411. The subscriber interfaces 38" are also connected to the second transmission line interface 392 through a second internal bus 402 and a second timing signal line 412.
The baslc TDM signal on each of the subscriber lines 20 has a selection signal in each of the "S" bits , as described in connection with Fig. 11. The selection 9ignal is also input to the basic TDM signal by provision o~ a selection signal input switch and 20 selection signal multiplexer in TEs on the subscriber line 20 in the similar manner to those shown at 71 and 72 in Figs. 11 and 12.
The basic TDM signal i5 applied from the subscriber line 20 to the corresponding one of 25 subscriber interfaces 38". Each of the ubscriber interfaces 38~' detects the selection signal in the basic TDM signal and transmits the basic TDM signal in the time division ashion to that one of the first and -the ,, :;'. ''' -.: -'''' ' - , .

~3~7~3 second transml~sion lines ~71 and 272 which is indicated by the selection signal, Referring to Fig. 14, the subscriber interface 38" is similar to the subscriber interface 38 in Fig. 6 S but further comprises a selection signal detector 80 connected to the channel multiplexer/demultiplexer 44.
The channel multiplexex/demultiplexer 44 functions similar to the B/D channel multiplexer/demultiplexer 44 in Fig. 5 and also functions to separate the "S" bit 10 signal from the basic TDM signal. The "S" bit signal represents the selection signal as described above. The "S" bit signal is applied to the selection signal detector ao which decides "1" or "2" of the selection signal to produce an enabling signal and an unabling 15 signal when the selection signal is ~ecided.
The subscriber interace 38" further comprises a ~irst and a second B channel transmitter/receiver circuit q91 and 492 which is similar to each other and similar to the B channel transmitter/receiver circuit 49 20 in Fig. 6. Those first and second B channel transmitter/receiver circuits 491 and 492 are connected to the channel multiplexer/demultiplexer 44 commonly.
Those first and secon~ B channel transmitter/receiver circuits 491 and 492 are also connected to the first and 25 second transmission line interfaces 391 or 392 through the first internal bus 401 and the first timing signal line 411 and through the second internal bus 402 and the second timing si~nal line 412, respectively.

~31)~iB

The subscriber interface 38" further comprises a first and a second D channel transmitter/receiver circuit 501 and 502 which is similar to each othar and similar to the D channel transmitter/receiver circuit 50 5 in Fig. 6. Those first and second D channel transmitter/receiver circuits 501 and 502 are connected to the primary LAPD controller 48 commonly. Those first and second D channel transmitter/receiver circuits 501 and 502 are also connected to the ~irst and second 10 transmission line interfaces 391 or 392 through the fixst internal bus 401 and the first timing signal line 411 and through the second internal bus 402 and the second timing signal line 412, respectively.
When the ~election signal cletector 80 decides lS "1"~ the selection signal detector 80 delivers the enabling signal to the first B channel transmitter~recelver circ~i~ 491 and the first D channel transmitter/receiver circuit S01 and delivers the unabling signal to the second B channel 20 transmitter/receiver circuit 492 and the second D
channel txansmitter/receiver circuit 502~ Accordingly, the subscriber interface 80 transmits and receives the primary TDM signal to and from the first transmission line 271 through the first B channel 25 transmitter/receiver circuit 491, the first D channel transmitter/receiver circuit 501, the first transmission line interface 391, in the similar manner of the embodiment in Figs. 5 and 6.

, j.

~' ' ','' ,. . .

When the selection signal detector 80 decides "0", the selection signal detector 80 delivers the enabling signal to the second B channel transmitter/receiver circuit 492 and the second D
5 channel transmitter/receiver circuit 502 and delivers the unabling si~nal to the first B channel transmitter/receiver circuit 491 and the first D channel transmitter/receiver circuit 501. Accordingly, the subscriber interface 38" transmits and receives the 10 primary TDM signal to and from the second transmission line 272 through the second B channel transmi-tt~r/receiver circuit 492, the second D channel transmitter/xeceiver circuit 502, the second transmission line interface 392, in the similar manner 15 of the em~odiment in Fi~8. 5 and 6.
Thus, when the selection ~ignal input switch is pu~hed down in a ~pecific one of T~s to select one of the irst tran~mission lines 271 and 272, the specific TE is connected~to the selected one of the first and 20 second transmission lines 271 and 272.

.

,

Claims (6)

1. In an ISDN (Integrated Services Digital Network) system comprising a plurality of subscriber lines associated with a plurality of terminal equipments, a subscriber line multiplexer connected to said plurality of subscriber lines, each of said subscriber lines transmitting a basic TDM (Time Division Multiplex) signal between said plurality of terminal equipments and said subscriber line multiplexer at a basic transmission rate, said terminal equipments having different terminal equipment TEIs (Terminal Endpoint Identifiers) assigned thereto as basic TEIs, respectively, said basic TDM signal comprising a plurality of basic information channels for transmitting information and a basic control channel for transmitting a basic control signal, the basic control signal comprising a specific one of said basic TEIs and control information from a specific one of said terminal equipments, a first ISDN digital switch having a TEI
list for registering TEIs, and a high rate transmission line coupling said subscriber line multiplexer and said first ISDN digital switch for transmitting a secondary TDM signal therebetween at a primary transmission rate higher than said basic transmission rate, said primary TDM signal comprising a plurality of primary information channels and a primary control channel, said subscriber line multiplexer statistically multiplexing the basic (Claim 1 continued) control channel signals on said plurality of subscriber lines into said primary control channel to thereby transmit said control information to said first ISDN
digital switch, the improvement wherein said subscriber line multiplexer comprises:
a plurality of subscriber interface means coupled to said plurality of subscriber lines, respectively; and internal bus means coupled to said high rate transmission line and commonly connected to said plurality of subscriber interface means;
each of said subscriber interfaces comprising:
channel separating means coupled with each one of said subscriber lines for separating said basic control channel signal from said basic TDM signal to produce a separated signal;
basic TEI list for memorizing said basic TEIs;
basic control means responsive to said separated signal for deriving said control information as a derived control information, said basic control means establishing a basic data link between said specific terminal equipment and said basic control means through said basic control channel in cooperation with said basic TEI list;
primary control means having a subscriber TEI
assigned to said each one of said subscriber line interface means as a primary TEI which is registered in (Claim 1 twice continued) said TEI list as a primary TEI list of said first ISDN
digital switch, said primary control means responsive to said derived control information for making a primary control signal which comprises said primary TEI and said derived control information without said specific basic TEI; and transmitting means for transmitting said primary control signal into said primary control channel through said internal bus in the statistical multiplexing fashion so that a primary data link is established between said primary control means and said first ISDN
digital switch through said primary control channel in cooperation with said primary TEI list.

2. An ISDN system as claimed in Claim 1, wherein said subscriber line multiplexer further comprises:
transmission line interface means coupled to said high rate transmission line for detecting said secondary TDM signal on said high rate transmission line to produce a timing signal, said transmission line interface means being connected to said internal bus means for coupling said internal bus means to said high rate transmission line; and a timing signal line connecting said transmission line interface means to said plurality of subscriber interface means for delivering said timing signal to said plurality of subscriber interface means;

(Claim 2 continued) said transmitting means detecting said primary control channel based on said timing signal.

3. An ISDN system as claimed in Claim 2, which further comprises:
a second ISDN digital switch and cross connecting means for selectively coupling said high rate transmission line to one of said first and second ISDN
digital switches;
each of said subscriber interface means further comprising setting means for setting a switch selection code for selecting one of said first and second ISDN
digital switches, said switch selection code being contained in said primary control signal together with said primary TEI;
said cross connecting means comprising detecting means for detecting said switch selection code in said primary control signal to deliver said secondary control signal to that one of said first and second ISDN digital switches which is identified by said switch selection code.

4. An ISDN system as claimed in Claim 3, wherein each of said subscriber interface means further comprises means for manually setting said secondary TEI.

5. An ISDN system as claimed in Claim 1, which further comprises:
an additional high rate transmission line coupled to said subscriber line multiplexer;

(Claim 5 continued) each of said terminal equipments being provided with means for generating line selection signal for selecting one of said high rate transmission line and said additional high rate transmission line, said selection signal being contained in said basic TDM
signal to be delivered to said subscriber multiplexer;
each of said subscriber line interface means further comprising:
additional transmitting means connected to said primary control means for transmitting said primary control signal onto said additional high rate transmission line; and selection signal detector means coupled to said each one of said subscriber lines for detecting said line selection signal to produce an enabling signal and an unabling signal, said enabling signal being delivered to one of said transmitting means and said additional transmitting means to transmit said primary control signal to one of said high rate transmission line and said additional high rate transmission line which is identified by said line selection signal, while said unabling signal being delivered to the other of said transmitting means and said additional transmitting means.

6. An ISDN system as claimed in Claim 1, which further comprises:
additional transmission line; and (Claim 6 continued) a plurality of line selector means connected to said plurality of subscriber lines, respectively:
each line selector comprising:
first, second and third ports;
a switching means for selectively connecting said first port to one of said second and third port, said first port being coupled with each one of said plurality of subscriber lines, said second port being coupled with said additional transmission line, and said third port being coupled with each one of said plurality of subscriber interface means;
input means for inputting a line selection signal for indicating one of said high rate transmission line and said additional transmission line;
switch control means responsive to said line selection signal for controlling said switching means so that each one of said subscriber lines is selectively coupled to one of said additional transmission line and said each one of said subscriber interface means connected to said high rate transmission line.