WO2009001972A2

WO2009001972A2 - Multi-channel formation methods for wired network

Info

Publication number: WO2009001972A2
Application number: PCT/KR2007/003088
Authority: WO
Inventors: Hyun Joo Woolsey
Original assignee: Hyun Joo Woolsey
Priority date: 2007-06-26
Filing date: 2007-06-26
Publication date: 2008-12-31
Also published as: WO2009001972A3

Abstract

Disclosed are multi-channel formation methods for assigning the same frequency band (A, B, C, D) to each cell at the location where respective cells (Cell 1-4) branch off in a cable broadcasting network or a cable Internet, which is divided into respective cells (Cell 1-4), thus enabling a limited frequency band to be reused several times. One of the methods provides the wired network dividing into a plurality of cells that include at least one subscriber terminal (1, 2, 3, 4) as a component thereof, dividing the frequency band, and providing the Internet communication channels to the subscriber terminals (1-4), wherein wired networks passing through respective cells (Cell 1-4), a frequency band assigned to the wired network at a location where the cells branch off is provided to respective cells (Cell 1-4), and a subscriber terminal (1, 2, 3, 4) belonging to one of the cells is assigned a same frequency band as a subscriber terminal (1, 2, 3, 4) belonging to another cell.

Description

[DESCRIPTION] [Invention Title]

FORMING METHOD OF MULTI CHANNEL FOR WIRE NETWORK [Technical Field] The present invention relates, in general, to a multi-channel formation method for a wired network, and, more particularly, to a multi-channel formation method for a wired network, which forms a multi^¬ channel for a wired network (for example, a cable network) having limited bandwidth, and repeatedly uses the same bandwidth through the formed multi-channel, thus improving line speed while increasing the frequency bandwidth that can be assigned to each subscriber. [Background Art]

To date, the Internet has been provided using an Asymmetric Digital Subscriber Line (ADSL) and a Very-high-bit-rate DSL (VDSL), which use an existing telephone network, a cable network that is deployed for local cable broadcasting, and a dedicated network (Tl/El lines). A telephone network is implemented using a pair of copper lines to transmit voice signals and is problematic in that the data transfer rate decreases in inverse proportion to the distance between a telephone office and a subscriber terminal. A dedicated network is generally used in Internet cafes and enterprises, but incurs a high cost, and thus it has not widely propagated to normal users. In contrast, a cable network is implemented using coaxial cable, previously deployed by a wired network service provider to transmit wired broadcasts, so that there is no need to deploy a new network. Further, since the bandwidth (800 MHz to 900 MHz) of the coaxial cable is wide, the cable network can provide suitable Internet speed. However, when Internet services are provided through the cable network, a plurality of subscribers must share and use the cable network therebetween, and thus there is a disadvantage in that the data transfer rate decreases in proportion to the increase in the number of subscribers. Since Internet services using the cable network typically allow respective subscribers to use frequency bands resulting from division of the frequency bandwidth assigned to the cable network, a frequency band of 6 MHz is assigned to each subscriber terminal, and the maximum transfer rate of the subscriber terminal is limited to about lOMbps.

FIG. 1 is a diagram conceptually showing a method of assigning frequencies to respective subscriber terminals in a conventional Hybrid Fiber Coaxial (HFC) network.

Reference numeral 10 denotes an CMJ disposed between an optical cable and a coaxial cable. The ONU 10 performs signal conversion between a cable broadcasting company, connected through an optical cable, and subscriber terminals, connected through a coaxial cable. Of the bandwidth provided from the ONU 10 to the coaxial cable, frequency bands A, B, C, and D, which can be assigned for the Internet, range from approximately 540 MHz to 900 MHz. The (MJ 10 provides the assigned frequency bands to a tap-off unit 20. The tap-off unit 20 distributes the assigned frequency bands to respective cells 1 to 4. That is, the frequency bands A, B, C, and D are assigned to the cells 1, 2, 3, and 4, respectively. Each of the cells is a region in which a cable network is assigned to a plurality of subscriber terminals. In the case of the cell 1, the frequency band A is divided into frequency bands Al, A2, A3 and A4, and the resulting frequency bands Al, A2, A3, and A4 are assigned to subscriber terminals 1 to 4, respectively. That is, in conventional Internet services provided through a wired network, the bandwidth assigned to each of the subscriber terminals 1 to 4 decreases in proportion to the number of subscribers connected to a unit cell (for example, cell 1), thus resulting in a decrease in the data transfer rate of each subscriber terminal. Further, since the cells 1 to 4 use frequency bands assigned to and distributed by the (MJ 10, there is a problem in that the number of subscriber terminals that can be covered by the ONU 10 is limited.

FIG. 2 is a conceptual block diagram showing frequency bands distributed to respective subscriber terminals in the cell of FIG.1.

As shown in FIG.2, in a cell (for example, cell 1), frequency bands distributed to respective subscriber terminals 1 to 4 cannot be obtained merely by dividing the frequency band assigned to the cell 1 by the number of subscriber terminals 1 to 4, and guard bands must be assigned to prevent the frequency bands of the subscriber terminals 1 to 4 from interfering with each other. Therefore, the conventional cable network cannot use the entire limited frequency band to provide Internet services. If the conventional cable network covers a plurality of subscriber terminals within a limited bandwidth, an expensive filter (band pass filter) for dividing a frequency band into frequency bands for respective subscriber terminals and performing filtering must be included in the QNU 10. As the width of guard bands is narrowed, a more expensive band pass filter for accurately discriminating the frequency bands of respective subscriber terminals must be used. According to the circumstances, a filter that costs as much as several million Won may be required. This problem is due to the fact that Internet services using the conventional cable network are provided such that a limited frequency band, assigned to the cable network, is divided equally, and resulting frequency bands are assigned to subscribers. FIG.3 is a diagram showing an example in which a cable network is deployed by a cable broadcasting company.

As shown in the drawing, since the conventional cable network is deployed to provide cable broadcasts to subscribers through a wired connection, it is disposed radially around the QNU 10, and the end terminals of the cable network do not meet each other. A typical network is configured to have a ring topology, in which end terminals of a network meet each other, in order to secure the stability of data transmission, while a cable network is typically configured in a radial topology, in which the end terminals of the cable network are not connected to each other, because the cable network only sends broadcasts from a cable broadcasting station to subscribers. The applicant of the present invention discards the conventional network configuration method of configuring a network in such a manner that the tap-off unit 20 equally assigns frequency bands, assigned to the CMJ 10, to the respective cells 1 to 4 equally, and proposes a multi-channel formation method for a wired network, in which the same frequency band is assigned to the cells 1 to 4, so that the same frequency band can be reused, thus increasing the frequency band that can be assigned to respective subscriber terminals and also increasing the data transfer rate of respective subscriber terminals. [Disclosure] [Technical Problem] Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a multi-channel formation method for a wired network, which enables a limited frequency band, assigned for Internet services in the wired network, to be reused several times, thus allowing a wide bandwidth to be assigned to subscriber terminals connected to the wired network. [Technical Solution]

In order to accomplish the above object, the present invention provides a multi-channel formation method for a wired network in a method of dividing a frequency band assigned to the wired network and establishing Internet communication channels in a plurality of subscriber terminals, wherein the wired network is divided into a plurality of cells that include at least one subscriber terminal as a component thereof, divide the frequency band, and thus provide the Internet communication channels to the subscriber terminals, and the multi-channel formation method is configured such that, when wired networks passing through respective cells are independent, a frequency band assigned to the wired network at a location where the cells branch off is provided to respective cells, and thus a subscriber terminal belonging to one of the cells is assigned a same frequency band as a subscriber terminal belonging to another cell.

Preferably, the wired network may be a cable broadcasting network. Preferably, the wired network may be a Hybrid Fiber Coaxial (HFC) network. Further, in order to accomplish the above object, the present invention provides a multi-channel formation method for a wired network in a method of dividing a frequency band assigned to the wired network and establishing Internet communication channels in a plurality of subscriber terminals, comprising partitioning the plurality of subscriber terminals into at least two cells, forming a first channel by dividing the frequency band assigned to the wired network into a predetermined number of frequency bands, and forming at least one second channel by dividing a frequency band in which the first channel is formed into a predetermined number of frequency bands, wherein each of the first and second channels corresponds to one of the plurality of cells so that a single channel corresponds to each cell.

Preferably, the wired network may be a cable broadcasting network.

Preferably, the first and second channels may be formed in a same frequency band.

Preferably, the first and second channels may be formed such that, in a cell to which each of the first and second channels belongs, the frequency band is divided equally and resulting frequency bands are provided to the subscriber terminals.

[Advantageous Effects]

The present invention assigns the same frequency band to each cell at the location where respective cells branch off in a cable broadcasting network or cable Internet, which is divided into respective cells, thus enabling a limited frequency band to be reused several times. Through the reuse of the frequency band, a frequency band wider than that of the prior art can be assigned to each of the subscriber terminals belonging to each cell, thus increasing the Internet communication speed of the subscriber terminal. Further, the number of channel cards is increased when the number of subscriber terminals requesting Internet communication increases, and thus a large number of subscriber terminals can be flexibly handled. [Description of Drawings]

FIG. 1 is a conceptual diagram showing a method of assigning frequencies to respective subscriber terminals in a conventional Hybrid Fiber Coaxial (HFC) network;

FIG. 2 is a conceptual block diagram showing frequency bands distributed to respective subscriber terminals in the cell of FIG. I;

FIG.3 is a diagram showing an example in which a cable network is deployed by a cable broadcasting company;

FIG. 4 is a diagram showing a multi-channel formation method according to the present invention;

FIG. 5 is a view showing the appearance of an MDU being prepared for commercialization by the present applicant; FIG. 6 is a conceptual block diagram showing an MDU when two or more channel cards correspond to a single port;

FIG.7 is a conceptual view showing the connection of channel cards with filters;

FIG. 8 is a conceptual block diagram showing the correspondence between respective ports 1 to 4 of the MDU of FIG.5 and channel cards;

FIG.9 is a flowchart showing a multi-channel formation method for a wired network according to an embodiment of the present invention; and FIG.10 is a flowchart showing a multi-channel formation method for a wired network according to another embodiment of the present invention. [Mode for Invention]

Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings.

FIG. 4 is a diagram showing a multi-channel formation method according to the present invention.

The multi-channel formation method of the present invention uses the network characteristic of a typical wired cable network, that is, the (independent) characteristic in which a cable broadcasting company assigns a frequency band using a tap-off unit, and divides and uses the assigned frequency band according to the number of subscriber terminals subscribing to each cell, and thus the end terminals of the cable networks deployed in respective cells are not connected to each other. This network characteristic was shown in FIG. 3 and described with reference thereto, and thus FIG.3 is also referred to. In the present invention, a Multi Dwelling Unit (MDU) 101 is disposed at the location where cells 1 to 4 first branch off, and the disposed MDU 101 provides the same frequency band to the respective cells 1 to 4. Generally, since a cable network is constructed to divide one frequency band and provides resulting frequency bands to the respective cells, the frequency band to be assigned to each of the subscriber terminals (for example, reference numbers 1 to 4) should be decreased when the number of subscribers increases, which results in a decrease in the data transmission path for each of the subscriber terminals. According to the. present invention, in order to solve the above-described problem, channel cards 111 to 114, which use the frequency band assigned by the cable broadcasting company, correspond to respective cells 1 to 4, and each of the channel cards 111 to 114 establishes a communication channel with each of the cells 1 to 4 through the same frequency band. For example, if the channel card 111 uses a frequency band from 800 MHz to 900 MHz, the channel cards 112, 113, and 114 also use the same frequency band from 800 MHz to 900 MHz used by the channel card 111. Each of the channel cards 111 to 114 has the form of an Ethernet card, which may be attachable/detachable to/from a motherboard 110 having a bus structure, and has a function of performing Radio Frequency (RF) communication with each of the cells 1 to 4. The shown MDU corresponds to the cells 1 to 4. Accordingly, the MDU 100 for forming a multi-channel according to the present invention has a structure that is different from that of an existing ONU. In order to distinguish it from the existing CMJ, this component is designated a Multi Dwelling Unit (MDU) 100.

The shown MDU 100 corresponds to each of the cells 1 to 4, and includes channel cards 111 to 114 for dividing the same frequency band and providing communication channels to the subscriber terminals (for example, 1 to 4), an Optical Transmitter (O)CT) connected to the cable broadcasting company through an optical cable and configured to convert data, received from the channel cards 111 to 114, into optical signals and transmit the optical signals to the cable broadcasting company, an Optical Receiver (ORX) for converting optical data, received from the cable broadcasting company, into electronic signals, and a motherboard 110 configured to have a bus structure and connected to the channel cards 111 to 114.

In the drawing, the channel cards 111 to 114 use the same frequency band A, B, C, and D. The channel card 111 provides the frequency band, composed of sections A, B, C, and D to a tap-off unit 20, which is positioned in the cell 1. The tap-off unit 20 divides the frequency band into four frequency sections A, B, C, and D, and distributes the frequency sections to the respective subscriber terminals 1 to 4. In the case where the subscriber terminals are four in number, each of the subscriber terminals 1 to 4 may perform data communication via a frequency section that is obtained by dividing the frequency band, assigned by the channel card 111, into four equal frequency sections. This structure can realize an effect in which the bandwidth becomes about four times as high as that of the prior art of FIG. 1 under the same conditions, and, theoretically, the communication speed of each of the subscriber terminals 1 to 4 is increased by about four times. Further, when the number of channel cards mounted on the motherboard 110 is increased, the number of subscriber terminals that must be managed by each of the channel cards 111 to 114 is decreased, and thus the frequency bandwidth that can be assigned to each subscriber terminal is further increased. For example, when 8 channel cards are mounted on the motherboard 110, the frequency bandwidth assigned to respective subscriber terminals (for example, subscriber terminals 1 to 4) under the same condition is approximately 8 times as wide as that of the prior art (refer to FIG. 1). As the number of channel cards mounted on the motherboard 110 is increased, the manufacturing cost of the MDU 100 is predicted to increase, but, in the present invention, the channel cards 111 to 114 use the same frequency band, so that the same type of band pass filter can be used for the channel cards 111 to 114, and thus the cost of the band pass filters is decreased. Through the decrease in the cost of the band pass filters, the increase in the cost of the channel cards 111 to 114 is compensated for. Furthermore, since the frequency band assigned to each subscriber terminal is greatly increased, there is no need for the precision of the filter to be higher than that of a conventional band pass filter. This also results in a decrease in the manufacturing cost of the filter, and thus there is no difference in cost between the MDU 100 of the present invention and the conventional CMJ. The band pass filter will be described in detail with reference to FIG.6. FIG. 5 is a view showing the appearance of an MDU being prepared for commercialization by the present applicant.

The shown MDU 100 denotes an MDU 100 formed to have four ports, wherein ports 1 to 4 correspond to cells 1 to 4, respectively. Each of the ports 1 to 4 forms an output terminal by grouping the outputs of two channel cards into one. This construction is slightly different from the construction of FIG. 4, in which each channel card manages an independent cell. This difference is due to the fact that, in a region managed by a single MDU 100, four or more cells are seldom required in many cases, and that the MDU has an appearance similar to that of the QNU (typically provided with two or four ports) used by a typical cable network service provider. It is apparent that the MDU may be provided with a number of ports corresponding to the number of channel cards mounted on the MDU 100. A description of the case where two or more channel cards are connected to a single port (for example, port 1) will be made with reference to FIG.6.

FIG.6 is a conceptual block diagram showing an MDU 100 when two or more channel cards correspond to a single port.

The MDU 100 includes channel cards 111 to 114 provided in a motherboard 110, filters 101 to 104 provided for respective frequency bands of the channel cards 111 to 114, and a splitter 130 disposed between the filters 101 to 104 and a tap-off unit. In the drawing, each of the channel cards 111 to 114 is assigned 1/3 of the frequency band assigned to the MDU 100, and divides the assigned frequency band into a plurality of frequency bands according to the number of subscriber terminals. A data communication channel is established between the channel card (for example, 111) and an interface unit on the subscriber terminal side (SU) for a corresponding frequency band resulting from division. The frequency band divided by the tap-off unit can be divided again by another tap-off unit according to the number of subscribers. The motherboard 110 is provided with a gigabit Ethernet controller (or Ethernet card) 106 connected to the channel cards 111 to 114 through a bus, and is connected to the Cable Modem Terminal System (CMTS) of an external Internet service provider for providing gigabit-level transfer rate or a local cable broadcasting company having an optical cable. In the drawing, an example in which four channel cards 111 to 114 and the gigabit Ethernet controller (or Ethernet card) 106 are assigned to a single cell (for example, cell 1) is shown, but a plurality of channel cards and filters may be mounted on the motherboard 110 to correspond to coverage managed by a single MDU 100. It is apparent that, through this structure, the same frequency band can be assigned to other cells (for example, cells 2 to 4), and thus communication channels can be established with user terminals belonging to the cells. The drawing shows that a plurality of channel cards can be assigned to a single cell (for example, cell 1).

FIG.7 is a conceptual view showing the connection of channel cards to filters.

The shown embodiment is an embodiment in which 8 channel cards 111 to 118 are mounted on a motherboard 110, and are divided into two groups, and resulting channel cards are caused to correspond to two cells (for example, cell 1 and cell 2). The channel cards 111 to 118 are mounted on the single motherboard 110, and the channel cards 111 to 114 and the channel cards 115 to 118 use the same frequency band assigned to the MDU 100. Therefore, a band pass filter (F_l) 101 applied to the channel card 111, and a band pass filter (F_l) 101a applied to the channel card 115 have the same characteristics based on the same specification. Similarly, band pass filters applied to the channel cards 112 and 116, band pass filters applied to the band pass filter 113 and 117, and band pass filters applied to the channel cards 114 and 118, respectively, have the same characteristics. In the drawing, two cells are shown and described, but, when the MDU 100 has ports for a plurality of cells, the number of band pass filters of the same type, connected to the channel cards, corresponds to the number of ports, so that band pass filters having the same characteristics can be connected to respective channel cards. Moreover, as described above with reference to FIG.4, from the standpoint of the fact that the frequency band assigned to each subscriber terminal is wider than that of the prior art through the characteristics of the present invention, in which the same frequency band can be used several times, the band pass filters 101 to 104 and 101a to 104a connected to the channel cards 111 to 118 are advantageous in that they can be less precise than the band pass filters mounted on a typical ONU.

FIG. 8 is a conceptual block diagram showing the correspondence between respective ports 1 to 4 of the MDU 100 of FIG. 5 and channel cards.

In the drawing, respective channel cards 111 to 118 correspond to four ports so that four channel cards correspond to two ports. The channel cards 111, 112, 113, and 114 correspond to ports 1 and 2, and the channel cards 115, 116, 117, and 118 correspond to ports 3 and 4. In the lower portion of the drawing, frequency bands corresponding to the channel cards 111, 112, 113, and 114, and guard bands between respective frequency bands are shown. As shown in the drawing, a frequency band from 975 MHz to 1025 MHz is assigned to the channel card 111, a frequency band from 1125 MHz to 1175 MHz is assigned to the channel card 112, a frequency band from 1300 MHz to 1350 MHz is assigned to the channel card 113, and a frequency band from 1475 MHz to 1525 MHz is assigned to the channel card 114. In the drawing, an example in which a multi-channel is formed in a frequency band higher than that of a typical cable broadcasting company, that is, a frequency band from 975MHz to 1525 MHz, is shown. Accordingly, the channel cards 111 to 118 can be connected to more subscriber terminals using a frequency band which is wider than that of the prior art. Further, guard bands from 100 MHz to 125 MHz are disposed between respective frequency bands assigned to the channel cards 111 to 114, thus minimizing the interference between the channel cards. If the conventional QNU divides the entire assigned frequency band according to the number of subscriber terminals, the widths of the guard band and the frequency band assigned to each user terminal must decrease as the number of subscriber terminals increases. When the width of the frequency band assigned to each user terminal and the frequency bandwidth of the guard band are decreased, the precision of the band pass filters connected to the channel cards 111 to 118 must be greatly increased. According to the circumstances, high precision band pass filters having a price of several million Won may be required. However, when the multi-channel formation method for a wired network according to the present invention is used, the frequency band assigned to a unit cell (for example, cell 1) can be increased, and thus the above problem can be flexibly handled. Further, when the number of subscribers for each unit area (one or more cells may be formed in the unit area) increases, the present invention can handle a large number of subscribers by reducing the size of the cells and by increasing the number of channel cards. Of course, it is apparent that the data transfer rate at which the subscriber terminal transmits or receives data over the Internet is greatly improved compared to the prior art.

FIG.9 is a flowchart showing a multi-channel formation method for a wired network according to an embodiment of the present invention.

First, in order to form a multi-channel according to the present invention, a node where cells for dividing a frequency band and providing resulting frequency bands to respective subscriber terminals are connected in common is detected in an area in which a wired network (for example, a cable network) is deployed at step S401. In the present invention, a node where respective cells are connected in common functions as a branch node where which respective cells branch off. Typically, in the case of a cable broadcasting network, subscriber terminals are grouped on the basis of certain areas to form cells, and subscriber terminals connected to a single cell are assigned a frequency band by the tap-off unit arranged in the cell.

Next, whether the end terminals of respective cells, which branch off at the wired network, are connected to each other (that is, whether respective cells are independent) is determined at step S402. Since the wired network is deployed to provide cable broadcasts, it is not formed of a network having a ring topology, unlike the typical structure of the Internet. As a result of the determination, if the end terminals of respective cells, that is, wired networks passing through the cells, are determined to be independent (for example, they do not have a ring topology), the node where respective cells branch off provides the same frequency band to the cells at step S403. For this operation, the MDU 100, as described above with reference to FIGS.4 to 7, must be arranged at the location where the cells branch off.

FIG. 10 is a flowchart showing a multi-channel formation method for a wired network according to another embodiment of the present invention. First, subscriber terminals, subscribing to a wired network (for example, a cable network), are partitioned and divided into two or more cells at step S501. The cells can be formed by partitioning the area in which the subscriber terminals are located into a number of regions. Preferably, the cells are partitioned according to the number of subscriber terminals that can be accommodated by the MDU 100. Next, whether respective cells are independent is determined at step S502. If respective cells are determined to be independent, the MDU 100 is installed at the node at which respective cells branch off at step S503. The installed MDU 100 has been described in detail with reference to FIGS.4 to 7, and thus a detailed description thereof is omitted. Next, a first channel is formed by distributing the frequency band assigned to the MDU 100 according to the number of subscriber terminals subscribing to the cell at step S504. Thereafter, the MDU 100 forms a second channel having the same frequency band as the first channel at step S505. The second channel denotes at least one channel having the same frequency band as the first channel. The MDU 100 can form two or more channels. If the first channel is formed for the cell 1, the plurality of second channels formed by the MDU 100 is assigned the same frequency band as the first channel, and then assigns the frequency band assigned to it to the cells connected thereto. Finally, the MDU 100 causes the first channel and the plurality of second channels to correspond to given cells, thus establishing communication channels required for Internet access in the subscriber terminals at step S506.

Claims

[CLAIMS] [Claim 1]

A multi-channel formation method for a wired network in a method of dividing a frequency band assigned to the wired network and establishing Internet communication channels in a plurality of subscriber terminals, wherein: the wired network is divided into a plurality of cells that include at least one subscriber terminal as a component thereof, divide the frequency band, and thus provide the Internet communication channels to the subscriber terminals; and the multi-channel formation method is constructed such that, when wired networks passing through respective cells are independent, a frequency band assigned to the wired network at a location where the cells branch off is provided to respective cells, and thus a subscriber terminal belonging to one of the cells is assigned a same frequency band as a subscriber terminal belonging to another cell.

[Claim 2]

The multi-channel formation method according to claim 1, wherein the wired network is a cable broadcasting network.

[Claim 3]

The multi-channel formation method according to claim 1, wherein the wired network is a Hybrid Fiber Coaxial (HFC) network.

[Claim 4]

A multi-channel formation method for a wired network in a method of dividing a frequency band assigned to the wired network and establishing Internet communication channels in a plurality of subscriber terminals, comprising: partitioning the plurality of subscriber terminals into at least two cells; forming a first channel by dividing the frequency band assigned to the wired network into a predetermined number of frequency bands; and forming at least one second channel by dividing a frequency band in which the first channel is formed into a predetermined number of frequency bands, wherein each of the first and second channels corresponds to one of the plurality of cells so that a single channel corresponds to each cell.

[Claim 5]

The multi-channel formation method according to claim 4, wherein the wired network is a cable broadcasting network.

[Claim 6]

The multi-channel formation method according to claim 4, wherein the first and second channels are formed in a same frequency band.

[Claim 7] The multi-channel formation method according to claim 4, wherein the first and second channels are formed such that, in a cell to which each of the first and second channels belongs, the frequency band is divided equally and resulting frequency bands are provided to the subscriber terminals.