The traditional PSTN was based on an analog connection between the customer premises and the local exchange, also called the local loop. The analog circuits introduce limitations on the bandwidth that can be obtained on the local loop. Circuit restrictions do not permit analog bandwidths greater than approximately 3000 Hz. ISDN technology permits the use of digital data on the local loop, providing better access speeds for the remote users.

傳統的PSTN是建基於客戶住宅及本地交換機樓之間的一個類比連接，亦稱為本地迴圈。 這些類比電路會引致由本地迴圈所產生頻寬上的限制。一般電路的限制並不容許有超過約3000赫茲的類比頻寬，但ISDN技術使數位資料能應用在本地迴圈上，並為遠端使用者提供更高的存取速度。

Telephone companies developed ISDN with the intention of creating a totally digital network. ISDN allows digital signals to be transmitted over existing telephone wiring. This became possible when the telephone company switches were upgraded to handle digital signals.

電話公司發展ISDN的背後是為了建立一個完全的數位網路。既然ISDN容許數位訊號被傳送於現有的電話接線上，只要電話公司的交換器都被提升至可處理數位訊號便行了。

ISDN brings digital connectivity to remote sites. The following list provides some of the benefits of ISDN:

ISDN能為網路之遠端帶來數位連接的能力. 下列乃是ISDN所提供的一些優點：

• Carries a variety of user traffic signals, including data, voice, and video
• Offers much faster call setup than modem connections
• B channels provide a faster data transfer rate than modems
• B channels are suitable for negotiated Point-to-Point Protocol (PPP) links

• 運送多種不同的使用者流量訊號，包括數據，話音及影像
• 提供比數據機連線更快的呼叫建立
• B通道提供比數據機更快的資料傳輸速度
• B通道適用於協商式點對點協定(PPP)鏈結

ISDN is a versatile service able to carry voice, video, and data traffic. It is possible to use multiple channels to carry different types of traffic over a single connection.

ISDN是一項能提供數據，話音及影像傳輸的多功能服務。利用單一連線中的多條通道來運送不同類型的資訊絕對是可行的。

ISDN standards define two main channel types, each with a different transmission rate.

The bearer channel, or B channel, is defined as a clear digital path of 64 kbps. It is said to be clear because it can be used to transmit any type of digitized data in full-duplex mode. For example, a digitized voice call can be transmitted on a single B channel.

The second channel type is called a delta channel, or D channel. There can either be 16 kbps for the Basic Rate Interface (BRI) or 64 kbps for the Primary Rate Interface (PRI). The D channel is used to carry control information for the B channel.

ISDN標準共定立兩項主要的通道種類，它們分別有不同的傳輸速度。傳送通道(或稱B通道)是被定義為一條64kbps的無障礙資料路徑。它被形容為無障礙，是因為它可用來以全雙工模式傳輸任何種類的數位化資料。例如一個數位化話音呼叫能被傳輸於一條B通道上。第二個通道種類名叫delta通道(或稱D通道)。這裡可能是16kbps的基本速率介面(BRI)或是64kbps的主要速率介面(PRI)。 這D通道是用來運送控制資訊給B通道使用的。

When a TCP connection is established, there is an exchange of information called the connection setup. This information is exchanged over the path on which the data will eventually be transmitted. Both the control information and the data share the same pathway. This is called in-band signaling. ISDN however, uses a separate channel for control information, the D channel. This is called out-of-band signaling.

當要建立一個TCP的連線時，當中有一項資訊上的交換，稱為連接建立。那進行資訊交換的路徑，最終會用來傳輸資料，所以控制資訊及資料是共用一條路徑的，這就是所謂頻帶內傳訊。然而，ISDN則利用一條獨立的通道(即D通道)來傳輸控制資訊，就稱為頻帶外傳訊。

ISDN specifies two standard access methods, BRI and PRI. A single BRI or PRI interface provides a multiplexed bundle of B and D channels.

ISDN指定了兩個標準的存取方法，分別是BRI和PRI。每一個BRI或PRI介面都提供一個由B與D通道編成的多工組合。

BRI uses two 64 kbps B channels plus one 16kbps D channel. BRI operates with many Cisco routers. Because it uses two B channels and one D channel, BRI is sometimes referred to as 2B+D.

BRI是利用兩條64kbps的B通道加上一條16kbps 的D通道來運行在許多Cisco路由器之間的。由於它採用2條B通道及一條D通道，故BRI有時候會被寫成2B+D。

The B channels can be used for digitized speech transmission. Also, the B channels can be used for relatively high-speed data transport. In this mode, the information is carried in frame format, using either high-level data link control (HDLC) or PPP as the Layer 2 protocol. PPP is more robust than HDLC.

B通道能用作數位化的音訊傳輸。此外, B通道還可用作較為高速的資料傳輸。在此模式中, 資訊是以訊框的格式來運送, 所採用的第二層協定乃是高階資料鏈結控制(HDLC)或者是PPP。PPP比HDLC更為實用。

ISDN is considered a circuit-switched connection. The B channel is the elemental circuit-switching unit.

ISDN可算是電路交換連接的一種，而B通道就是那個基本的電路交換單元。

The D channel carries signaling messages, such as call setup and teardown, to control calls on B channels. Traffic over the D channel employs the Link Access Procedure on the D Channel (LAPD) protocol. LAPD is a data link layer protocol based on HDLC.

D通道所運送的是傳訊訊息，例如是用來控制B通道傳呼的呼叫建立與解除。在D通道上的流量須應用在D通道上的鏈路存取程序(LAPD)之協定。LAPD建基於HDLC之上，也屬於資料鏈結層協定之一種。

In North America and Japan, PRI offers twenty-three 64 kbps B channels and one 64 kbps D channel. A PRI offers the same service as a T1 or DS1 connection. In Europe and much of the rest of the world, PRI offers 30 B channels and one D channel in order to offer the same level of service as an E1 circuit. PRI uses a Data Service Unit/Channel Service Unit (DSU/CSU) for T1/E1 connections.

在北美和日本，PRI可提供廿三條64kbps的B通道和一條64kbps的D通道。每個PRI都能提供與T1或DS1連線相同的服務。在歐洲及世界上許多其他地方，PRI利用卅條B通道及一條D通道來提供與一條E1電路同級的服務。PRI採用資料服務單位/通道服務單位(DSU/CSU)作為T1/E1連線。

ISDN utilizes a suite of ITU-T standards spanning the physical, data link, and network layers of the OSI reference model:

ISDN所採用的是一套橫跨OSI參考模型的實體層、資料鏈結層和網路層的ITU-T標準：

• The ISDN BRI and PRI physical layer specifications are defined in ITU-T I.430 and I.431, respectively.
• ISDN的BRI和PRI的實體層規格分別是定義於ITU-T的I.430和I.431中.

• The ISDN data link specification is based on LAPD and is formally specified in the following:

  ISDN的資料鏈結層規格乃建基於IAPD之上，且為正式訂立如下:

  • ITU-T Q.920
  • ITU-T Q.921
  • ITU-T Q.922
  • ITU-T Q.923

• The ISDN network layer is defined in ITU-T Q.930, also known as I.450 and ITU-T Q.931, also known as I.451. These standards specify user-to-user, circuit-switched, and packet-switched connections.
• ISDN之網路層是被定義於ITU-T的Q.930(亦稱I.450)及Q.931(亦稱I.451)之中。這些標準訂立了使用者對使用者的、電路交換的及分封交換的連線。

BRI service is provided over a local copper loop that traditionally carries analog phone service. While there is only one physical path for a BRI, there are three separate information paths, 2B+D. Information from the three channels is multiplexed into the one physical path.

BRI服務是在運送傳統類比電話服務的本地銅線迴圈上提供的。當這裡只有一條實體路徑給BRI使用，即是說這裡共有三條獨立的資訊路徑(2B+D)。從這三條通道而來的資訊是經過多工化而成為這一條實體路徑。

Note that the physical bit rate for the BRI interface is 48*4000 = 192 kbps. The effective rate is 144 kbps = 64 kbps + 64 kbps + 16 kbps (2B+D).

注意這個BRI介面的實體層的位元速率是48*4000=192kpbs，而有效速率則是144 kpbs = 64 kbps + 64 kbps + 16 kbps (2B+D)。

Layer 2 of the ISDN signaling channel is LAPD. LAPD is similar to HDLC. LAPD is used across the D channel to ensure that control and signaling information is received and flows properly.

ISDN的傳訊通道之第2層是LAPD。LAPD跟HDLC相似。LAPD是用於D通道之上，以確保控制與傳訊資料能正常地接收和流動。

ISDN standards define functional groups as devices or pieces of hardware that enable the user to access the services of the BRI or PRI. Vendors can create hardware that supports one or more functions. ISDN specifications define four reference points that connect one ISDN device to another. Each device in an ISDN network performs a specific task to facilitate end-to-end connectivity.

ISDN的標準把功能群組定義成一些設備或硬件的組件，好讓使用者存取BRI或PRI的服務。廠商們更能創造可支援一個或多功能的硬件。ISDN的規格共定義了四個參考點來將ISDN的設備一個一個的連接起來。 ISDN網路中的每一個設備都會執行特定的工序，使促成端對端的連接。

To connect devices that perform specific functions, the interface between the two devices needs to be well defined. These interfaces are called reference points. The reference points that affect the customer side of the ISDN connection are as follows:

要將會執行特定功能的設備連接起來，就必須對兩個設備的介面定義清楚。這些介面稱為參考點。 跟ISDN連線的客戶端有關的參考點如下：

• R — References the connection between a non-ISDN compatible device Terminal Equipment type 2 (TE2) and a Terminal Adapter (TA), for example an RS-232 serial interface.
• S — References the points that connect into the customer switching device Network Termination type 2 (NT2) and enables calls between the various types of customer premises equipment.
• T — Electrically identical to the S interface, it references the outbound connection from the NT2 to the ISDN network or Network Termination type 1 (NT1).
• U — References the connection between the NT1 and the ISDN network owned by the telephone company.

• R — 為終端設備2型(TE2)這種非兼容ISDN的設備及終端介面卡(TA)之間的連接作參考。
• S — 為接進網路終端2型(NT2)這類客戶交換設備的接點作參考，促成不同種類的用戶租用設備間之傳呼。
• T — 在電力上等同S介面，為由NT2至ISDN網路或網路終端1型(NT1)之外向連接作參考。
• U — 為NT1與電話公司的ISDN網路間之連接作參考。

Because the S and T references are electrically similar, some interfaces are labeled S/T interfaces. Although they perform different functions, the port is electrically the same and can be used for either function.

由於S和T參考點在電力上是相似的，有些介面會被稱為S/T介面。雖然它們會執行不同的功能，但連接埠在電力上是相同的，也能供兩者使用。

In North America, the NT1 is part of the Customer Premise Equipment (CPE). This means that the customer must supply an NT1 device or a device with integrated NT1 functionality. In North America, ISDN routers are typically equipped with ISDN BRI-U interface cards to provide NT1 functionality. In Europe, the service provider supplies a separate NT1 device. Therefore, the customer supplies an ISDN capable device to connect to the NT1, such as a router with an ISDN BRI-ST interface. 

在北美，NT1是用戶租用設備(CPE)的一部份。意思即是客戶必須自行提供一個NT1或一個能提供綜合的NT1功能的設備。在北美，ISDN的路由器是獨有地內置了ISDN的BRI-U介面卡，從而提供NT1的功能。在歐洲，服務供應商會提供一個獨立的NT1設備。這樣客戶只須提供一個兼容ISDN的設備，例如是一個擁有ISDN的BRI-ST介面的路由器，便可連上這個NT1設備了。

If the router has a connector labeled BRI then it is already ISDN-enabled. With a native ISDN interface already built in, the router is a TE1 and will need to connect to an NT1. If the router has a U interface, it also has a built-in NT1.

假如這路由器有一個標籤著BRI的連接器，它便算是兼容ISDN了。以一個內置的ISDN介面，這路由器算是一個TE1，並須要連接到NT1去。如果路由器擁有一個U介面，它應該已內置有NT1了。

If the router does not have a connector labeled BRI, and it is a fixed-configuration, or non-modular router, then it must use an existing serial interface. With non-native ISDN interfaces such as serial interfaces, an external TA device must be attached to the serial interface to provide BRI connectivity. If the router is modular it may be possible to upgrade to a native ISDN interface, providing it has an available slot.

假如這路由器並沒有一個標籤著BRI的連接器，並且是一個已固定結構，或非模組化的路由器，那麼它必須要使用現存的串聯介面，而一個外置的TA設備必須要連接到這個串聯介面來提供BRI連接。但如果這路由器是模組化的，且有空置的槽位，它便可能被升級成擁有原裝的ISDN介面了。