Module 2: WAN Technologies 
  
2.1 WAN Technologies Overview 
   
2.1.1 WAN technology

Devices on the subscriber premises are called customer premises equipment (CPE). The subscriber owns the CPE or leases the CPE from the service provider. A copper or fiber cable connects the CPE to the service provider’s nearest exchange or central office (CO). This cabling is often called the local loop, or "last-mile". A dialed call is connected locally to other local loops, or non-locally through a trunk to a primary center. It then goes to a sectional center and on to a regional or international carrier center as the call travels to its destination.

在用戶端場所的裝置叫做用戶端設備(CPE)。用戶擁有CPE或來自服務供給者的租線。銅線或光纖纜線連結CPE到服務供給者最近的交換點或中央機房(CO)。這個纜線常被稱為本地迴路或"最後的－哩"。一個被撥接的呼叫會連接連結本地區域到其他的本地迴路，或透過主幹到主要的中心的非本地區域。於是經由區域性到地域的或國際的傳送中心的呼叫旅行到它的目的地。

2.1.2 WAN devices

WANs are groups of LANs connected together with communications links from a service provider. Because the communications links cannot plug directly into the LAN, it is necessary to identify the various pieces of interfacing equipment.

廣域網路是一群本地區域網路經由一個服務供給者接在一起的的通訊鏈結。因為通訊鏈結不能夠直接掛進本地區域網路，它需要識別各種不同的介面裝備。

2.1.3 WAN Standards

WANs use the OSI reference model, but focus mainly on Layer 1 and Layer 2. WAN standards typically describe both physical layer delivery methods and data link layer requirements, including physical addressing, flow control, and encapsulation. WAN standards are defined and managed by a number of recognized authorities.

廣域網路使用OSI參考模型，但是主要集中在第一層和第二層。廣域網路標準典型地描述實體層的傳送方法和資料鏈結層的需求，包括實體層定址，流量控制和封裝。廣域網路標準被幾個公認授權的組織定義及管理。

2.1.4 WAN encapsulation

Data from the network layer is passed to the data link layer for delivery on a physical link, which is normally point-to-point on a WAN connection. The data link layer builds a frame around the network layer data so the necessary checks and controls can be applied. Each WAN connection type uses a Layer 2 protocol to encapsulate traffic while it is crossing the WAN link. To ensure that the correct encapsulation protocol is used, the Layer 2 encapsulation type used for each router serial interface must be configured. The choice of encapsulation protocols depends on the WAN technology and the equipment. Most framing is based on the HDLC standard.

為了在在實體鏈接上傳送，來自網路層的資料被送到資料連結層，這通常是在廣域網路上點對點的連接。資料鏈結層在網路層資料之外建立一個訊框，如此一來必要的檢查和控制才能被套用。當它跨越廣域網路鏈接的時候，每個廣域網路連接使用第二層協定來封裝流量。必須確定正確的封裝協定被使用，每個路由器序列介面的第二層封裝類型一定要被設定組態。封裝協定的選擇依據廣域網路技術和設備。大多數的訊框以HDLC標準為基礎。

2.1.5 Packet and circuit switching

Packet-switched networks were developed to overcome the expense of public circuit-switched networks and to provide a more cost-effective WAN technology.

分封交換網路被發展用來克服昂貴的公眾電路交換網路並提供一種較有成本效益的廣域網路技術。

2.1.6 WAN link options

Circuit switching establishes a dedicated physical connection for voice or data between a sender and receiver. Before communication can start, it is necessary to establish the connection by setting the switches. This is done by the telephone system, using the dialed number. ISDN is used on digital lines as well as on voice-grade lines.

電路交換為聲音或資料在傳送端和接收端之間建立一個了專用的實體連接。在通訊開始之前，藉由設定交換器建立連接是必需的。這被用於撥號電話系統。整合服務數位網路在數位線路上被使用就如同在聲音等級的線路上。

To avoid the delays associated with setting up a connection, telephone service providers also offer permanent circuits. These dedicated or leased lines offer higher bandwidth than is available with a switched circuit. Examples of circuit-switched connections include:

為避免與建立連接有關的延遲，電話服務供給者通常提供永久的線路。這些專用或租線比一個交換式電路可允許提供較高的頻寬。電路交換連接的例子包括：

In these networks, the data is transmitted in labeled cells, frames, or packets through a packet-switched network. Because the internal links between the switches are shared between many users, the costs of packet switching are lower than those of circuit switching. Delays (latency) and variability of delay (jitter) are greater in packet-switched than in circuit-switched networks. This is because the links are shared and packets must be entirely received at one switch before moving to the next. Despite the latency and jitter inherent in shared networks, modern technology allows satisfactory transport of voice and even video communications on these networks.

在這些網路中，資料以標記的細胞，訊框或封包中經過一個分封交換的網路被傳輸。因為交換器之間的內部鏈結被許多使用者分享，分封交換的費用比電路交換更低。更棒的是分封交換的延遲(等待時間)和延遲的變化(劇跳)是超越電路交換網路的。因為連線是被分享，並且封包一定要被一個交換器完全地收到後才會移動到下一個交換器。儘管在分享的網路中的延遲和劇跳是本來就有的，然而現代的技術讓聲音和影像的通訊被滿意的傳送在這些網路上。

To connect to a packet-switched network, a subscriber needs a local loop to the nearest location where the provider makes the service available. This is called the point-of-presence (POP) of the service. Normally this will be a dedicated leased line. This line will be much shorter than a leased line directly connected to the subscriber locations, and often carries several VCs. Since it is likely that not all the VCs will require maximum demand simultaneously, the capacity of the leased line can be smaller than the sum of the individual VCs. Examples of packet or cell switched connections include:

為了要連接到一個分封交換網路，用戶需要到服務供給者能提供服務的一個本地迴路最近的地點。這叫做出現點(POP)。通常這是一條專用的租線。這條線路將直接被連接到用戶端這比一條專線短許多，並且通常能攜帶幾個虛擬電路。因為不是所有的虛擬電路都有可能同時地需要最大的要求，租線的能力可以比個別的虛擬電路的總和更小。分封或細胞交換的例子包括：

2.2 WAN Technologies 
   
2.2.1 Analog dialup 
 
公眾交換電話網路(PSTN)。
2.2.2 ISDN 
 
整合服務數位網路(ISDN)
2.2.3 Leased line

When permanent dedicated connections are required, leased lines are used with capacities ranging up to 2.5 Gbps.

當持續的專用連接是被須要，租線被使用的容量範圍可以達到2.5 Gbps。 

2.2.4 X.25

X.25 can be very cost effective because tariffs are based on the amount of data delivered rather than connection time or distance. Data can be delivered at any rate up to the connection capacity. This provides some flexibility. X.25 networks are usually low capacity, with a maximum of 48 kbps.

X.25可以是非常有成本效益因為費率是基於傳輸資料的總量而非連接時間或距離。資料能在任何速率的連接能力下被傳送。這提供了一些彈性。X.25網路通常是低容量，最大值是48 kbps。

X.25 technology is no longer widely available as a WAN technology in the US. Frame Relay has replaced X.25 at many service provider locations.

X.25技術在美國不再是廣泛使用的廣域網路技術。許多服務供給者已經改用訊框中繼以替換掉X.25。

2.2.5 Frame Relay

With increasing demand for higher bandwidth and lower latency packet switching, communications providers introduced Frame Relay. Although the network layout appears similar to that for X.25, available data rates are commonly up to 4 Mbps, with some providers offering even higher rates.

因為較高的頻寬及較低延遲的分封交換的需求增加，通訊提供者引入了訊框中繼。雖然網路的設計看起來接近於 X.25，允許的資料傳輸率一般是高於 4Mbps，某些提供者甚至提出更高的速率。

Frame Relay differs from X.25 in several aspects. Most importantly, it is a much simpler protocol that works at the data link layer rather than the network layer.

訊框中繼在幾個方面是不同於 X.25。最重要的，它是一個更簡單的協定工作在資料連結層而不是網路層。

Frame Relay implements no error or flow control. The simplified handling of frames leads to reduced latency, and measures taken to avoid frame build-up at intermediate switches help reduce jitter.

訊框中繼本身並沒有實作錯誤控制或流量控制。這個被簡化的訊框控制可以導致延遲降低，且在量測上避免訊框塞滿在中間的交換器幫助降低信號劇跳。

2.2.6 ATM

Communications providers saw a need for a permanent shared network technology that offered very low latency and jitter at much higher bandwidths. Their solution was Asynchronous Transfer Mode (ATM). ATM has data rates beyond 155 Mbps. As with the other shared technologies, such as X.25 and Frame Relay, diagrams for ATM WANs look the same.

通訊提供者看到在一個固接分享網路技術在更高的頻寬上提供非常低的延遲及劇跳的需要，其解決方案就是非同步傳輸模式，非同步傳輸模式能超越 155Mbps 的資料傳輸率。如同其他分享技術，例如 X.25 及訊框中繼，非同步傳輸模式廣域網路的圖示看起來是一樣的。

2.2.7 DSL

Digital Subscriber Line (DSL) technology is a broadband technology that uses existing twisted-pair telephone lines to transport high-bandwidth data to service subscribers. DSL service is considered broadband, as opposed to the baseband service for typical LANs. Broadband refers to a technique which uses multiple frequencies within the same physical medium to transmit data. The term xDSL covers a number of similar yet competing forms of DSL technologies:

數位用戶迴路(DSL) 技術是一種使用現成的雙絞線傳送高頻寬資料以服務用戶的寬頻技術，數位用戶迴路服務被視為寬頻是相對於典型本地區域網路的基頻服務。寬頻採用在同一實體媒介中多個頻率來傳送資料的技術。xDSL 這個術語涵蓋了數種相似卻呈現競爭的數位用戶迴路技術 :

2.2.8 Cable modem

Coaxial cable is widely used in urban areas to distribute television signals. Network access is available from some cable television networks. This allows for greater bandwidth than the conventional telephone local loop.

同軸纜線廣泛的用於都會地區以分配電視的訊號。網路存取可以來自於一些有線電視網路，其允許比一般常見的本地電話迴路有較大的頻寬。

2.3 WAN Design
  
2.3.1 WAN communication

Because the WAN is merely a set of interconnections between LAN based routers, there are no services on the WAN. WAN technologies function at the lower three layers of the OSI reference model. Routers determine the destination of the data from the network layer headers and transfer the packets to the appropriate data link connection for delivery on the physical connection.

因為廣域網路只是在區域網路基礎路由器間互連的集合，所以沒有服務是在廣域網路上。廣域網路技術功能在 OSI 參考模型最低的三層。路由器決定來自網路層表頭的資料的方向及在實體連接上轉送封包到適當的資料鏈路連線。

2.3.2 Steps in WAN design

Designing a WAN can be a challenging task, but approaching the design in a systematic manner can lead to superior performance at a reduced cost.

設計一個廣域網路是一個具有挑戰性的工作，而以系統性的方法完成設計在降低的成本下仍可以導致較高的效能。

2.3.3 How to identify and select networking capabilities

Many WANs use a star topology. As the enterprise grows and new branches are added, the branches are connected back to the head office, producing a traditional star topology. Star end-points are sometimes cross-connected, creating a mesh or partial mesh topology. This provides for many possible combinations for interconnections. When designing, re-evaluating, or modifying a WAN, a topology that meets the design requirements must be selected.

許多廣域網路使用星狀拓樸，當企業成長且新的分支開始加入，分支連回總公司，就產生了一個傳統的星狀拓樸。星狀的端點有時候是交叉連接產生了全網狀(mesh)或部份網狀(partial mesh)拓樸，這樣提供了許多互連的可能組合。當設計,重新評估或修改一個廣域網路,一個迎合設計需求的拓樸必須被選擇。

2.3.4 Three-layer design model

A systematic approach is needed when many locations must be joined. A hierarchical solution with three layers offers many advantages.

當許多位置必須被加入時一個系統性的方法是必需的。一個三層的階層式解決方案提供許多優點。

2.3.5 Other layered design models

Many networks do not require the complexity of a full three-layer hierarchy. Simpler hierarchies may be used.

許多網路並不需要複雜的完整三層階層式。較簡單的階層式也許就可以使用。

2.3.6 Other WAN design considerations  

Many enterprise WANs will have connections to the Internet. This poses security problems but also provides an alternative for inter-branch traffic.

許多企業的廣域網路與網際網路連接，這造成了安全性的問題但是同時也提供了內部分支流量的替代。