Frame Relay is an International Telecommunication Union Telecommunications Standardization Sector (ITU-T) and American National Standards Institute (ANSI) standard. Frame Relay is a packet-switched, connection-oriented, WAN service. It operates at the data link layer of the OSI reference model. Frame Relay uses a subset of the high-level data-link control (HDLC) protocol called Link Access Procedure for Frame Relay (LAPF). Frames carry data between user devices called data terminal equipment (DTE), and the data communications equipment (DCE) at the edge of the WAN.

訊框中繼是國際電訊聯盟的電訊標準化部門和美國國家標準局的標準，是分封交換和連接導向式的廣域網路服務。訊框中繼在OSI model的第二層（Data link）作用，使用高階數據鏈路控制（HDLC）底下的連接存取程序（Link Access procedure）， 訊框在資料終端設備（DTE）之間帶有所需的資訊（資料），在廣域網路的邊緣有資料通訊設備（DCE）。

A Frame Relay network may be privately owned, but it is more commonly provided as a service by a public carrier. It typically consists of many geographically scattered Frame Relay switches interconnected by trunk lines.

訊框中繼可以是私有的但大部分都是由公用載體（carrier）所提供，基本上載體包含地理上散亂的訊框中繼交換單元，它們被主要的線路（Trunk line）所連接。

Frame Relay is often used to interconnect LANs. When this is the case, a router on each LAN will be the DTE. A serial connection, such as a T1/E1 leased line, will connect the router to a Frame Relay switch of the carrier at the nearest point-of-presence for the carrier. The Frame Relay switch is a DCE device. Frames from one DTE will be moved across the network and delivered to other DTEs by way of DCEs.

訊框中繼經常使用在各個區域網路的連結，區域網路上的路由器被當成資料終端設備，像T1或E1此類串列式連結的專線會連接到一個載體的訊框中繼轉接點，訊框中繼轉接點是一種資料通訊設備。訊框從一個資料終端設備移動到另一個資料終端設備是藉由各個資料通訊設備的路徑。

The connection through the Frame Relay network between two DTEs is called a virtual circuit (VC). Virtual circuits may be established dynamically by sending signaling messages to the network. In this case they are called switched virtual circuits (SVCs). However, SVCs are not very common. Generally permanent virtual circuits (PVCs) that have been preconfigured by the carrier are used. A VC is created by storing input-port to output-port mapping in the memory of each switch and thus linking one switch to another until a continuous path from one end of the circuit to the other is identified.

在資料終端設備建立的連結稱為虛擬電路，虛擬電路可被動態的建立藉由傳送訊號訊息到網路中這種例子我們稱為交換虛擬電路（switched virtual circuits）。但是虛擬交換電路還不普遍，通常都使用固定的虛擬線路（permanent virtual circuits）。虛擬電路的建立是藉由儲存記憶體中input-port到output-port的連續應對，及從一個交換器到另一個可辨識交換器的連通。沒有在區域網路上的計算設備也會在訊框中繼網域傳送資料，但它是以其他訊框中繼設備（Frame Relay Access Device）當成是資料終端設備。

Because it was designed to operate on high-quality digital lines, Frame Relay provides no error recovery mechanism. If there is an error in a frame, as detected by any node, it is discarded without notification.

訊框中繼是建立在有較高品質的傳輸媒介上面，所以它沒有錯誤復原的技術，假設有任何錯誤的訊框發生，就直接把此訊框丟棄而不用任何回報。

The router connected to the Frame Relay network may have multiple virtual circuits connecting it to various end points. This makes it a very cost-effective replacement for a mesh of access lines. With this configuration, each end point needs only a single access line and interface. More savings arise as the capacity of the access line is based on the average bandwidth requirement of the virtual circuits, rather than on the maximum bandwidth requirement.

連接到訊框中繼網路的訊框中繼存取設備或路由器可以有多重虛擬電路連接到不同的端點，這跟網狀聯結的方式比起來更有經濟效益，依照此方式每個端點只需要一條單一的存取線路或介面。存取線的容量是基於虛擬電路的平均頻寬需求而不是最大的頻寬需求。

The various virtual circuits on a single access line can be distinguished because each VC has its own Data Link Connection Identifier (DLCI). The DLCI is stored in the address field of every frame transmitted. The DLCI usually has only local significance and may be different at each end of a VC.

在同一條存取線上的不同虛擬電路是可以被分辨出來的，因為它們有自己的資料連接頻道辨識器（Data Link Channel Identifier） ，資料連接頻道辨識器是存在每個訊框的位址欄，資料連接頻道辨識器只在局部上有較重要的意義，在每個虛擬電路上都有可能不同。

Frame Relay functions by doing the following:

• Takes data packets from a network layer protocol, such as IP or IPX
• Encapsulates them as the data portion of a Frame Relay frame
• Passes them to the physical layer for delivery on the wire

訊框中繼有下列的功能：
 

在網路層協定「ex IP或IPX」中抓取資料封包。
 

把它們當成是資料的部分壓縮在訊框終。
 

The physical layer is typically EIA/TIA-232, 449 or 530, V.35, or X.21. The Frame Relay frame is a subset of the HDLC frame type. Therefore it is delimited with flag fields. The 1-byte flag uses the bit pattern 01111110. The Frame Check Sequence (FCS) is used to determine if any errors in the layer 2 address field occurred during transmission. The FCS is calculated prior to transmission and the result is inserted in the FCS field. At the distance end, a second FCS value is calculated and compared to the FCS in the frame. If the results are the same, the frame is processed. If there is a difference, the frame is discarded. No notification is sent to the source when a frame is discarded. Error control left to the upper layers of the OSI model.

實體層可能有下 EIA/TIA-232、449 or 530，V.35，X.21。訊框中繼的訊框是高階數據鏈路控制的一種訊框，使用旗標欄來分界。旗標欄使用1位元組01111110。訊框檢查順序（Frame Check Sequence）用來檢測在第二層的位址欄在傳輸時是否有錯誤發生，在傳送前就必須計算訊框檢查順序並把結果存在訊框檢查順序欄位，在一個距離的端點，第二個訊框檢查順序會被計算並且跟前一個作比較，假如比較結果是一樣的，這個訊框就會被傳送，如果不同就放棄此訊框，放棄時不會通知來源地點，錯誤控制交給OSI模式的較上層去管理。

The serial connection or access link to the Frame Relay network is normally a leased line. The speed of the line is the access speed or port speed. Port speeds are typically between 64 kbps and 4 Mbps. Some providers offer speeds up to 45 Mbps.

訊框中繼的串列連線或存取線路通常是一條專線，線路的速度稱為存取速度或阜速度，阜速度通常在64kbps和4Mbps之間，有些提供到45Mbps。

Usually there are several PVCs operating on the access link with each VC having dedicated bandwidth availability. This is called the committed information rate (CIR). The CIR is the rate at which the service provider agrees to accept bits on the VC.

在同一條線路上可能有好幾個固定的虛擬線路，每條線路有固定的頻寬大小，這稱為約定之資訊速率（committed information rate CIR）。CIR是服務提供端同意去接收的位元數。

Individual CIRs are normally less than the port speed. However, the sum of the CIRs will normally be greater than the port speed. Sometimes this is a factor of 2 or 3. Statistical multiplexing accommodates the bursty nature of computer communications since channels are unlikely to be at their maximum data rate simultaneously.

每個CIR都小於阜速度，然而全部CIR的總合會大於阜速度，因為每個頻道同時間不會有最大的資料速率所以經過統計可以容納超出的原本電腦可通信的量。

While a frame is being transmitted, each bit will be sent at the port speed. For this reason, there must be a gap between frames on a VC if the average bit rate is to be the CIR.

訊框在傳輸的時候，每個位元會以阜速度在傳送，因此，假如平均的位元速率為CIR則在虛擬電路傳送的訊框之間必須要有一個間隙（GAP）。

The switch will accept frames from the DTE at rates in excess of the CIR. This effectively provides each channel with bandwidth on demand up to a maximum of the port speed. Some service providers impose a VC maximum that is less than the port speed. The difference between the CIR and the maximum, whether the maximum is port speed or lower, is called the Excess Information Rate (EIR).

The time interval over which the rates are calculated is called the committed time (T_c). The number of committed bits in T_c is the committed burst (B_c). The extra number of bits above the committed burst, up to the maximum speed of the access link, is the excess burst (B_e).

Although the switch accepts frames in excess of the CIR, each excess frame is marked at the switch by setting the Discard Eligible (DE) bit to "1" in the address field.

這個時間的間隔稱為約定時間Tc（Committed time）。在約定時間裡的所有位元稱為約定突發（Committed Burst）Bc，其他超出約定突發在存取連結上到達最大速度的稱超抑溢突發（excess burst）Be。雖然轉接點可接受這些超出CIR的訊框，但是這些訊框在位址欄裡面可選擇丟棄的位元（Discard Eligible（DE） bit）設為1。

The switch maintains a bit counter for each VC. An incoming frame is marked DE if it puts the counter over B_c. An incoming frame is discarded if it pushes the counter over B_c + B_e. At the end of each T_c seconds the counter is reset. The counter may not be negative, so idle time cannot be saved up.

轉接點對每個虛擬電路有一個位元記數器對應之，假設記數器被改為Bc 則接收的訊框標示為DE。假設記數器為Bc+Be則訊框標示為丟棄，在每個Tc的最後記數器被重新設置。

Frames arriving at a switch are queued or buffered prior to forwarding. As in any queuing system, it is possible that there will be an excessive buildup of frames at a switch. This causes delays. Delays lead to unnecessary retransmissions that occur when higher-level protocols receive no acknowledgment within a set time. In severe cases this can cause a serious drop in network throughput.

當一個訊框到達一個轉接點要往下一個點前進時必須先排隊或被緩衝起來。在任何的排隊系統中轉接點可能超過可以負載的訊框量，這會造成延遲，延遲會造成較上層協定接收不到東西而有重新傳送的動作，傳輸的效能就會降低。

To avoid this problem, Frame Relay switches incorporate a policy of dropping frames from a queue to keep the queues short. Frames with their DE bit set will be dropped first.

為了解決此問題，訊框中繼的轉接點會有丟棄訊框的策略，有標示為DE在緩衝中的訊框會先被丟棄。

When a switch sees its queue increasing, it tries to reduce the flow of frames to it. It does this by notifying DTEs of the problem by setting the Explicit Congestion Notification (ECN) bits in the frame address field.

再轉接點排隊的訊框增加時，它會設法降低訊框流進的量，藉由設定擁塞通知（Explicit Congestion Notification）位元來來告知資料終端設備來達成此動作。

The Forward ECN (FECN) bit is set on every frame that the switch receives on the congested link. The Backward ECN (BECN) bit is set on every frame that the switch places onto the congested link. DTEs receiving frames with the ECN bits set are expected to try to reduce the flow of frames until the congestion clears.

If the congestion occurs on an internal trunk, DTEs may receive notification even though they are not the cause of the congestion.

The DE, FECN and BECN bits are part of the address field in the LAPF frame.

前向擁塞通知位元會被設定在每個訊框，轉接點則在擁塞的鏈結上接收它，後向擁塞通知位元也會被設定在每個訊框，轉接點在擁塞的鏈結上放置它。資料終端設備接收ECN位元時就知道要減少流量以降低擁塞。

Frame Relay is unlikely to be cost-effective when only two sites are interconnected with a point-to-point connection. Frame Relay is more cost-effective where multiple sites must be interconnected.

訊框中繼在只有兩個端點的點對點連結是沒有效率的,在多個端點間的連結才會有效率。

WANs are often interconnected as a star topology. A central site hosts the primary services and is connected to each of the remote sites needing access to the services. In a hub and spoke topology the location of the hub is chosen to give the lowest leased line cost. When implementing a star topology with Frame Relay, each remote site has an access link to the frame relay cloud with a single VC. The hub has an access link with multiple VCs, one for each remote site. Because Frame Relay tariffs are not distance related, the hub does not need to be in the geographical center of the network.

廣域網路通常使用星狀拓撲，中間的區域是作為其他端點服務的主區域，要實作訊框中繼的星狀拓撲時，每個遠端區域有一個存取連結到訊框叢聚中，每個叢有一個虛擬電路。 需要用到網狀拓撲服務有兩個主要原因，各個連接點在地理位置上是分散的和高可靠度的存取，每一個區域都有線路和其它區域相連接。不像專線的互連，網狀可以不經任何硬體設備達到訊框中繼的功能。它必須要使用更多的虛擬電路，我們可以從存在的星狀拓撲去改進成網狀拓撲，使用更多的虛擬電路是訊框中繼可以達到最高效率的一個原則。

A full mesh topology is chosen when services to be accessed are geographically dispersed and highly reliable access to them is required. With full mesh, every site is connected to every other site. Unlike with leased line interconnections, this can be achieved in Frame Relay without additional hardware. It is necessary to configure additional VCs on the existing links to upgrade from star to full mesh topology. Multiple VCs on an access link will generally make better use of Frame Relay than single VCs. This is because they take advantage of the built-in statistical multiplexing.

當服務是從分散的地方和高可靠度的存取須選擇完全的網狀拓撲,當使用完全的網狀拓撲時,每個地點都互相連結.不像專線的連接,這可以不需要額外的硬體而只用訊框傳輸達到目的.在固定的連結上由星狀到網狀拓撲, 設定額外的虛擬電路是必須的.在存取線路使用多重虛擬電路會比單一虛擬電路有效率,因為這使用到內建統計多工的優勢。

For large networks, full mesh topology is seldom affordable. This is because the number of links required for a full mesh topology grows at almost the square of the number of sites. While there is no equipment issue for Frame Relay, there is a limit of less than 1000 VCs per link. In practice, the limit will be less than that, and larger networks will generally be partial mesh topology. With partial mesh, there are more interconnections than required for a star arrangement, but not as many as for a full mesh. The actual pattern is very dependant on the data flow requirements.

對較大的網域而言，很好會使用網狀拓撲，因為要建立的連結數是以平方的速度變化。架設我們對增加設備無義論的時候，每條連結的最大虛擬電路數目應該小於1000，就實際而言，虛擬電路的數目都是小於1000，且大型網域通常使用部分的網狀拓撲，使用部分網狀拓撲的連結量不像像全部網狀拓撲那麼多，又有比星狀拓撲優異的特性，而使用部分網狀拓撲需求是依照資料流量需求而定。

In any Frame Relay topology, when a single interface is used to interconnect multiple sites, there may be reachability issues. This is due to the nonbroadcast multiaccess (NBMA) nature of Frame Relay. Split horizon is a technique used by routing protocols to prevent routing loops. Split horizon does not allow routing updates to be sent out the same interface that was the source of the route information. This can cause problems with routing updates in a Frame Relay environment where multiple PVCs are on a single physical interface.

在任何訊框中繼的架構下，當一個單一的介面連接到多重區域時必須達成某些要求，因為訊框中繼沒有廣播多重存取（NBMA）的功能，我們使用Split horizon的技術，Split horizon技術是不准對相同的來源介面作重複傳送的功能，它是依據來源介面的路由資訊；但是對單一介面上有多個固定虛擬電路會產生某些問題。

Whatever the underlying topology of the physical network, a mapping is needed in each FRAD or router between a data link layer Frame Relay address and a network layer address, such as an IP address. Essentially, the router needs to know what networks are reachable beyond a particular interface. The same problem exists if an ordinary leased line is connected to an interface. The difference is that the remote end of a leased line is connected directly to a single router. Frames from the DTE travel down a leased line as far as a network switch, where they may fan out to as many as 1000 routers. The DLCI for each VC must be associated with the network address of its remote router. This information can be configured manually by using map commands. The DLCI can also be configured automatically using Inverse ARP.

不管在任何實體網路的拓撲下，訊框中繼設備和路由器都必須要在網路層作應對的動作，基本上路由器必須知道特定的介面連接到各種不同網域，相同的問題也存在一般專線連接當中，不同的地方在專線是由遠端連接到特定的路由器。訊框藉由網路轉接點一直傳送出去，最多可經過1000個路由器。虛擬電路的DLCI必須跟遠端路由器的網路位址有關，這些資訊可藉由應對指令可由人工操作完成。

Frame Relay was designed to provide packet-switched data transfer with minimal end-to-end delays. Anything that might contribute to delays was omitted. When vendors implemented Frame Relay as a separate technology rather than as one component of ISDN, they decided that there was a need for DTEs to dynamically acquire information about the status of the network. This feature was omitted in the original design. The extensions for this status transfer are called the Local Management Interface (LMI).

訊框中繼提供最小延遲在分封交換的環境下，當使用訊框中繼當成是分散技術時而不是ISDN的一部份，資料終端設備是動態要求網路的狀態資訊，這個特點在原來的設計被刪去，狀態轉移的延伸稱為區域管理介面（Local Management Interface (LMI)）。

The 10-bit DLCI field allows VC identifiers 0 through 1023. The LMI extensions reserve some of these identifiers. This reduces the number of permitted VCs. LMI messages are exchanged between the DTE and DCE using these reserved DLCIs.

The LMI extensions include the following:

• The keepalive mechanism, which verifies that a VC is operational
• The multicast mechanism
• The flow control
• The ability to give DLCIs global significance
• The VC status mechanism

10位元的DLCI欄位可以讓虛擬電路可以有0到1023個識別碼，LMI的延伸保留某些識別碼，這可降低認可的虛擬電路量，LMI的訊息在資料終端設備和資料通信設備間轉換藉由使用保留的DLCI欄位。
LMI延伸的功能包含以下： 

There are several LMI types, each of which is incompatible with the others. The LMI type configured on the router must match the type used by the service provider. Three types of LMIs are supported by Cisco routers:

• Cisco - The original LMI extensions
• Ansi - Corresponding to the ANSI standard T1.617 Annex D
• q933a - Corresponding to the ITU standard Q933 Annex A

不同的LMI介面都互不相容，路由器上的LMI設定需配合服務提供端。Cisco路由器有3種LMI：Cisco、Ansi、q9332。

LMI messages are carried in a variant of LAPF frames. This variant includes four extra fields in the header so that they will be compatible with the LAPD frames used in ISDN. The address field carries one of the reserved DLCIs. Following this are the control, protocol discriminator, and call reference fields that do not change. The fourth field indicates the LMI message type.

There are one or more information elements (IE) that follow the header. Each IE consists of the following:

• A one byte IE identifier
• An IE length field
• One or more bytes containing actual data that typically includes the status of a DLCI

不同的LAMP訊框攜帶不同的LMI訊息，這種特性在標頭地方包含4個額外的欄位，所以LAMP訊框可相容於ISDN網路。位址欄包含保留的DLCI訊息，其他本來的欄位接保留下來，第四個欄位為LMI訊息。
在標頭檔之後跟隨很多資訊單元（information elements），資訊單元包含如下：

Status messages help verify the integrity of logical and physical links. This information is critical in a routing environment because routing protocols make decisions based on link integrity.

狀態訊息可幫助驗證和檢查邏輯、實體的鏈結，這種資訊對有路由器的環境是非常重要的因為路由的協定是基於正確的鏈結。

LMI status messages combined with Inverse ARP messages allow a router to associate network layer and data link layer addresses.

包含反轉位址解析協定（inverse ARP）LMI狀態訊息使路由器跟網路層和資料鏈結層有關。

When a router that is connected to a Frame Relay network is started, it sends an LMI status inquiry message to the network. The network replies with an LMI status message containing details of every VC configured on the access link.

當路由器連接到訊框中繼的網域時，會先傳送LMI狀態詢問訊息給此網域，網域也是以LMI狀態回傳虛擬電路設定的詳細訊息給路由器。

Periodically the router repeats the status inquiry, but subsequent responses include only status changes. After a set number of these abbreviated responses, the network will send a full status message.

路由器每隔一段時間會重複傳送詢問訊息，隨後的回應會包含狀態改變的資訊。

If the router needs to map the VCs to network layer addresses, it will send an Inverse ARP message on each VC. The Inverse ARP message includes the network layer address of the router, so the remote DTE, or router, can also perform the mapping. The Inverse ARP reply allows the router to make the necessary mapping entries in its address to DLCI map table. If several network layer protocols are supported on the link, Inverse ARP messages will be sent for each.

假如路由器想把虛擬電路應對到網路層位址，它會在每個虛擬電路上回傳反轉位址解析協定。轉位址解析訊息包含路由器的網路層位址，所以資料終端設備或路由器可以執行位址應對DLCI應對表。假如鏈結上支援多個網路層協定，轉位址解析協定會送給每個協定訊息。
　

5.2 Configuring Frame Relay 
   
5.2.1 Configuring basic Frame Relay 

5.2.2 Configuring a static Frame Relay map 

5.2.3 Reachability issues with routing updates in NBMA 

5.2.4 Frame Relay subinterfaces 

5.2.5 Configuring Frame Relay subinterfaces 

The Frame Relay service provider will assign the DLCI numbers. These numbers range from 16 to 992, and usually have only local significance. This number range will vary depending on the LMI used. DLCIs can have global significance in certain circumstances.

訊框中繼服務端會分配DLCI號碼，這些號碼從19到992，從局部的角度去看是有用的，只有在某些情況下在全域環境中是較重要的。號碼範圍會依照LMI的使用而有變化。

In the figure, Router A has two point-to-point subinterfaces. The s0/0.110 subinterface connects to router B and the s0/0.120 subinterface connects to router C. Each subinterface is on a different subnet. To configure subinterfaces on a physical interface, the following steps are required:

• Configure Frame Relay encapsulation on the physical interface using the encapsulation frame-relay command
• For each of the defined PVCs, create a logical subinterface

在此圖中，路由器A有兩個點對點的子介面，s0/0.110子介面聯接道路由器B，s0/0.120子介面聯接道路由器C。每個子介面都是不同的子集合，設定實體介面中的子介面有下列步驟：
 

使用encapsulation frame-relay，在實體介面中設定訊框中繼資訊
 

創造一個邏輯的子介面給每個已定義的固定虛擬電路

router(config-if)#interface serialnumber.subinterface-number [multipoint | point-to-point]

To create a subinterface, use the interface serial command. Specify the port number, followed by a period (.), and then by the subinterface number. Usually, the subinterface number is chosen to be that of the DLCI. This makes troubleshooting easier. The final required parameter is stating whether the subinterface is a point-to-point or point-to-multipoint interface. Either the multipoint or point-to-point keyword is required. There is no default. The following commands create the subinterface for the PVC to router B:

routerA(config-if)#interface serial 0/0.110 point-to-point

使用interface serial指令創造子介面。在（.）之後指定一個阜號碼，接著是子介面的號碼，子介面的號碼是DLCI的號碼，這使除錯容易許多。最後需要參數來說明此子介面是點對點或點對多點的介面。要使用Multipoint和point-to-point這兩個關鍵字，下面的指令創造路由器B的固定虛擬電路子介面：routerA(config-if)#interface serial 0/0.110 point-to-point

If the subinterface is configured as point-to-point, then the local DLCI for the subinterface must also be configured in order to distinguish it from the physical interface. The DLCI is also required for multipoint subinterfaces for which Inverse ARP is enabled. It is not required for multipoint subinterfaces configured with static route maps. The frame-relay interface-dlci command is used to configure the local DLCI on the subinterface

router(config-subif)#frame-relay interface-dlci dlci-number

假如子介面被設定為point-to-point則局部的DLCI也要被設定，是為了要從實體介面中去分辨出來。多點子介面在inverse ARP中也需要DLCI，多點子介面不需要靜態的路由應對表。frame-relay interface-dlci 是用來在子介面中設定DLCI。
router(config-subif)#frame-relay interface-dlci dlci-number

5.2.6 Verifying the Frame Relay configuration 

5.2.7 Troubleshooting the Frame Relay configuration