CCNA Study Guide

This tutorial explains basic concept of Frame Relay such as VC, PVC, SVC, DTE, DCE, access link, LMI types, DLCI numbers, FECN, BECN, access rate and CIR rate in detail with examples.

Basic concept of Frame Relay

Frame Relay is one of the most popular WAN service deployed over the past decade. Even though several advanced technologies (such as VPN, ATM) are available today, Frame Relay still rocks and will be in near future due to its features, benefits and lower cost in comparison with other point to point wan services. For example have look on following figure that illustrates a network with simple point to point leased line connection.

Frame Realy network

There are four routers in this network. To connect these routers with each other, total six leased lines and three serial interfaces on each router are used. We can use following formula to figure out how many connections are required:-

(N × (N – 1)) / 2 [Here N is the number of routers]

In our example we have four routers so we need (4 x (4-1)) /2 = 6 leased lines.

If we have 100 routers then we need (100 x (100-1)) /2 = 4950 lease lines and 99 serial interfaces on each router. Forget about low end routers, even a 7700 series router does not have sufficient physical interfaces to handle this requirement.

Here comes Frame Relay. Frame Relay turns physical interface in virtual interfaces. With virtual interface Frame Relay can effectively handle this network or even bigger network with single serial interface. Have a look on following figure that illustrate above network with Frame Relay

frame relay network example

With Frame Relay implementation, we still need 6 connections to connect all these routers with each other. But instead of physical lines, Frame Relay uses virtual lines to connect all these locations. The biggest benefit of these virtual lines is that we do not need equal physical interfaces on router to connect them. We can connect multiple virtual lines with single interface.

Frame Relay access link share

This tutorial is the fourth part of our article “WAN Terminology Explained with Encapsulation Protocols and Methods”. You can read other parts of this article here.

WAN Tutorial – Basic WAN Switching Concept Explained

This tutorial is the first part of article. This part explains basic wan concepts including terminology, encapsulation methods, switching concepts and encapsulation protocols in detail with example.

HDLC Protocol and Encapsulation method Explained

This tutorial is the second part of the article. This part explains HDLC (High-Level Data Link Control) protocol and encapsulation method in detail with examples including step by step configuration guide.

PPP Protocol and Encapsulation method Explained

This tutorial is the third part of the article. This part explains PPP (Point to Point) protocol and encapsulation method in detail with examples including step by step configuration guide.

How to configure Frame Relay Step by Step Guide

This tutorial is the last part of the article. This part provide step by step guide on how to configure Frame Relay in Cisco routers.

Frame Relay VC, PVC and SVC

In Frame Relay terminology virtual connection lines are known as Virtual Circuits (VCs). There are two types of VCs; PVCs and SVCs.

Differences between Frame Relay PVCs and Frame Relay SVCs

Frame Relay PVCs (Permanent Virtual Circuits) Frame Relay SVCs (Switched Virtual Circuits)
PVC is just like a leased line that is once configured will stay there until we manually reconfigure it. SVC is just like a telephone connection that is dynamically built whenever we have data to transmit and once transmission is over it will be terminated.
If we have regular data for transmission then PVC is the best choice. If we have periodical data for transmission then SVC is the right choice.
PVCs need a lot of manual configuration. SVCs need less configuration in comparison with PVCs.
Once PVC is built there is no delay before data transmission. Since SVC is built each time whenever we send data, therefore a small delay before data transmission is expected.
Whether we use it or not, we have to pay for entire billing cycle. We need to pay only when we actually use it.

SVC is not tested in any CCNA level exam. So I am not going to include it in rest of the article. After this wherever VC or PVC is referred please take that for PVC only.

Frame Relay Network Type

A frame relay network is considered fully meshed when all sites (routers) are connected with each other via direct link. When all sites do not have direct link with each other then it would be considered as partially meshed frame relay network.

Frame Relay network type

Frame Relay Terminology

Frame Relay uses a lot of terms to describe its components and functions. In this section we will understand these terms in detail. Have a look on following figure that illustrate a simple Frame Relay network

frame relay terminology


DTE (Data Terminal Equipment) is a device (usually a router or PC) that converts data frame into signals and reconvert received signals in data frame. DTE device communicates with DCE device.


A CSU/DSU (Channel Service Unit/Data Service Unit) is a device that converts data signal between LAN network and WAN network. LAN network and WAN network uses separate communication technology. A CSU/DSU understands both technologies. DSL and cable modems are the example of CSU/DSU.


DCE (Data circuit terminating equipment) is a device (usually modem, CSU/DSU or Frame Relay switch) that provides clock rate and synchronization.

Access Link

Connection line between DTE and DCE.

Frame Relay cloud

Frame Relay cloud refers Telco companies internal infrastructure.


A VC is the logical path between two endpoint DTEs.

Access Rate

This is the maximum speed of purchased connection. Access link should be clocked on this speed. Access rate is the maximum speed at which data can be transmitted.

CIR (Committed Information Rate)

This is the guaranteed bandwidth that we will get from provider for a VC. In congestion we could be sure about this bandwidth. CIR is the maximum bandwidth at which data will be delivered guaranteed.

Let’s understand these value with an example. Suppose, there are three networks connected with a frame relay switch sharing single path. Network1 and Network2 purchased a connection with access rate of 128Kbps and CIR of 64 Kbps. Network3 purchased connection with access rate of 64Kbps and CIR of 64 Kbps.

If access rate and CIR rate is equal then Frame relay connection is pretty much works like a leased line. Network3 is paying for 64Kbps speed and in return, getting a guaranteed speed of 64Kbps from frame relay company. So for network3 this connection will work just like a leased line of 64Kbps where you will get what you will pay for.

Instead of fix bandwidth, network1 and network2 opt for a flexible connection where access rate and CIR rate is different. For this connection they have to spend a little extra money. They will be charged for 64Kbps guaranteed (CIR) + additional bandwidth (if available under certain terms and conditions). Additional bandwidth will be provided on share basis. If all other users are transferring data at any particular time then they will get a minimum bandwidth of 64Kbps at that time.

Frame Relay access rate

If no other user is transferring data at any specific time then they will get a maximum bandwidth of 128 Kbps on that time. For example if no other user is transferring data at any specific time then R1 is allowed to use additional 64Kbps bandwidth at that particular time.

frame relay cri
  • If no other user is transferring data then maximum (128 Kbps) bandwidth would be available.
  • If all other users are transferring data then minimum (64 Kbps) bandwidth would be available.
  • If some users are transferring data then bandwidth speed may be anywhere between 64Kbps and 128 Kbps.

Anything beyond the CIR is marked as burst. There are two types of burst:-

BC (committed burst rate)

A small amount of additional bandwidth that is allowed to handle small burst in traffic.

BE (excessive burst rate)

Remaining amount of bandwidth. If Telco allows, we can also set bandwidth at this rate. Connection rarely works at this speed.


When we add up all CIRs (CIR + Bc + BE ) and sum exceed the access rate then it would be considered as oversubscription.

Usually oversubscription is not allowed. So any data that fall in oversubscription category will be dropped.

frame relay oversubscription

Frame Relay Congestion Control

Since users share bandwidth, congestion is common in Frame Relay. We should avoid sending additional data if network is facing congestion at any particular time. Frame Relay uses three bits to manage congestion:-

Discard Eligibility (DE)

Any packet beyond CIR is eligible to discard if Frame Relay network is facing congestion. DE bit is set in header. During congestion, Frame Relay switch will drop all the packets that are marked (set to on) with DE bit. If there is no congestion, packet will be allowed to cross the frame relay network.

Forward Explicit Congestion Notification (FECN)

If there is congestion in network then frame relay switch will set FECN bit to on (1) in data frame header. This way destination router will learn about congestion in VC.

Backward Explicit Congestion Notification (BECN)

Once the frames with congestion bit on arrived at destination router (DTE) , the destination router will send back a frame in reverse direction with BECN bit on in header of frame. Once source receive this frame it will learn about the congestion and slow down the data transmission on that VC.

frame relay congestion control

If Frame Relay carrier experiences less or no congestion, you will get a good speed and great service at competitively low price. If Frame Relay carrier experiences constantly congestion, you will get a poor service since most of your fames will be dropped.

Frame Relay LMI (Local Management Interface) protocol

Before data transmission DTE confirms the status of remote end. It sends data only if remote end is up. To know the status of each other’s, devices exchange Keepalive messages. If one end does not receive a Keepalive message from other end in specified time then it would assume that remote end is down. Keepalive messages are exchanged between directly connected devices. For example in leased line where two devices connect with each other via direct link, will exchange Keepalive messages. But in Frame Relay devices connect with each other via the Frame Relay switches, so they will exchange Keepalive message with Frame Relay switches.

frame relay keepalive LMI message

Frame Relay uses LMI protocol to exchange the Keepalive messages between DTE (connection end point) and DCE (last frame relay switch that is directly connected with the end point). DTE (Routers) send LMI status enquiry messages to the connected DCE (Frame Relay switch). If DCE (Frame Relay) is up then it will respond with LMI status reply message. If DTE does not get response form DCE then it will assume that either access link or frame relay switch is down.

Besides LMI status enquiry DTE also asks for full status updates. In response DCE respond with all information that is related to DTE. This information includes the status of VCs which are connected to the DTE and their configuration values (CIR, BC, BE and DLCIs).

lmi status inquiry

LMI status enquiry :- A simple query asking simple question “Are you there”. Response of this query is also simple “Yes I am here”.

LMI full status enquiry :- A complete query seeking full information “Tell me everything that is related to me”. Response of this query contains all information that is related to DTE “Here is all information which is related to you”.

There are three types of LMI Cisco, ANSI, and Q.933A. Each LMI type is slightly different from other two. Therefor they are not compatible with each other. We must have to use same LMI option on both ends. In next part of this article we will learn how to configure LMI type.

On Job

Unless you configure LMI type, routers use autosense feature. In autosense feature router automatically figure out which LMI type is Frame Relay switch is using and configure itself accordingly.

Frame Relay DLCI (data link connection identifiers) explained

Frame Relay allows us to connect multiple VCs with single physical access link. In first example of this tutorial, we connected six VCs with single physical link ( serial interface). Basically we divided a serial interface in six sub-interfaces and assigned one VC with each sub interface. Frame Relay must need to know which sub-interface is connected with which VC before it can transmit the data. Frame Relay uses DLCI (data link connection identifiers) number to map the interface with VC.

Since a VC has two ends it need two DLCI number, one for each end. DLCI value is provided by Telco. Probably we may get same or different DLCI number for both ends. DLCI number need to be unique only between Frame Relay switch and DTE router. If we received different DLCI number for both end then Frame Relay would convert DLCI number in midway.

frame relay dlci

That’s all for this part. In next part of this article we will configure Frame Relay in Cisco router.

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