Overview: Frame Relay Basics
Goal: Help beginners visualize Frame Relay flows: configure PVC, frame switching by DLCI.
Topology: Two DTE devices (router R1, R2) connected to a Frame Relay network of 4 nides (FR1~FR4)
Steps: 1) Configure a PVC for R1 and R2. Configure necessaty entries for DLCI switching tables
2) R1 sends a packet to R2. It is switched hop-by-hop by Frame Relay switches FR1, FR2, FR3
3) Configure OPVC2 between R and R2.
4) R1 sends 2 packets to R2 via PVC1 and PVC2. Observe how multi-DLCIs are mapped to PVCs.
In this topology, there are two types of devices and two paths.:
- DTE: Routers R1 and R2 are connected to a Frame Relay network. They are DTE (Data Termoinal
Equipment). DTE are devices that not owned by the phone company. E.g., computers, routers..
- DCE: Frame Relay network contains 4 switches FR1 ~ FR4.. They are DCE
(Data Communication Equipment). For example, modem, telephone switches.
- R1 and R2 are connected by 2 physical paths: R1 - FR1 - FR2 - FR3 - R2 and R1 - FR1 - FR4 - FR3 - R2.
R1 and R2 are logically conneected by two PVCs..
R1 wants to send data to R2.
- First, select a physical path that connects R1 and R2: R1-FR1-FR2-FR3-R2.
- Then configure a PVC to connect R1 and R2 by creating one VC (called DLCI) per physical link segment:
102 for R1-FR1, 112 for FR1-FR2, 213 for FR2-FR3, and 201 for FR3-R2.
PVC1 can be presented by its DLCIs. E.g., PVC1=201, 112, 213, 201.
Note: See following tutorials for configuration commands. See FAQ for PVC, VC, DLCI.
Configure switching table
PVC1 contains 4 DLCIs: 102, 112, 213, 201. The next step is to confgure DLCI switching tables on 3 switches.
- FR1 switching table: Forward frames from DLCI 102 (a VC at link R1-FR1 ) to DLCI 112 (link FR1 - FR2）
- FR2 switching table: From DLCI 112 (FR1 - FR2） forward to DLCI 213 （FR2 - FR3）
- FR3 switching table: From DLCI 213 (FR2 - FR3) forwrad to DLCI 201 （FR3 - R2）
From these 3 DLCI switching tables, we can see how frames can be switched along PVC1 hop by hop: (102>112), (112> 213), (213> 201)
R1 sends a frame to R2
Now R1 isready to send a frame to R2. It encpasulates the frame with DLCI 102, which is the ID of VC configured on the physical link that connects R1 and FR1.
FR1 switches frame to DLCI 112**
When the frame arrives at FR1, its DLCI is compared with the From DLCIs in the switching table.
- A match is found. FR1 switches the incoming frame from interface 0 (DLCI 102) to interface 1 (DLCI 112).
- FR1 encapsulates the frame with DLCI 112.
- FR1 transmits the frame from interface 1.
Switch to DLCI 213, 201
When FR2 receives the frame, it does a DLCI lookup and forwards the frame to interface 2 (DLCI 213).
When FR3 receives the frame, it looks up switching table and forwards the frame to interface 1 (DLCI 201).
The frame arrrives at R2, the other end-point of the PVC.
Configure PVC2, switching tables
Now configure another path, PVC2, to connect R1 and R2. PVC2's physical line path is R1-FR1-FR4-FR3-R2. PVC2 is made of 4 DLCI 15, 16, 17, 18 for the corresponding physical link segments R1-FR1, FR1-FR4, FR4-FR3, and FR3-R2 respectively.
Then configure DLCI switching entries in FR1, FR4, FR3 respoectively..
R1 sends frames over 2 PVCs
Now R2 sends 2 frames, Packet1 and Packet2, to R2.
R1 sends Packe1s over PVC1 and encapsulates it with DLCI 201.
R1 sends Packet2 over PVC2 and encapsulates it with DLCI 15.
It's clear now that line R1-FR1 is configured 2 VCs (DLCI 102, 15) taht are associated with 2 PVCs.
FR1 forwards frames to different interfaces
When receiving Packet1 (DLCI 102), FR1 DLCI lookup got a match (102, 0, 112, 1), and forwards Packet1 to interface 1.
When FR1 receives Packet2 (DLCI 15), DLCI lookupo got a match (15, 0, 16, 2), and forwards Packet2 to interface 2.
Reviiew: PVC, DLCI, link.
Animation shows that two flows are transmitted over two different PVCs, which illustrate some abstract conecpts:
- PVC is a logical path over a phycial path. The phycal path connects two end-points.
- PVC is made of VCs (DLCI) from consecutiive physical links that connects 2 end-points.
- A physical link can have several DLCIs, one for each PVC. For example, link R1-FR1 has 2 DLCIs 102 and 15.
DLCI 102 is part of PVC1 and DLCI 15 is part of PVC2.
- DLCI is only locally significant as it is used by hop-by-hop DLCI switching. After a frame is switched to a next
hop link, the frame's DLCI is replaced by the next hop link's DLCI.
Frame Relay structure for beginners
Many people are more familair witth LAN than WAN. Here are some Frame Relay tips for beginners:
- In Frame Relay, data is transmitted between end-points via PVCs (permanent virtual circuit). PVC is a virtual
path that connects two end-points.
- End-point connections. End-points are called DTE (Data Terminal Equipment). Routers are typical end-point
devices.. End-point are conneted to Frame Relay switches via serial interfaces. Each interface is
configured manually with VCs (virtual circuit). A PVC is made of VCs on the physical path.
- DLCI (Data Link Connection Identifier) is a VC on a local link. A PVC is made of several DLCI, one per link
along the phsycai path.
When a source end-point sends aframe, it includes its local DLCI in the frame's link headers.
When a Frame Relay switches receives a frame, it uses the frame's DLCI to lookup its switching table
and forwards the frames to next hop switch.
When the remote end-point receives the frame, it uses the frame's DLCI to determine which PVC this frame
is coming from. This helps iy to determine which DLCI to use when respond.
DCE (Data Communication Equipment) are devices owned by telephone companies. For example, modem, telephone switches.
DLCI (Data Link Connection Identifier) identifies the virtual connection so that the receiving end knows which information connection a frame belongs to. DLCI has only local significance. A single physical line can multiplex several different virtual connections.
DTE (Data Termoinal Equipment) are end-point devices that transmit and receive data over Frame Relay netwoirk. DTE devices are owned by customers, not the phone company. For example, computers, routers.
PVC (permanent virtual circuit) let the customer see a continuous, dedicated connection without having to pay for a full-time leased line.
VC (Virtual Circuit). Frame Relay switches create virtual circuits to connect remote LANs to a WAN. There are two types of VC: PVC (Permanent Virtual Circuit) and SVC (Switched Virtual Circuits (SVCs).
X.25. This is the first data communication protocol that uses public phone network infrastructure. Frame Relay has its technical base in the older X.25 packet-switching technology.
Note: This glossary only covers hwat's mentioned in this tutorial. For more detaisls, see wikpedia Frame Relay at http://en.wikipedia.org/wiki/Frame_relay
1) What is Frame Relay?
Frame relay is a wide area network protocol that specifies the physical and logical link layers, It is derived from X.25 with many improvement for modern Internet applications (file transfer, voice, interactions.)
Network providers use Frame Relay for voice and data services between LANs over a wide area network (WAN). Each end-user gets a private line to a frame-relay node.
Frame Relay has been popular for two reasons: cheapness (compared to leased lines), and simplicity of configuring user equipment.
2) How does Frame Relay work?
- Frame relay network has low propagatipon delay since it does not check for errors. Only customer end point devices check errors.
- Frame Relay uses PVC (permanent virtual circuit) to connect user end points. Circuits are set up by installation and remain up permanently.
- Frame relay network topology is better understood from two perspectives: topological view, a logical connection map.
a) Topological view. A simplified Frame Relay topology looks like: R1-----FR1------------- FR2-----R2
where R1 and R2 are routers. They are DTE.
FR1, FR2 are DCE (Data Communications Equipment.) They form a Frame Relay network.
The lines of R1-FR1 and FR2-R2 represent the connections from the frame relay provider (phone
company) to customers. These lines ranging in speed from 56000 bps to T-1 (1.544 Mbps).
b) Logical connection map: (from DLCI, from interface, to DLCI, to interface)
Each physical link is configured with one or more VCs (called DLCIs).
A PVC is made of DLCIs associated with physical links connecting two DTE end-points.
- Frame Relay uses HDLC to transmit data in "frames." A frame contains 4 fields: Flags (synchronization
bits), Address (DLCI, Control,) Data, and CRC.
3) Why Frame Relay?
Internet started in 1980s. That was 90 years after the first telephone. In orderr to transmit data over long distance, the obvious solution is using telephone network. That is how X.25 got started in the 1970s. It transmits data over a voice network. X.25 is complicated and slow. When telephone network became faster and more reliable, Frame Relay was emerrging in the 1980's to replace X.25. It is able to send bulk data quickly and supports interactive application.
In today's Internet, Frame Relay and ATM are two popular networks for WAB,.
4) What is PVC, DLCI?
PVCs (permanent virtual circuit) are used to form logical end-to-end links mapped over a physical network.
Customer gets a PVC as a continuous, dedicated connection that connects two end points.without paying the full price for a leased line.
Service-provider figures out the route each frame travels to its destination and can charge based on usage.
5) How to configure PVC?
6) How does a Frame relay node forward frames
7) What are the LMI states?
- Active state: Everything is up, and routers can exchange information.
- Inactive state: The router’s interface is up and working with a connection to the switching office, but the remote router isn’t up.
- Deleted state: No LMI information is being received on the interface from the switch, which could be due to a mapping problem or a line failure.
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