Frame Relay and Packet-Switching Services
Exam: Cisco 640-822 - Interconnecting Cisco Networking Devices Part 1
Different types of WAN services are offered by the various Service Providers. These are different from the leased lines and can be classified as packet switching services. Physical WAN connectivity exists in a packet switching service which is identical to a leased line. Multiple routers can be connected to packet switching service. This can be done by utilizing a single link between a router and the packet switching service. Once the connection is established, packets can be sent by the router to other routers just as in the case of a switch or an Ethernet hub.
There are two types of packet switching services that are popular-
- Frame Relay and
- Asynchronous Transfer Mode (ATM)
Frame Relays being more common. Frame Relay has advantages over point-to-point links. The primary advantages are -
- Scalability;
- Cost effectiveness
The article discusses Frame Relays in detail.
Frame Relay Basics
Frame Relays are loaded with features and benefits in comparison to simple point-to-point WAN links. The only word of caution that needs to be considered is that the Frame Relay protocols are highly detailed. Being multi-access networks, allowing more than two devices to be connected to the network, it is imperative that the protocols be more detailed. The figure illustrated below, shows the basic components for Frame Relay.
Frame Relay Components
It is clear from this figure that the same features as of Layer1 point-to-point are used by Frame Relay. Between a router and a nearby Frame Relay switch, a leased line is required to be installed known as access links. Same speed and signaling standards identical to point-to-point lease lines are used.
A major point of difference between Point-to-Point links and Frame Relays is that the examination of the data frame that is forwarded by the router is done by the equipment with the telecom company. The data link of the Frame Relay is also defined with a header and a trailer and each header holds an address field known as DLCI - Data Link Connection Identifier. Depending on the DLCI, the frame is forwarded by the WAN switch. The frame passes through the network of the provider till it reaches the remote site router which is on the other end of the Frame Relay Cloud.
The equipment available with the telecom company transmits one frame to one remote site and the other to another remote site, this results in the Frame Relay to be viewed as a type of packet switching. This allows the service provider to choose the destination of the data.
Frame Relay Protocols closely resemble OSI Layer 2 protocols. Bits that are forwarded by a Layer 2 device are termed as frame. The general terminology that is used is packet switching while frame switching service is also used for Frame Relay.
DTE and DCE are terms with alternative meanings with reference to packet switching or frame switching service. In the case of Frame Relays, the relay switches are known as DCE, the equipment lying at the end of the customer (router) is known as DTE. DCE refers to the device that is providing services while DTE refers to the device that requires the switching services.
The logical path that the frame travels between a pair of routers is termed as Frame Relay VC. The details pertaining to a VC are preconfigured and are known as PVC-Permanent Virtual Circuits. When a packet needs to be forwarded by R1 to R2, the Layer 3 packet is encapsulated into a Frame Relay header and trailer before transmitting the frame. DLCI, a Frame Relay address is used by R1. The DLCI identifies the appropriate VC to the provider allowing the switch to deliver the frame to R2. Details of Layer 3 packet are ignored and only the header and the trailer are looked at. The VCs are one of the main advantages of using Frame Relays over Point-to-Point leased lines. A VC acts like a virtual point-to-point circuit. VCs also allow sharing of the accesslink and the Frame Relay network.
