Frame Relay is an example of a packet-switched technology. Packet-switched networks enable end stations to dynamically share the network medium and the available bandwidth. The following two techniques are used in packet-switching technology:
•
Variable-length packets
•Statistical multiplexing
Frame Relay Devices
Devices attached to a Frame Relay WAN fall into the following two general categories:
•Data terminal equipment (DTE)
•Data circuit-terminating equipment (DCE)
Frame Relay Example Configuration
service udp-small-servers
service tcp-small-servers
!
hostname Atlanta
!
enable secret cisco
!
ip subnet-zero
ip domain-lookup
ip name-server 129.250.35.250 129.250.35.251
!
interface Ethernet0
ip address 209.39.6.1 255.255.255.0
!
interface Serial0
no ip address
encapsulation frame-relay
frame-relay lmi-type ansi
!
interface Serial0.16 point-to-point
description Frame Relay to Boston
ip address 199.1.138.2 255.255.255.252
frame-relay interface-dlci 16
!
ip http server
ip classless
ip route 0.0.0.0 0.0.0.0 serial0.16
!
line con 0
password console
login
line aux 0
line vty 0 4
password telnet
login
!
DTEs generally are considered to be terminating equipment for a specific network and typically are located on the premises of a customer. In fact, they may be owned by the customer. Examples of DTE devices are terminals, personal computers, routers, and bridges.
DCEs are carrier-owned internetworking devices. The purpose of DCE equipment is to provide clocking and switching services in a network, which are the devices that actually transmit data through the WAN. In most cases, these are packet switches. Figure 10-1 shows the relationship between the two categories of devices.
Figure 10-1 DCEs Generally Reside Within Carrier-Operated WANs
The connection between a DTE device and a DCE device consists of both a physical layer component and a link layer component. The physical component defines the mechanical, electrical, functional, and procedural specifications for the connection between the devices. One of the most commonly used physical layer interface specifications is the recommended standard (RS)-232 specification. The link layer component defines the protocol that establishes the connection between the DTE device, such as a router, and the DCE device, such as a switch. This chapter examines a commonly utilized protocol specification used in WAN networking: the Frame Relay protocol.
Frame Relay Virtual Circuits
Frame Relay provides connection-oriented data link layer communication. This means that a defined communication exists between each pair of devices and that these connections are associated with a connection identifier. This service is implemented by using a Frame Relay virtual circuit, which is a logical connection created between two data terminal equipment (DTE) devices across a Frame Relay packet-switched network (PSN).
Virtual circuits provide a bidirectional communication path from one DTE device to another and are uniquely identified by a data-link connection identifier (DLCI). A number of virtual circuits can be multiplexed into a single physical circuit for transmission across the network. This capability often can reduce the equipment and network complexity required to connect multiple DTE devices.
A virtual circuit can pass through any number of intermediate DCE devices (switches) located within the Frame Relay PSN.
Frame Relay virtual circuits fall into two categories: switched virtual circuits (SVCs) and permanent virtual circuits (PVCs).
Switched Virtual Circuits
Switched virtual circuits (SVCs) are temporary connections used in situations requiring only sporadic data transfer between DTE devices across the Frame Relay network. A communication session across an SVC consists of the following four operational states:
•Call setup—The virtual circuit between two Frame Relay DTE devices is established.
•Data transfer—Data is transmitted between the DTE devices over the virtual circuit.
•Idle—The connection between DTE devices is still active, but no data is transferred. If an SVC remains in an idle state for a defined period of time, the call can be terminated.
•Call termination—The virtual circuit between DTE devices is terminated.
After the virtual circuit is terminated, the DTE devices must establish a new SVC if there is additional data to be exchanged. It is expected that SVCs will be established, maintained, and terminated using the same signaling protocols used in ISDN.
Few manufacturers of Frame Relay DCE equipment support switched virtual circuit connections. Therefore, their actual deployment is minimal in today's Frame Relay networks.
Previously not widely supported by Frame Relay equipment, SVCs are now the norm