Frame Relay Group Members Presented by Thong Jing

Frame Relay

Group Members Presented by: Thong Jing Wen n Stephanie Goh n Hoh Yun Yee n Pang Sook Shi n WET 020184 WET 020165 WET 020046 WET 020142

Frame Relay n n n Frame Relay is a high performance network protocol. Operates at the physical and data link layer of OSI reference model. Originally designed for use across ISDN. Used over a variety of other network interfaces. An example of packet-switched technology like X. 25

Frame Relay Process is streamlined n Does not perform error detection n Independent protocol – accept data from many different protocols n Transmission data is faster and more efficient n Entirely digital – more reliable n

Frame Relay vs. X. 25 Frame Relay X. 25 n. No error detection ->greater speeds n. Error n. Physical and data link layers. -> high performance, greater transmission n. Physical, n. Prepare and send frames n. Frames contain expanded address field -> direct frames to destinations with minimal processing n. Prepare n. Can n. Has dynamically allocate bandwidth detection -> error-free delivery data link and network layers and send packets n. Packets contain fields used for error and flow control fixed bandwidth available

What does Frame Relay Do? Frame Relay sends information in packets called frames n Each frame contains all information necessary to route it to correct destination n Each endpoint can communicate with many destinations over one access link to network n

Frame Relay Standardization n Initial proposals for the standardization of Frame Relay were presented to the Consultative Committee on International Telephone and Telegraph (CCITT) in 1984. Frame Relay was standardized by the International Telecommunication Union— Telecommunications Standards Section (ITU-T). In the United States, Frame Relay is an American National Standards Institute (ANSI) standard.

Frame Relay Devices n Devices attached to a Frame Relay WAN fall into two general categories: (a) Data terminal equipment (DTE) (b) Data circuit-terminating equipment (DCE)

Data terminal equipment (DTE) considered to be terminating equipment for a specific network n typically are located on the premises of a customer, they may be owned by the customer. n Examples of DTE devices are terminals, personal computers, routers, and bridges. n

Data circuit-terminating equipment (DCE) DCEs are carrier-owned internetworking devices. n 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. n In most cases, these are packet switches. n

Relationship between DTE & DCE

Frame Relay Virtual Circuit n Frame Relay is a virtual circuit, which is a logical connection created between two data terminal equipment (DTE) devices across a Frame Relay packet-switched network (PSN). n Therefore, it does not use physical addresses to define the DTE but virtual circuit identifier that operates at the data link layer. n Virtual circuits provide a bidirectional communication path from one DTE device to another and are uniquely identified by a number called data link connection identifier (DLCI).

Frame Relay Virtual Circuit n A virtual circuit can pass through any number of intermediate DCE devices (switches) located within the Frame Relay PSN. n When a virtual circuit is established by the network, a DLCI number is given to a DTE in order to access the remote DTE. n The local DTE uses this DLCI to send frames to the remote DTE. n Frame Relay virtual circuits fall into two categories: switched virtual circuits (SVCs) ¨ permanent virtual circuits (PVCs). ¨

Frame Relay Virtual Circuit. Switched Virtual Circuits (SVCs) n Switched virtual circuits (SVCs) are temporary connections. n A new virtual circuit connection will be established each time a DTE wants to make a connection with another DTE. n A communication session across a SVC consists of the following four operational states: ¨ Call setup ¨ Data transfer ¨ Idle ¨ Call termination

Frame Relay Virtual Circuit. Switched Virtual Circuits (SVCs) n Call setup—The virtual circuit between two Frame Relay DTE devices is established. n Data transfer—Data is transmitted between the DTE devices over the virtual circuit. n 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. n Call termination—The virtual circuit between DTE devices is terminated.

Frame Relay Virtual Circuit. Permanent Virtual Circuit (PVCs) n Permanent virtual circuits (PVCs) are permanently established connections by the network provider that are used for frequent and consistent data transfers between DTE devices across the Frame Relay network. n PVC does not require the call setup and termination states that are used with SVCs. n PVCs always operate in one of the following two operational states: Data transfer—Data is transmitted between the DTE devices over the virtual circuit whenever they are ready because the circuit is permanently established. ¨ Idle—The connection between DTE devices is active, but no data is transferred. Unlike SVCs, PVCs will not be terminated under any circumstances even in an idle state. ¨

Frame Relay Virtual Circuit. Data Link Connection Identifier n Frame Relay virtual circuits are identified by data-link connection identifiers (DLCIs). n DLCIs are assigned to define the virtual circuit between: ¨ DTE and DCE ¨ Two DCEs n DLCI values typically are assigned by the Frame Relay service provider (for example, the telephone company). n Frame Relay DLCIs have local significance, which means that their values are unique in the LAN, but not necessarily in the Frame Relay WAN.

Congestion-Control Mechanisms n n Implemented to reduce network overhead Frame Relay networks includes the following elements in congestion-control: ¨ ¨ ¨ n Admission Control Committed Information Rate Committed Burst Size Frame Relay implements 2 congestion-notification mechanisms: ¨ ¨ Forward-explicit congestion-notification (FECN) Backward-explicit congestion-notification (BECN)

FECN n n Part of the Address field in the Frame Relay frame header. FECN mechanism is initiated when a DTE device sends Frame Relay frames into the network. If network is congested, DCE devices set the value of the frames’ FECN bit to 1. When the frames reach destination, the Address field indicates the frame experienced congestion in the path from source to destination.

BECN Part of the Address field in the Frame Relay frame header. n DCE devices set the value of the BECN bit to 1 in frames traveling in the opposite direction of frames with their FECN bit set. n This informs the receiving DTE device that a particular path through the network is congested. n

Frame Relay Discard Eligibility (DE) Part of the Address field in the Frame Relay frame header. n Used to indicate that a frame to has lower importance than other frames by set DE bit of a frame to 1 of DTE device. n When network become congested, DCE devices will discard frames with the DE bit set before discarding those that do not. n

Frame Relay Error Checking n Frame Relay uses a common errorchecking mechanism ¨ Cyclic n Redundancy Check (CRC) Frame Relay reduces network overhead by implementing error checking.

Frame Relay Local Management Interface (LMI) A set of enhancements to the basic Frame Relay specification. n Key Frame Relay LMI extensions include n ¨ Global addressing ¨ Virtual circuit status messages ¨ multicasting

Global Addressing n n n Gives Frame Relay data-link connection identifier (DLCI) value global rather than local significance. DLCI values become DTE addresses that are unique in the Frame Relay WAN. The global addressing extensions adds functionality and manageability to Frame Relay internetworks. Individual network interfaces and the end nodes attached to them. The entire Frame Relay network appears to be a typical LAN to routers on its periphery.

Virtual Circuit Status Messages Provide communication and synchronization between Frame Relay DTE and DCE devices. n These messages are used to periodically report on the status of PVCs n

Multicasting Allows multicast group to be assigned. n Saves bandwidth by allowing routing updates and address-resolution messages to be sent only to specific groups of routers. n Transmits reports on the status of multicast groups in update messages. n

Standard Frame Relay Structure

n n Flags – delimits the beginning and end of the frame. The value of this field is always the same and is represented either as the hexadecimal number 7 E or as the binary number 01111110. Address/Frame Relay Header – contains the following information: ¨ DLCI – 10 -bit DLCI field represent the address of the frame and correspond to a PVC. ¨ C/R – designates whether the frame is a command or response. ¨ EA – Extended Address field signifies up to two additional bytes in the Frame Relay header, thus greatly expanding the number of possible addresses. ¨ Forward-explicit Congestion Notification (FECN) - is a single-bit field that can be set to a value of 1 by a switch to an end DTE device, such as a router, that congestion was experienced in the direction of the frame transmission from source to destination.

¨ Backward-explicit n n Congestion Notification (BECN) – is a single-bit field that, when set to a value of 1 by a switch, indicates that congestion was experienced in the network in the direction opposite of the frame transmission from source to destination. ¨ Discard Eligibility (DE) – is set by the DTE device, such as a router, to indicate that the marked frame is lesser importance relative to other frames being transmitted. Data/Information – contains encapsulated upperlayer data. Frame Check Sequence (FCS) – ensures the integrity of transmitted data. This value is computed by the source device and verified by the receiver to ensure integrity of transmission.

LMI Frame Format

LMI Frame Format n n n Flag – delimits the beginning and end of the frame LMI DLCI – identifies the frame as an LMI frame instead of a basic Frame Relay frame. Unnumbered Information Indicator – sets the poll/final bit to zero. Protocol Discriminator – always contains a value indicating that the frame is an LMI frame. Call Reference – always contains zero. This field currently is not used for any purpose. Message Type – labels the frame as one of the following message types : Status-inquiry message – allows a user device to inquire about the status of the network. ¨ Status message – responds to status-inquiry messages. It include keepalives and PVC status messages. ¨

LMI Frame Format n Information Elements – contains a variable number of individual information elements (IEs). IEs consist of the following fields: IE Identifier – Uniquely identifies the IE. ¨ IE Length – Indicates the length of the IE. ¨ Data – Consists of 1 or more bytes containing encapsulated upper-layer data. ¨ Frame Check Sequence (FCS) – Ensures the integrity of transmitted data. ¨

Summary n n Frame Relay is a networking protocol that works at the bottom two levels of the OSI reference model : the physical and data link layers. Two general categories : Data terminal equipment (DTEs), which include terminals, personal computers, routers and bridges. ¨ Data circuit-terminating equipment (DCEs), which transmit the data through the network and are often carrier-owned devices. ¨

Summary n Two connection types : ¨ Switched virtual circuits (SVCs), which are temporary connections that are created for each data transfer and then are terminated when data transfer is complete. ¨ Permanent virtual circuits (PVCs), which are permanent connections.

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