Trish Report Special Topics

FRAME RELAY TECHNOLOGY Frame Relay is a simple connection-oriented, virtual circuit, packet service. It can provide both Switched Virtual Circuits (SVCs) and Permanent Virtual Circuits (PVCs) It operates at the physical and data link layers of the OSI model. It used variable-length packets. Frame Relay services provide bandwidth on demand. Main Approaches used by the Frame Relay • • •

Circuit Switching using TDM Unchannelized T-1 STDM

- Statistical Multiplexing Techniques – the advantage of this technique is that it accommodates more flexibility and more efficient use of bandwidth. -it does not perform error detection and correction on each hop between pairs of switches as does packet switching. The premise of frame relay is that modern communications systems do not require the additional operations for error correction. Due to major Trends in the communications industry frame relay developed the following: - Increased need for speed across the network platforms within the end-user and the carrier networks - Improved transmission facilities - Increased intelligence of the devices attached to the network - Increased need to connect LANs and WANs and the Internet-working capabilities X.25 It was the first universal data communications protocol. It stimulated growth in data communications traffic, largely due to its reliability and robustness. It was developed when the quality of telecommunications lines was much worse than today and communication errors were more common It was designed for analog signals FRAME RELAY VERSUS X.25 A. In frame relay, when error is detected in a frame, it is simply “dropped” (thrown away), the end-points are responsible for error detecting and transmitting dropped frames. For X.25, the network switches performed error-checking operations on all traffic, including retransmissions in the event of errors and transmissions of messages to the user in the event of failed retransmission.

B. X.25 and frame relay both perform error detection, but X.25 will request a retransmission, whereas Frame relay will discard a bad frame. C. Frame Relay operates at Layers1 and 2 of the OSI model. Since X.25 operates at OSI Layers 1 to 3. D. Frame Relay data transfer protocol provides a high degree of transparency to the higher layer protocols that are carried. This contrast with X.25, where the scope for destructive interference with higher layer protocols often causes problem and ca seriously impair performance and throughput. E. Instead of using a PAD to convert packets, as in X.25, Frame Relay uses a Frame Relay Assembler/Disassembler (FRAD) To summarize, Frame relay does the following: - Utilizes less overhead, and therefore is faster than X.25 - Assumes that the network is error free - Assumes that intelligent devices are on each end of the connection FRAME RELAY ARCHITECTURE There are two separate planes of operation for the Frame Relay protocol: 1. Control Plane – involved in the establishment and the termination of logical connections. - control plane protocols are between a subscriber and the network. 2. user Plane – responsible for the transfer of user data between subscriber. User plane protocols provide end-to-end functionality. FRAME RELAY NETWORKS Frame Relay is a virtual, value-added, private network service where end users are not required to leases dedicated lines. It is mainly used for LAN-to-LAN DATA transmissions across wide areas and is cost effective for organizations with more that four sites. It uses multiple virtual connections over a single cable medium. Two types of virtual connections exist within Frame relay 1. Switched Virtual Circuits – temporary connections used in situations that require only sporadic data transfer between DTE devices across the Frame Relay network. Four Operational States - Call setup - Data transfer - Idle - Call Termination *The main benefit of a SVC arrangement is that individual data links need only be established when needed and may be cleared afterwards.

2. Permanent Virtual Circuits – permanently established connections that are used for frequent and consistent data transfers between DTE devices across the Frame Relay network. Two operational states - Data transfer - Idle FRAME RELAY FRAME Frame Relay operates at the data link layer of the OSI model rather than at the network layer. Frame Check Sequence (FCS) field allows each switch to check for errors in the Frame Relay header. If it finds an error, the switch discards the frame. Committed Information Rate (CIR) – refers to the maximum transmission speed of a user over to the frame relay network THROUGHPUT AND CAPACITY A potential problem associated with Frame Relay is that carriers tend to oversell the capacity, calculating that the network will not be used by everyone at the same time. Over Subscription - When the sum of the data arriving over all virtual circuits exceeds the access rate. Port Speed – maximum rate at which data can be transmitted. 2 factors that comprise the CIR 1. Committed Burst rate (Bc) – describes the maximum amount of data that a user is allowed to offer to the network during some time interval. 2. Excess Burst rate (Be) – describes the amount of data that a user may send that exceeds the committed burst rate during the time interval. 2 Simple Congestion-notification Mechanisms implementing in Frame Relay 1. Forward Explicit congestion Notification (FECN) – warns the destination recipient of impending network. 2. backward Explicit congestion Notification (BECN) – sent back to the original source user to throttle back its transmission to prevent network congestion. Discard Eligibility (DE) bit – which is used to identify less important traffic that can be dropped during periods of congestion. Frame relay Error Checking Frame Relay uses a common error-checking mechanisms known as cyclic Redundancy Check(CRC). It is stored in the Frame Relay Check Sequence (FCS) field of the frame relay frame.

FRAME RELAY COMPONENTS A frame relay packet assembler/disassembler (FRAD) is required at the end of the Frame Relay circuit. The FRAD provide for the conversion of continuous bit stream oriented data signals into a frame format. The FRAD function can take place in several CPE devices, namely a router, switch, bridge or gateway. FRADs are also available as standalone units. Frame Relay Switches Frame relay switches are available in basically two versions – central office switch and customer network switch.