FRAME RELAY. What is Frame Relay? high-performance WAN protocol operates at the physical and data...
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Transcript of FRAME RELAY. What is Frame Relay? high-performance WAN protocol operates at the physical and data...
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FRAME RELAY
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What is Frame Relay?• high-performance WAN protocol• operates at the physical and data link layers• Originally designed for use across ISDN interfaces• An example of packet-switched technology• described as a streamlined version of X.25
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Introduction• Frame Relay (FR) is a high-performance WAN
protocol that operates at the physical and data link layers of the OSI reference model.
• FR originally was designed for use across Integrated Service Digital Network (ISDN) interfaces.
• Today, it is used over a variety of other network interfaces as well.
• FR is an example of a packet-switched technology.• Packet-switched networks enable end stations to
dynamically share the network medium and the available bandwidth. 3
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Frame Relay Devices• Devices attached to a Frame Relay WAN fall into the
following two general categories:• Data terminal equipment (DTE)
• DTEs generally are considered to be terminating equipment for a specific network and typically are located on the premises of a customer.
• Example of DTE devices are terminals, personal computers, routers, and bridges.
• Data circuit-terminating equipment (DCE)• DCEs are carrier-owned internetworking devices.• The purpose of DCE equipments is to provide clocking and
switching services in a network, which are the devices that actually transmit data through the WAN.
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Frame Relay Devices (cont.)
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Frame Relay vs. X.25• Frame Relay is a Layer 2 protocol suite, X.25 provides services
at Layer 3• Frame Relay offers higher performance and greater
transmission efficiency than X.25
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Frame Relay Devices
Figure 1 Frame Relay Devices
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Circuit-Switching• Long-haul telecom network designed for voice• Network resources dedicated to one call• Shortcomings when used for data:
• Inefficient (high idle time)• Constant data rate
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Packet-Switching• Data transmitted in short blocks, or packets• Packet length < 1000 octets• Each packet contains user data plus control info (routing)• Store and forward
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Advantages with compared to Circuit-Switching• Greater line efficiency (many packets can go over shared link)• Data rate conversions• Non-blocking under heavy traffic (but increased delays). When
traffic becomes heavy on a circuit-switching network, some calls are blocked.
• Priorities can be used.
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Disadvantages relative to Circuit-Switching• Packets incur additional delay with every node they pass
through• Jitter: variation in packet delay• Data overhead in every packet for routing information, etc• Processing overhead for every packet at every node traversed
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Switching Technique• Large messages broken up into smaller packets• Datagram
• Each packet sent independently of the others• No call setup• More reliable (can route around failed nodes or
congestion)• Virtual circuit
• Fixed route established before any packets sent• No need for routing decision for each packet at each
node
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Frame Relay Virtual Circuits• provides connection-oriented data link layer communication• a logical connection between two data terminal equipment
across a Frame Relay packet-switched network• provide a bi-directional communications path from one DTE
device to another
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Frame Relay Virtual Circuits• Switched virtual circuits (SVCs)
• temporary connections requires sporadic data transfer between DTE devices across the Frame Relay network
• Call Setup• Data Transfer• Idle• Call Termination
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Frame Relay Virtual Circuits• Permanent Virtual Circuits (PVCs)
• used for frequent and consistent data transfers between DTE devices across the Frame Relay network
• Data Transfer• Idle
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Congestion Control Mechanism• Forward-explicit congestion notification (FECN)• Backward-explicit congestion notification (BECN)
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Forward-explicit congestion notification (FECN)
• initiated when a DTE device sends Frame Relay frames into the network
• When the frames reach the destination DTE device, the frame experienced congestion in the path from source to destination
• flow-control may be initiated, or the indication may be ignored
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Backward-explicit congestion notification (BECN)• DCE devices set the value of the BECN bit to 1 in frames
traveling in the opposite direction, informs the receiving DTE device that a particular path through the network is congested
• flow-control may be initiated, or the indication may be ignored
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Frame Relay Discard Eligibility (DE)• (DE) bit is used to indicate that a frame has lower importance
than other frames• When the network becomes congested, DCE devices will
discard frames with the DE bit
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Frame Relay Error Checking• common error-checking mechanism known as the cyclic
redundancy check (CRC)• CRC compares two calculated values to determine whether
errors occurred during the transmission
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Frame Relay Network Implementation• consists of a number of DTE devices connected to remote
ports on multiplexer equipment via traditional point-to-point services
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Public Carrier-Provided Networks
• Frame Relay switching equipment is located in the central offices of a telecommunications carrier
• The DCE equipment also is owned by the telecommunications provider
• The majority of today’s Frame Relay networks are public carrier-provided networks
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Private Enterprise Networks• the administration and maintenance of the network are the
responsibilities of the enterprise• All the equipment, including the switching equipment, is
owned by the customer
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Frame Relay Frames
Figure 3 Frame Relay Frame
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Frame Relay Frames• Flags indicate the beginning and end of the frame• Three primary components make up the Frame Relay frame
• the header and address area• the user-data portion• the frame-check sequence (FCS)
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Frame Relay Frames• The address area (2 bytes)
• 10 bits represents the actual circuit identifier• 6 bits of fields related to congestion management
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Frame Relay Frame Formats• Standard Frame Relay Frame• LMI Frame Format
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Standard Frame Relay Frame• Flags
• Delimits the beginning and end of the frame• The value of this field is always the same (7E or 01111110)
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Standard Frame Relay Frame• Address – contains the following information
• DLCI: The 10-bit DLCI is the essence of the Frame Relay header, values have local significance only, devices at opposite ends can use different DLCI values for the same virtual connection
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Standard Frame Relay Frame• Address
• Extended Address (EA): used to indicate whether the byte in which the EA value is 1 is the last addressing field, the eighth bit of each byte of the Address field is used to indicate the EA
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Standard Frame Relay Frame• Address
• Congestion Control: consists of the three bits; FECN, BECN, and DE bits
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Standard Frame Relay Frame• Data – Contains encapsulated upper-layer data
• serves to transport the higher-layer protocol packet (PDU) through a Frame Relay network
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Standard Frame Relay Frame• Frame Check Sequence
• Ensures the integrity of transmitted data
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LMI Frame Format
Figure 4 Nine fields comprise the Frame Relay that conforms to the LMI format
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LMI Frame Format• 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
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LMI Frame Format• Protocol Discriminator - Always contains a value
indicating that the frame is an LMI frame• Call Reference - Always contains zeros. This field
currently is not used for any purpose• Message Type
• Status-inquiry message: Allows a user device to inquire about the status of the network
• Status message: Responds to status-inquiry messages. Status messages include keep-alives and PVC status messages
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LMI Frame Format
• Information Elements—Contains a variable number of individual information elements (IEs)• IE Identifier: Uniquely identifies the IE• IE Length: Indicates the length of the IE• Data: Consists of one or more bytes containing
encapsulated upper-layer data
• Frame Check Sequence (FCS) - Ensures the integrity of transmitted data
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Frame Relay Inverse ARP and LMI Operation (cont.)
Hello, I am 172.168.5.7.4
5Frame Relay Map
172.168.5.5 DLCI 400 Active
Frame RelayCloudDLCI=100 DLCI=400
Frame Relay Map
172.168.5.7 DLCI 100 Active5
172.168.5.5 172.168.5.7