FRAME RELAYInstructor:Musfara Farooqui

LECTURE - OBJECTIVES Introduction to Frame Relay Technology Frame Relay Devices Frame Relay Virtual Circuits Data Link Connection Identifiers (DLCIs) Frame Relay Congestion Control (FECN, BECN, DE) Frame Relay Frame Format Committed Information Rate (CIR) Local Management Interface (LMI) X.25 vs. Frame Relay.

FRAME RELAY – AN INTRODUCTION Frame Relay is a high performance WAN protocol that

operates at the physical and data link layer of OSI model It was originally designed for transmitting data over fixed

lines (not dial up lines). It provides digital data transfer over long distances and at

higher data transfer rates Supports Lower delay and Higher throughput ITU-T recommend frame relay above 2Mbps

FRAME RELAY It is an efficient packet switched service Uses variable-length data link layer packets called frames

to send information through a shared Frame Relay network.

No hop by hop error or flow control End to end flow and error control (if used) are done by

higher layer Single user data frame sent from source to destination

and ACK (from higher layer) sent back

FRAME RELAY NETWORK

FRAME RELAY STACK

TECHNIQUES USED IN PACKET SWITCHING Two techniques are used in packet-switching technology:

Variable-length packets Flexible data transfer

Statistical multiplexing Efficient use of resources Bandwidth is not wasted

FRAME RELAY STANDARDIZATION Internationally, 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.

BURSTY DATA Burst Transmission: Any relatively high-bandwidth

transmission over a short period. Some wide area networks provide its user a fixed data rate. For example, a T-1 line is designed for a user who wants to

use the line at consistent 1.544 Mbps. This type of service is not suitable for many users today

that need to send bursty data. For example, a user may want to send data at 6 Mbps for 2

seconds, 0 Mbps (nothing) for 7 seconds, and 3.44 Mbps for 1 second for a total of 1.544 Mbps during a period o 10 seconds.

Although, the average data rate is still 1.544 Mbps, the T-1 line can’t accept this type of demand because it is designed for fixed-rate data, not bursty data.

Bursty data requires what is called bandwidth on demand. The user needs different bandwidth allocation at different times.

FIXED-RATE VERSUS BURSTY DATA

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)

Data Terminal Equipment (DTE) DTEs are connected to a Frame Relay service from a

telecommunications company and typically reside at sites used by the company buying the Frame Relay service.

Data circuit-terminating equipment (DCE) Frame Relay switches are DCE devices. DCEs are also

known as data circuit-terminating equipment. DCEs are typically in the service provider’s network.

FRAME RELAY DEVICES

DCE Devices Generally Reside Within Carrier-Operated WANs

FRAME RELAY VIRTUAL CIRCUIT Virtual Circuit Identifier

A VCI is a small number that used by a frame between two switches. It is the identifier that is actually used for data transfer is called

virtual circuit identifier (VCI). When a frame arrives at a switch, it has one VCI, when it leaves, it

has another. Three Phases

To communicate, a source and destination need to go through three phases:

Setup Data transfer Teardown

VIRTUAL CIRCUIT NETWORKS A virtual circuit (VC) consists of:

A path between the source and destination hosts Virtual circuit numbers, one number for each link along the

path Entries in the virtual circuit VC-number translation tables in

each packet switch along the path. Example

Consider host A requests that the network establish a VC between itself and host B.

Suppose the network chooses the path and assigns the VC numbers on three links as follows:

Path: A --------- Switch1 --------- Switch2 --------- BVC #: 12 22 32

VIRTUAL CIRCUIT NETWORKS A simple virtual circuit network

VIRTUAL CIRCUIT NETWORKS Why a packet doesn’t just keep the same VC number on

each of the link along its route? First, by replacing the number from link to link, the length of the

VC field is reduced. Second, by permitting a different VC number for each link along

the path of the VC, a network management function is simplified. i.e. with multiple VC numbers, each link in the path can

choose a VC number independently of what the other links in the path choose.

If a common number were required for all the links along the path, the switches would have to exchange and process a substantial number of messages to agree on the VC number to be used

Switches in the VC maintain the state information (create entries in translation tables) for the ongoing connections.

VIRTUAL CIRCUITS - DETAILS With VC approach, the source and destination

needs to go through three phases 1. VC Setup 2. Data Transfer 3. VC Teardown

VCI The identifier that is actually used for data transfer is

called Virtual Circuit Identifier (VCI) A VCI is a small number that only has switch scope

(local significance) It is used by a frame between two switches When a frame arrives at a switch, it has one VCI;

when it leaves it has another.

VC SETUP PHASE DETAILS To transfer a frame from source to destination, all the

switches need to have a table entry for this VC. In VC setup phase, “an outgoing VCI is given to the

source and an incoming VCI is given to the destination”.

If there is a need for duplex communication, two virtual circuits are established

VCI PHASES

FRAME RELAY VIRTUAL CIRCUITS Switched Virtual Circuits

Call Setup Data Transfer Call Termination

Permanent Virtual Circuits Data Transfer Idle

SETUP PHASE How does a switch create an entry for a virtual circuit? There are two approaches here:

Permanent virtual circuit (PVC) Switched virtual circuit (SVC)

Permanent virtual circuit (PVC) In this case, the connection setup is simple. The corresponding table

entry is recorded for all switches by the administrator (remotely and electronically)

An outgoing VCI is given to the source, and an incoming VCI is given to the destination.

The source always uses the VCI to send frames to particular destination.

If there is a need for duplex communication, two virtual circuits are established.

The PVC is like a leased telephone line. If there is a leased telephone line between party A and party B, A can pick up the receiver and talk with B without dialing.

SETUP PHASE… Switched virtual circuit (SVC)

PVC connections have two drawbacks. They are costly because two parties pay for the connection all the time

even when it is not in use. A connection is created from one source to one single destination. If a

source needs connections with several destinations, it needs a PVC for each connection.

An alternate approach is SVC. The SVC creates a temporary, short connection that exists only

when data are being transferred between source and destination. An SVC requires a connection phase.

SVC SETUP REQUEST

SVC SETUP REQUEST Suppose source A needs to create a virtual circuit to B. Two

steps are required, the setup request and acknowledgement. Setup Request

A setup request frame is sent from the source to the destination.

1. Source A sends a setup frame to switch 1.2. Switch 1 receives the setup request frame. It knows that a

frame going from A to B goes out through port 3. How the switch has this information? The switch in setup phase, acts as a router; it has a routing table which is different from the switching table. For the moment, assume that it knows the output port. The switch creates an entry in its table for this virtual circuit, but it is only able to fill three of the four columns. The switch assigns the incoming port (1) and chooses an available incoming VCI, which will be found during the acknowledgement step. The switch then forwards the frame through port 3 to switch 2.

SVC SETUP REQUEST3. Switch 2 receives the setup request frame. The same events happen

here as at switch 1; three columns of the table are completed: in this case, incoming port 1, incoming VCI 66, and outgoing port 2.

4. Switch 3 receives the setup request frame. Again, three columns are completed: incoming port 2, incoming VCI 22, and outgoing port 3.

5. Destination B receives the setup frame, and if it is ready to receive frames from A, it assigns a VCI to the incoming frames that come from A, and not other sources.

SVC SETUP ACKNOWLEDGMENT

SVC SETUP ACKNOWLEDGMENT Acknowledgement

A special frame, called the acknowledgement frame, can complete the entries in the switching tables.

1. The destination sends an acknowledgement to switch 3. The acknowledgement carries the global source and destination addresses so the switch knows which entry in the table is to be completed. The frame also carries VCI 77, chosen by the destination as the incoming VCI for frames from A. Switch 3 uses this VCI to complete the outgoing VCI column for this entry. Note that 77 is the incoming VCI for destination B, but outgoing VCI for switch 3.

2. Switch 3 sends an acknowledgement to switch 2 that contains its incoming VCI in the table, chosen in the previous step. Switch 2 uses this as the outgoing VCI in the table.

SVC SETUP ACKNOWLEDGMENT3. Switch 2 sends an acknowledgement to switch 1 that contains its

incoming VCI in the table. Chosen in the previous step. Switch 1 uses this as the outgoing VCI in the table.

4. Finally switch 1 sends an acknowledgement to source A that contains its incoming VCI in the table, chosen in the previous step.

5. The source uses this as the outgoing VCI for the data frames to be sent to destination B.

DATA TRANSFER PHASE… To transfer a frame from a source to its destination, all

switches need to have a table entry for this virtual circuit. The table, in its simplest form, has four columns. This means

that the switch holds four pieces of information for each virtual circuit that is already set up.

The first figure shows a frame arriving at port 1 with a VCI of 14. When the frame arrives, the switch looks in its table to find port 1, and VCI 14. When it is found, the switch knows to change the VCI to 22 and send out the frame from port 3.

The second figure shows how a frame from source A reaches destination B and how its VCI changes during the trip. Each switch changes the VCI and routes the frame.

TEARDOWN PHASE In this phase, source A, after sending all frames to B,

sends a special frame called teardown request. Destination B responds with a teardown confirmation

frame. All switches erase the corresponding entry from their

tables.

SOURCE-TO-DESTINATION DATA TRANSFER

VC ENTRIES IN SWITCHING TABLES

VC ENTRIES IN SWITCHING TABLES

DATA-LINK CONNECTION IDENTIFIER Frame Relay virtual circuits are identified by data-link

connection identifiers (DLCIs). DLCI values typically are assigned by the Frame Relay

service provider (for example, the telephone company). Frame Relay DLCIs have local significance, which means

that their values are unique in the LAN, but not necessarily in the Frame Relay WAN.

DATA-LINK CONNECTION IDENTIFIER A virtual circuits in Frame Relay are identified by data-link

connection identifiers (DLCIs). A switch assigns a DLCI to each virtual connection in an

interface. This means that two different connections belonging to

two different interfaces may have the same DLCIs. In other words, DLCIs are unique for a particular

interface.

The PVC DLCI are permanent and assigned by frame relay network provider.

The SVC DLCI are temporary and assigned by frame relay during connection phase.

DLCI INSIDE A NETWORK

FRAME RELAY SWITCH

CONGESTION-CONTROL MECHANISMS

congestion being full buffers, over subscribed port, overloaded resources, etc,

Frame Relay implements two congestion-notification mechanisms:

Forward-explicit congestion notification (FECN) Backward-explicit congestion notification (BECN)

FECN AND BECN BITS FECN

A FECN tells the receiving device that the path is congested so that the upper layer protocols should expect some delay.

It indicates that the available network bandwidth (at that time) is not as great as can be supported by the destination terminal.

BECN The BECN tells the transmitting device that the Frame Relay

network is congested and that it should "back off" to allow better throughput.

It means the available network bandwidth (at that time) is not as great as can be supported by the source.

FRAME RELAY FRAME… Forward Explicit Congestion Notification (FECN)

The Forward Explicit Congestion Notification (FECN) bit can be set by any switch to indicate the traffic is congested in the direction in which the frame is traveling. This bit informs the destination that congestion has occurred.

Backward Explicit Congestion Notification (BECN) The Backward Explicit Congestion Notification (BECN) bit is set to

indicate a congestion problem in the direction opposite to the one in which the frame is traveling. This bit informs the sender that congestion has occurred.

FOUR CASES OF CONGESTION

FRAME RELAY DISCARD ELIGIBILITY The Discard Eligibility (DE) bit

used to indicate that a frame has lower importance than other frames.

The DE bit is part of the Address field in the Frame Relay frame header.

When the network becomes congested, DCE devices will discard frames with the DE bit set before discarding those that do not.

This reduces the likelihood of critical data being dropped by Frame Relay DCE devices during periods of congestion.

FRAME RELAY ERROR CHECKING Frame Relay uses a common error-checking mechanism

known as the cyclic redundancy check (CRC). Error correction can be left to higher-layer protocols

running on top of Frame Relay. Frame Relay doesn’t implement Error Control or Flow

Control, it is the responsibility of upper layers

ACCESS LINK A leased line is installed between the router and a nearby

Frame Relay switch; this link is called the access link. To ensure that the link is working, the device outside the

Frame Relay network, called the data terminal equipment (DTE), exchanges regular messages with the Frame Relay

switch. These keep alive messages, along with other messages,

are defined by the Frame Relay Local Management Interface (LMI) protocol.

The routers are considered DTE, and the Frame Relay switches are data communications equipment (DCE).

FRAME RELAY FRAME FORMATS

FRAME RELAY FRAME

FRAME RELAY FRAME… The description of the fields are as follows: Address (DLCI) filed

The first 6 bits of the first byte make up part 1 of the DLCI. The second part of the DLCI uses the first four bits of the second byte. These bits are parts of the 10-bit data link connection identifier defined by the standard.

Command/Response (C/R) The command/response (C/R) bit is provided to allow upper layers to

identify a frame as either a command or response. It is not used by the Frame Relay protocol.

FRAME RELAY FRAME… Extended Address

The extended address (EA)bit indicates whether the current byte is the final byte of the address. An EA of 0 means that another address byte is to follow. An EA of 1 means that the current byte is the final one.

To increase the range of DLCIs, the Frame Relay address has been extended from the original 2-byte address to 3- or 4-byte address.

The figure shows the different addresses. Note that the EA field defines the number of bytes; it is 1 in the last byte of the address, and it is 0 in the other bytes. Note that in the 3- and 4-byte formats, the bit before the last bit is set to 0.

OTHER FEATURES Voice Support

Frame Relay is now capable of supporting voice Use of SVCs

It can also create switched virtual circuits (SVCs) in addition to permanent virtual circuits (PVCs)

An SVC can be created dynamically by the customer. Good for short-term connections.

FRAME RELAY SERVICE CHARGES A customer contacts a frame relay provider who

creates a PVC between the customer and to whomever the customer wants to be connected.

The customer pays for three services: the PVC the access to the frame relay network (port charge) the telephone line that gives them access to the port.

FRAME RELAY VS. INTERNET Frame relay has many advantages over the

Internet guaranteed throughput minimum delay better security.

Internet has the advantage of: being practically everywhere Cheaper simpler to create connections (no PVCs necessary).

CIR Committed Information Rate (CIR)

It is an agreement between Frame Relay customer and the service provider.

The customer requests a particular transmission speed, and if the customer does not exceed that rate, the the service provider will guarantee accurate and timely delivery of data frames.

CIR The user and frame relay service would agree upon a

committed information rate (CIR). The CIR states that if the customer stays within a

specified data rate (standard rate plus a burst rate) the frame relay provider will guarantee delivery of 99.99% of the frames.

The burst rate cannot be exceeded for longer than 2 seconds.

CIR For example: If a company agrees to a CIR of 512 Kbps

with a burst rate of 256 Kbps, the company must stay at or below 512 Kbps, with an occasional burst up to 768 Kbps, as long as the burst does not last longer than 2 seconds.

If the company maintains their end of the agreement, the carrier will provide something like 99.99% throughput and a network delay of no longer than 20 milliseconds.

If the customer exceeds its CIR, and the network becomes congested, the customer’s frames may be discarded.

LOCAL MANAGEMENT INTERFACE LMI is a set of enhancements to the basic Frame

Relay specification. The LMI was developed in 1990 by Cisco

Systems, Northern Telecom and other org. Features Included (called extensions):

global DLCI addressing Virtual circuit status messages Multicasting

LMI MESSAGES Perform a keep alive function between the DTE and

DCE. If the access link has a problem, the absence of keep alive messages implies that the link is down.

Signal whether a PVC is active or inactive. Even though each PVC is predefined, its status can change. An access link might be up, but one or more VCs could be down. The router needs to know which VCs are up and which are down. It learns that information from the switch using LMI status messages.

LMI LMI is the protocol used between a DCE and DTE to manage

the connection. Frame relay was originally designed to provide PVC

connections. There was not, therefore, a provision for controlling or managing interfaces.

Local management interface (LMI) is protocol added recently to the Frame Relay protocol to provide more management features. In particular, LMI can provide A keep-alive mechanism to check if data are flowing. A mechanism to allow an end system to check the status of a

switch (e.g, to see if switch is congested). A multicast mechanism to allow a local end system to send

frames to more than one remote end system.

EVOLUTION OF FRAME RELAY

FRAME RELAY TERMS AND CONCEPTS

FRAME RELAY TERMS AND CONCEPTS

FRAME RELAY TERMS AND CONCEPTS…

LEASED LINES Most home computer users use PSTN lines and

conventional modems to connect to other computer systems using dial up services.

If you need a faster service, or need one that is always on, then you can get a leased line service.A basic leased line, or tie line, gives you a 56 Kbps data transfer rate.

A T-1 (or T1) service gives you a 1.544 Mbps rate and is used by businesses to connect their in-house telephone systems (PBX) and data networks to the outside world.

T1 - SERVICE A T-1 service is a digital, synchronous TDM stream used

by businesses and telephone companies. A T-1 service is always on and always transmitting. One T-1 service can support up to 24 simultaneous

channels. These channels can be either voice or data (PBX support).

A T-1 service can also be provisioned as a single channel delivering 1.544 Mbps of data (LAN to ISP connection).