Frame Relay Option for Service Providers - QoS Mechanisms and SLA

Pre-conference workshop

International IT Conference 2002 Colombo, Sri Lanka 4 th October 2002

R. Jayanthan.

Bsc.Eng.(Hons), MIEEE, AMIEE, AMIE(SL) MCP, CCNA, NNCDS, NNCAS

Team Leader – Design & Consultancy

[email protected]

© 2002

Objectives



At the end of this workshop you will be able to:

 Define how Frame Relay defers from other communication technology  Describe the features & benefits of Frame Relay technology  Explain the Quality of Service mechanism built-in to Frame Relay technology  Discuss the Service Level Agreement Parameters and Measurements related to Frame Relay © 2002

What is not Frame Relay ?



Frame Relay is

not

a networking protocol !



Frame Relay

 Is an Interface Protocol used in wide area networking  Frame Relay User - Network Interface (UNI)  Frame Relay Network - Network Interface (NNI) © 2002

Frame Relay Interfaces

CPE Frame Relay Network FR-NNI FR-UNI UNI – Between Customer and Operator NNI – Between two Operators Frame Relay Network CPE FR-UNI

© 2002

Leased Line based Network

Router Router Router Multiple Interfaces, DSU/CSUs and Links Router Router

© 2002

Leased Line based Network



Advantages

 Simple  Totally Managed by the Customer organization  The links are ‘private’ to the organization  Considered to be secure 

Disadvantages

 Not optimum in bandwidth utilization  High number of links & physical ports required.

 Hence expensive to the customer  Confined to LAN traffic (IP/IPX)  Does not provide extensive QoS features © 2002

Frame Relay based Network

FRAD DTE FRAD DTE Single Physical Link FRAD DTE UNI Frame Relay Switch DCE Frame Relay Switch DCE Frame Relay Frame Relay Switch DCE Frame Relay Switch DCE Frame Relay Switch DCE Virtual Circuits FRAD DTE FRAD DTE

© 2002

A Shared Network

Access Link

Frame Relay Switch DCE Frame Relay Switch DCE Frame Relay Switch DCE Frame Relay Switch DCE Frame Relay Switch DCE

Network Trunk Link

© 2002

Frame Relay based Network



Network bandwidth is shared



Single physical port at CPE



Virtual Circuits are configured through software



Built-in QoS mechanism



Less frame overhead hence fast switching



Support for switched virtual circuit enables on demand services viz. voice & video calls

© 2002

TDM vs. Frame Relay

TDM Approach

4500 4000 3500 3000 2500 2000 1500 1000 500 0

Time of day

3000 2500 2000 1500 1000 500 0

Virtual Circuit Approach Time of day

© 2002

Types of Virtual Circuits



Permanent Virtual Circuits



Switch Virtual Circuits SVC CPE PVC

LAN

CPE

LAN

CPE

LAN

© 2002

FR-UNI

Physical Link Permanent Virtual Circuits PVC

Frame Relay Switch DCE © 2002

DLCI



Data Link Control Identifier



Identifies each PVC within a FR-UNI

Router DTE DLCI 16 DLCI 17 DLCI 18 DLCI 19 FR-UNI Frame Relay Switch DCE Frame Relay Network © 2002

DLCI (Cont…)



Assigned unique to each logical channel (PVC) within one FR-UNI



DLCI has only local significance



DLCI values has to be provided by the Network Operator



Can be from 16 to 1023 in value

 (DLCI 0 - 15 are reserved) © 2002

Frame Relay Frame Structure

1 byte Flag 2 bytes Header Variable byte I field 2 bytes FCS 1 byte Flag DLCI DLCI 8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1 •

C/R

•

EA

•

FECN

•

BECN

•

DE Command /Response Address Extension Forward Explicit Congestion Notification Backward Explicit Congestion Notification Discard Eligible

Frame Relay supports 2, 3 or 4 byte headers resulting in more DLCI’s per FR-UNI. However 2 byte header is the most commonly implemented.

LAPF Frame

© 2002

Need for Congestion Management

Output buffer Frame Handler Input buffer Subscriber Switch Node © 2002

Congestion Management

No congestion Mild congestion Severe congestion Delay Throughput Offered Load

ITU Recommendation I.370 defines the frame relay congestion

© 2002

Congestion Control Techniques



Discard Strategy

 Providing guidance to the network regarding which frames to discard; by way of

CIR

and

DE

bit.



Congestion Avoidance

 Providing guidance to the end systems about the congestion in the network; by way of

FECN

,

BECN

and

CLLM

. This is called explicit control.



Congestion Recovery

 End system infers congestion from frame loss; by way of higher level protocol function. This is called implicit control.

© 2002

FECN / BECN

User data FECN=0 / BECN=0 Client Server User data FECN=1 / BECN=0 Congestion User data FECN=0 / BECN=0 Frame Relay Network User data FECN=0 / BECN=1 Congestion in the direction of Server The end stations (or Transport protocol such as TCP) shall take care of FECN/BECN to avoid congestion

© 2002

CLLM Message



Consolidated Link Layer Management message

 Is a variation of BECN  Used when no reverse traffic is available  Carries congestion information of multiple virtual circuits © 2002

Implicit Control



When a higher layer protocol detects frame discards, it can adapt rate control such as using sliding window technique.



This function is independent of Frame Relay technology and usually handled by transport layer protocol like TCP.

© 2002

Service Parameter Definition



CIR Committed Information Rate:

The guaranteed throughput provided by the network for the user traffic under normal operation 

T c Committed Rate Measurement Interval

or

Bandwidth Interval



B c Committed Burst Size:

The maximum amount of data the network agrees to transfer, under normal conditions, over the measurement interval of T c 

B e Excess Burst Size:

The maximum amount of data in excess of B c the network will attempt to transfer, over a period of T c

B c + B e B c Time T c

© 2002

Example Service Parameters

Access Rate = 2.048 Mbps T c = 1.125 s CIR = 128 kbps Permits a burst rate B c = 144 kbps

•

These parameters are defined per virtual circuit

•

Though the CIR is 128 kbps, user data is fed in to the network at 2.048 Mbps resulting in low latency; 15 times faster in this example.

•

A single physical link can carry several virtual circuits and the service parameters are configured according to the:

•

Type of traffic, viz. real-time, transaction, database backup & replication, etc.

•

Bandwidth required

© 2002

Measurement Intervals

CIR >0 >0 =0 B c >0 >0 =0 B e >0 =0 >0 T c T c = (B c /CIR) T c = (B c /CIR) T c = (B e /access rate)

© 2002

Discard Eligible (DE) bit

The DE bit is used to mark a frame as Discard Eligible at the ingress port of the Frame Relay switch first Frame Relay switch if the input data rate exceed the committed burst rate

 

DE=0: The frame is guaranteed to be delivered.

DE=1: The frame delivered if possible CPE CPE

Ingress port

FR Switch DE=1 Frame Relay Network FR Switch

Egress Port © 2002

Traffic Management using

CIR

and

DE

bit

B c + B e B c Frame 1 DE = 0 Frame 2 DE = 0 Frame 3 DE = 1 Frame 4 Discarded T c Time

© 2002

CIR Gauge

Current Rate CIR Maximum Rate 0

Guaranteed transmission Transmission if possible

Discard all excess Access Rate

© 2002

Leaky Bucket Algorithm

Input data Limit C = B c +B e Discard any incoming data while C is at its threshold B e

(set DE=1 and forward)

B c C C = Counter; increases with incoming data Decrement C by MIN [C, B c ] every T c time units B c CIR = ---- T c

© 2002

Service Level Agreement

Frame Relay service offerings are available from multiple service providers. Each provider describes the offering by specifying user information transfer parameters. End-users of the service utilize these parameters to: 

Compare different service providers



Measure the quality of specific service



Enforce contractual commitments

© 2002

FRF.13 Implementation Agreement



Frame Relay Forum Implementation Agreement FRF.13 specifies the SLA parameters that describes frame service performance

 Frame Transfer Delay ( FTD )  The time required to transfer data through the network  Frame Delivery Ratio ( FDR,FDR c , FRD e )  Effectiveness in transporting offered load in one direction in a single virtual circuit  Data Delivery Ratio ( DDR, DDR c , DDR e )  Effectiveness in transporting payload  Service Availability ( FRVCA, FRMTTR, FRMTBSO ) © 2002

FRF.13 Connection Components

Access Circuit Section Access Network Section Internetwork Circuit Section Transit Network Section Internetwork Circuit Section Access Network Section Access Circuit Section FR-DTE FR-UNI Frame Relay Network FR-NNI Frame Relay Network FR-NNII Frame Relay Network FR-UNI FR-DTE © 2002

FRF.13 Reference Points

SrcRP Source FR-DTE (Optional) Measurement Function Frame Relay End System EqiRP TpRP EqoRP IngRP L1/L2 Function Traffic Policing Function Ingress Node Intermediate Nodes Public Frame Relay Network Egress Queue Function Egress Node DesRP (Optional) Measurement Function Destination FR-DTE Frame Relay End System © 2002

FRF.13 Scopes

FR-DTE FR-UNI End-to-end Scope Edge-to-edge Interface Scope Edge-to-edge Queue Scope Public Frame Relay Network (s) FR-UNI FR-DTE FR-UNI Private FR Network FR-UNI / NNI © 2002

Delay

Frame Transfer Delay

FTD = t 2 – t 1 t 1 t 2

– – time when the frame left the source (ms) time the frame arrived at the destination (ms)

Measurement Domain

End-to-end Edge-tp-edge Interface

Source

SrcRP IngRP

Edge-to-edge Egress Queue

IngRP

Destination

DesRP EqoRP EqiRP

© 2002

Frame Delivery Ratio (

FDR

)

FDR

= (

FramesDelivered c

(

FramesOffered c

+

FramesDelivered e

) +

FramesOffered e

)

FDR c

= = (

FramesDelivered c+e

) (

FramesOffered c+e

) (

FramesDelivered c

) (

FramesOffered c

)

[FDR for load consisting of frames within CIR]

FDR e

= (

FramesDelivered e

) (

FramesOffered e

)

[FDR for load in excess of CIR]

© 2002

Data Delivery Ratio (

DDR

)

DDR

= (

DataDelivered c

(

DataOffered c

+

DataDelivered e

) +

DataOffered e

) = (

DataDelivered c+e

) (

DataOffered c+e

)

DDR c

= (

DataDelivered c

) (

DataOffered c

)

[FDR for load consisting of frames within CIR]

DDR e

= (

DataDelivered e

) (

DataOffered e

)

Data = Frame – Header - FCS [FDR for load in excess of CIR]

© 2002

Service Availability

Frame Relay virtual connection availability

FRVCA

=

IntervalTime - ExcludedOutageTime – OutageTime IntervalTime - ExcludedOutageTime

* 100 Frame Relay mean time to repair for virtual connection when

OutageCount

> 0

FRMTTR

=

OutageTime OutageCount

Frame Relay mean time between service outage for virtual connection when

OutageCount

> 0

FRMTBSO

=

IntervalTime – ExcludedOutageTime – OutageTime OutageCount

When

OutageCount

= 0, then

FRMTTR

= 0 and

FRMTBSO

= 0 © 2002

Summary



Frame Relay standards only define Interface Protocols.



It enables network sharing and bandwidth optimization.



The Frame Relay UNI & NNI have traffic management And congestion control mechanisms built-in.



SLA parameters are defined to measure the network performance on a per virtual circuit basis.

© 2002

Reference



Further reading:

 Uyless Black:

Frame Relay Networks

, McGraw-Hill, 1998  William Stallings:

ISDN and Broadband ISDN with Frame Relay and ATM

, Prentice Hall, 2000 

Configuring Frame Relay Services

: BayRS documentation- WAN Suite Protocols.

 Nortel MAGELAN Training Manual: Network Engineering Volume 1 & 2  Frame Relay Forum web site www.frforum.com

© 2002