Packet Network Performance

Frame Relay Performance

Peter HicksActing Rapporteur Q2/17 “ Data Network

Performance”

Tel: + 61 3 9253 6308Fax: + 613 9253 6777

Email: [email protected]

Why worry about Network Performance• Importance:

– Network Management & Planning– Both customers & network operators have an interest

(contractual arrangements / SLAs)

• Benefits of understanding performance– Able to provide Performance Guarantees for customers– Accurate and cost effective dimensioning and network

provisioning– Customer reporting– Compliance with International Standards / National Regs.

ITU-T Performance Objectives Framework • End to end performance• 3 x 3 matrix

Network(X.25, FR, ATM, IP)

TERM. TERM.

Access

InformationTransfer

CriteriaFunction

Speed Accuracy Dependability

Disengage-ment

•Call setup delay

Availability (function of the primary parameters

•Packet, Frame, cell transfer delay•Throughput

• Residual error rate• User information misdelivery prob.

• Call setup error probability

• Call setup failure probability

• Call Clearing (Delay)

• Premature disconnect probability

• Call clear failure probability

• User information loss probability - eg Frame Loss Ratio

Access/call setup

Info Transfer

Disengagement call clearing

Dependability

Accuracy

Speed

Availability

Only the Info transfer stage is applicable to IP networks

Packet Network Performance• ATM, Frame Relay, X.25 all examples of Packet

Switching technologies• Connection oriented; simultaneous circuits (virtual

circuits) able to be supported on a single access line• For FR and ATM, no acknowledgment of frames / cells

sent into the network (requires transport layer protocol to ensure end to end data integrity)

• Can essentially use the same techniques for measuring and quantifying performance

• Performance (Loss & delay) is very dependent on the network architecture, the capacity provided within the network (buffering and transmission trunk speed) and traffic loading.

What are the important parameters that can be readily measured?

• X.25: – throughput and packet transfer delay, Call setup delay

• Frame Relay: for CIR or EIR traffic– Frame transfer delay , Frame delay jitter & Frame loss ratio

• ATM– Cell transfer delay, Cell delay variation, Cell loss ratio

• IP (best effort / connectionless)– packet transfer delay, traffic flows– packet loss ratio– packet delay jitter

Frame Relay Performance Parameters• User information transfer performance

parameters for the FR PVC services defined in X.144

• Key Primary Performance Parameters:– User information Frame Transfer Delay– Frame Delay Jitter– Frame Loss Ratio– Residual Frame Error Ratio– Extra Frame Rate

Frame Relay Performance Parameters• Connection set-up performance parameters for

the FR SVC services defined in X.145• Key Performance Parameters

– Connection Set-up Delay– Disconnect Delay– Release Delay– Connection set-up error probability– Connection set-up failure probability

Frame Relay Performance Parameters• Frame Transfer Delay

– time taken for a frame to traverse the network. Time taken commences when the first bit of frame is transmitted and ends when the last bit of the frame is received

• Frame Loss Ratio (FLRc or FLRe)– The ratio of lost frames to total number sent for either the

committed or excess traffic streams• Frame Delay Jitter = FTDMax – FTDMin

• Residual Frame Error Rate– The rate of errored frames arriving at the destination

• Extra Frame Rate– The rate at which extra frames which were not part of the

source traffic are detected in the destination traffic stream

Frame Relay Performance Parameters• Frame Based Conformant Traffic Distortion:

– distortion from the traffic contract, measure of clumping or spacing of traffic bursts

• Connection Set-up Delay– Time interval between the occurrence of a Set-up message

and the occurrence of the corresponding return Connect message

• Dis-connect Delay– one way delay based on the transport of the a disconnect

message from the clearing to the cleared end terminal.

FR Performance Objectives• Objective values for frame transfer delay, frame loss

ratio & frame delay jitter specified in ITU-T Recommendation X.146

• End to end objectives (not including contributions of the access line) apply to an international frame relay data connection.

• National Network (portion) allocated 34.5% of the end to end objective for FTD & FLR

• International Network (portion) allocated 31% of the end to end objective for FTD & FLR

• 4 Quality of Service classes specified

X.146: FLR, FTD FDJ ObjectivesService Class FLR FTD: 256

byte Frames FDJ

0 No upper bound

No upper bound

Not applic.

1 < 1 X 10-3 95 %< 400 ms

95 %< 52 ms

2 < 3 X 10-5 95 %< 400 ms

95 %< 17 ms

3 < 3 X 10-5 95 %< 150 ms

95 %< 17 ms

FR Performance Objectives - Notes• All values are provisional and they need not be met by networks until

they are revised (up or down) based on real operational experience. The FTD objectives apply edge-to-edge.

• For FTD performance, all objectives apply to frames of size 256 (i.e. to frames with user information fields of 256 octets). If frames of size 128 are used to estimate compliance with these objectives, then the following tighter 95th percentile objectives of for FTD should be used; 380 ms for classes 1 & 2, and 130 ms for class 3.

– Frame Relay Forum have specified 128 octet frame size– Networks with high speed backbone should meet objective when

using 512 frame size • In the case of service class 3, if the international portion route length

exceeds 9300 km, an allowance of 6.25 ms per 1000 km of route length is allocated to the international portion.

Frame Transfer Delay

• Highly sensitive to network topology/architecture, internode trunk transmission speeds & traffic levels

• Can predict delay by simple model• Can measure using echo / loop back techniques

– avoids use of synchronised real-time clocks at remote sites– simple to setup, approach favoured / adopted by ITU– extensive testing of X.25 (and Frame relay & ATM)

• Can also use OAM techniques specified in X.148 and FRF.19 (Requires OAM to be implemented)

Impact of performance objectives on network design

• The transfer delay performance objectives and geographic span impose a maximum transit node limit .

– Network Architecture and Infrastructure impact• For example the mean frame transfer delay (Class 3)

objective across a national FR network (excluding the customers access lines) is specified in Recommendation X.146 as 34.5% of 150ms, and can be used to calculate the maximum number of nodes that a frame can transit.

CoreSwitch

AccessSwitch

AccessSwitch

t Propag

t Clocking

t Node

DTE DTE

CoreSwitch

t Propag t Propag

t Clocking

t Node

t Propag

t Clocking

t Node

t Propag

t Clocking

t Node

CoreSwitch

N L+ t P N (k-1)L+ (k-1) t P +KN N

t Clocking

L+ t P N

Simple model to calculate transit delay

Notes1. Model can be applied to X.25, Frame Relay, ATM or IP networks2. Propagation delay may or may not be a dominant component of the overall delay. This depends on distance, trunk transmission speeds and pkt size.

Transit Delay Model• The model consists of a concatenation of nodes and internode

transmission links. • Each FR switch can be characterised by a mean (or worst

case) processing delay of N ms. • The transmission time across the internode trunks is

dependent or the size of the data frame and the transmission speed and can also be expressed as a fixed delay of L ms.

• The propagation delay is distance dependent (5ms/1000 km) but can be readily calculated for each internode transmission section as tpi. Propagation delay may or may not be a significant component of the overall delay.

• Hence overall delay depends on distance (propagation delay), trunk transmission speeds & frame size.

Transfer Delay Model (cont)

• Using this model an active connection can be shown as:–a series of (k-1) transmission links (l1 to lk-1)

through k switching nodes (n1 to nk).. –each link li has a clocking delay of L ms –each link has a propagation delay of tpi & –each switch ni has a processing & queuing delay

of N ms.

Expression for mean transit delay• The mean or alternatively the upper-bound - worst case packet

transfer delay across the network is readily calculated as mean delay approx = (k-1)L + Propagation Delay + kNmean worst case delay < (k-1)L + Propagation Delay + kNworst-case

• With a high speed backbone (34 Mbits/s) the transmission (clocking) delay for a 1024 byte data Frame is 240 s. This delay reduces to 53s if the transmission backbone is 155 Mbits/s.

• For a 48 byte Frame the clocking delay is approximately 3s. The switching and queuing delay (the variable parameter) through a high speed ATM/FR switch is of the order of 1ms.

• Over long distances the dominant factor will be the propagation delay (Melbourne to Perth 17 ms, Melbourne to Sydney 5ms, Perth to Brisbane 28 ms).

• For “old style” X.25 networks with low speed (64kbit/s) transmission trunks, switching/clocking delay may dominate

_

Expression to calculate maximum number of switching nodes

From the above expressions we can also derive an expression for the maximum number of switching (or routing) stages within a network

(L+ N)( Delay Objective + L – Prop-delay )

Max number of hops k =

For example for a National FR network, delay objective = 52ms Assume Geographic span 4000 km -> Prop Delay = 20msFrame size 256 Bytes, Trunk transmission 34 Mb/s -> L = 61sFor N = 2 ms: k=15.6 maximum node of switches = 15

Clocking delay for various transmission rates and frame sizes

TransmissionSpeeds

Frame Size (FR Information Field)

64 bytes 128 bytes 256 bytes 512 bytes 1024 bytes 2048 bytes64 kbit/s 8 ms 16 ms 32 ms 64 ms 128 ms 256 ms

128 kbit/s 4 ms 8 ms 16 ms 32 ms 64 ms 128 ms256 kbit/s 2 ms 4 ms 8 ms 16 ms 32 ms 64 ms512 kbit/s 1 ms 2 ms 4 ms 8 ms 16 ms 32 ms

1024 kbit/s 0.5 ms 1 ms 2 ms 4 ms 8 ms 16 ms1544 kbit/s 0.35 ms 0.68 ms 1.35 ms 2.67 ms 5.3 ms 10.6 ms2048 kbit/s 0.25 ms 0.5 ms 1 ms 2 ms 4 ms 8 ms

34368 kbit/s 16 s 31 s 61 s 120 s 240 s 480s44736 kbit/s 12 s 24 s 46 s 92 s 184 s 367 s

155520 kbit/s 3.5 s 7 s 13 s 27 s 53 s 106 sTable 1 Clocking delay for various transmission rates and frame sizes.

The frame size refers to the size of the FR information field.Also a 2 byte FR header plus a 2 byte FCS are assumed

Effect of transmission delay and frame size on FTD

• Consider a national network which has a geographic span of 4000 km, consisting of 8 switching stages and inter-trunk transmission speeds of 2Mbit/s. Each switch contributes 1ms of queuing delay. Propagation delay 5ms / 1000km.

• For a 256 octet test frame, each trunk will contribute a clocking delay of 1 ms. (see Table 1). The total FTD is calculated as 8 x 1 ms + 4000 km x 0.005 + 7 x 1 ms = 35 ms. This network meets the national portion FTD objective allocation of 51.75 ms for Class 3 FR Services.

• For a 512 octet test frame, each trunk will contribute a clocking delay of 2 ms. (see Table 1). The total FTD is calculated as 8 x 1 ms + 4000 km x 0.005 + 7 x 2 ms = 42 ms. This network meets the national portion FTD objective allocation of 51.75 ms for Class 3 FR Services.

Effect of transmission delay & frame size on network architecture

• Question: Can a network with 10 switching stages and a trunk speed of 1.544 Mbit/s meet the national portion FTD objective allocation for class 3 if a frame size of 512 octets is used?

– what is the maximum frame size allowed in order to meet the objective

• Question: Show that that only in the case where the number of switching stages exceeds eight (8), and the inter-node trunk transmission speed is than 1.544 Mbit/s or less will the national portion FTD objective be exceeded when the test frame size is 512 octets

Practical Measurement of transit delay• Accurate measurement of transit delay requires

synchronised real time clocks located at appropriate locations. Very expensive

• alternative technique / practical low cost method required.

• measure the round trip delay time of a 256 byte test frame sent to an echo facility or loop back

– echo facility receives a frame and retransmits the frame on the same virtual connection

– echo technique standardised in Rec X.139 & ( X.148)• Use OAM (FRF.19 frames) techniques as per X.148

Measurement of Network Transit Delay using echo technique

Test DTE

Echo device

X.36X.36 FR network

Access lines can be at different speedsIdeally echo device retransmits after set period of time.Use 256 octet test frame.

Measurement of round trip delay time

• Define:Tr = round trip delay time to the echo facilityTd = access line transmission delay (1ms for 256 byte

frame transmitted at 2.048 kbit/s)Tnw = Network transit delayTech = Echo facility delay

• Assuming the echo facility is connected to destination pkt exchange by 2.048 kbit/s line

• Tnw ~ Tr/2 - 2Td - Tech/2

Application of echo technique to measure national & international transit delay

• National network delay can be made by locating echo device at the international gateway exchange

• establish logical channels to national gateway exchange and to a destination international gateway.

Define • t1 = round trip delay time to national gateway• t2 = round trip delay time to destination international

gateway• tint = (t2 - t1)/2 - tgw

where tgw = transit delay of the destination gateway

CoreSwitch

CoreSwitch

CoreSwitch

CoreSwitch

CoreSwitch

Perth

Canberra

Melbourne

Sydney

Brisbane

AccessSwitch

loopback(128 kbit/s)

AccessSwitch

AccessSwitch


AccessSwitch


AccessSwitch


AccessSwitch


AccessSwitch


Central MeasurementTest Equipment

155 Mbit/s Trunk between core switches

34 Mbit/s Trunk from core to access switch

d = distance [km]tp = propagation delay [ms]

Legend for transmission links:

d = 3500 kmtp = 17 ms

d = 500 kmtp = 3 ms

d = 500 kmtp = 3 ms

d = 1000 kmtp = 5 ms

d = 1000 kmtp = 5 ms

256 kbit/s access

A

B

AB

Arrangements for Performance testing for ATM & FR

Some Frame Relay Transfer Delay Results• FR traffic test source located at Melbourne, • used 512 octet test frame • one way delay to other capital cities

Sydney (1000 km) 11 ms Brisbane (2000 km)18 ms Canberra (500km) 11 ms Perth (3500 km) 24 ms

• each switch contributes in the order of 1 - 5 ms• end to end delay dominated by propagation delay,

but switching & clocking delay of the edge / access switches make noticeable contributions

Frame Loss Ratio Performance• Rec X.146 specifies 1 X 10-3 for Class 1 network• Measurement of Frame loss ratio for CIR traffic

– mean monthly figure < 1 X 10-5

– worst case 4 X 10-5

» Have we over dimensioned the network? Is our Class 0 network providing Class 3 service?

• Loss ratio very dependent on network dimensioning and traffic levels

• Also noting the large number of small frames (voice and TCP acknowledgment) causing some functional processors to be very heavily loaded. If the switch becomes overloaded it discards frame

What’s new from Q2/17

• New Recs covering:– FR Network Availability– Metrics for FR/ATM Service Inter-working– Performance of IP over FR– FR OAM

FR Network Availability• Availability is the percentage of time that the network

can successfully transfer frames. Availability is a Key parameter often specified in SLAs

• Traditional ITU approach is to choose a significant primary performance parameter (eg FLR) and assess the performance of a connection against a defined threshold for that parameter.

– For example: If the FLR > 10% over a period of time the connection is declared to be unavailable.

• New Rec X.147 provides a number of options for assessing availability – based on use of OAM Frames or Status messages

Availability vs Connectivity

• Availability (ITU)– represents the point at which the IP service is so bad as to be

unusable: eg. extremely high packet loss will impact on the achieved transfer rate (FTP or HTTP) but the transport layer protocols will still work.

– (a digit bearer is consider unavailable if BER>10-3 for 10 consecutive seconds : Rec G.826)

• Connectivity (IETF IPPM)– defines the period when there is no working route between

source and destination - nothing gets through.• Perhaps we need both

Performance of IP over FR

• What is the performance of an IP network when the backbone infrastructure (connectivity) is provided by FR connections

• Can the IP service classes defined by Y.1541 be supported?

• Propagation delay dominant for long distances. Difficult to achieve a user-to-user IPTD of 100ms on long IP Paths.

• New Rec X.FRIP provides guidelines for use of frame relay as the lower layer transport.

Metrics to characterise FR/ATM Service Interworking Performance

FR ATM

IWF

FRDTE

ATMDTE

The ATM DTE has noknowledge that it istalking to a FR DTE.

The FR DTE has noknowledge that it istalking to an ATM DTE.

FR / ATM Service Interworking

FR/ATM Service Interworking Metrics

• End-to-end performance or the performance of the IWF can be characterized by the following user layer parameters:–Data Block Delivery Ratio–Data Block Transfer Delay–Data Block Delay Jitter

• Parameters are independent of the FR or ATM Traffic Contracts

FR OAM• SG13 developed I.620 (1998) covering FR OAM

–Only basic functionality defined - detection of fault conditions using loop-back frames

–SG13 have agreed to withdraw I.620 in favour of FRF.19• Frame Relay Forum have developed FR.19

–Extensive capabilities to monitor primary performance parameters FTD, FLR (Data Delivery Ratio) and fault detection

• For completeness of the FR Recommendations propose new Rec X.FROAM (text to be technically aligned with FRF.19)

Any Questions?

Packet Network Performance

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