LANS, performance and Client/Server design issues

CP3397Network design and security Lecture 3

Basic performance definitions

Bandwidth Raw data rate of links

Capacity Theoretical limit of data transfer Measured over the network, sub-net or link

Throughput Actual data transmitted (e.g. packets per

second) Limited by protocol overhead, delays, latency

etc

Throughput v Capacity

Max

capacity

Thro

ughp

ut

Load

Optimum

Actual

100%

0%

Max throughput

Basic performance definitions

Latency End-to-end delay, comprising

propagation delay (near speed of light), transmission delay (media speed), store-and-forward delay (bridge/switch/router

buffering), processing delay (action on protocol elements)

Sensitivity to delay is application dependent video is very sensitive and virtual terminal (Telnet) is medium sensitive

(user-dependent)

Basic performance definitions

Jitter The variability of latency Buffering can smooth the delay

Media access delay LAN access delay depends on

Access scheme used No. of contending devices

Accuracy Data corruption Bit error rate on WAN links < 1 in 106 on LANs

Key performance relationships

Payload (TCP/IP over Ethernet) Payload = MTU – (TCPOverhead + IPOverhead+ MACOverhead) MTU is maximum transmission unit Overheads are: TCP 20 bytes; IP 20 Bytes; MAC 18

bytes

Maximum packet ratePPSmax =Channel Speed

(8 bits x PDUsize )

For example at 64 kbps with 128 byte PDUs

PPSmax =64000/(8 x 128) = 62.5 pps

Performance issues

Different network types have different maximum packet/frame sizesOverlarge packets need fragmentation and re-assembly to be transmitted limits throughput reduces performance

Compression can be used to improve performance on slower speed links

Key performance relationships

Packet rate and link speed Ensure links do not exceed PPSmax

Error probability and frame size Larger packets are more likely to contain an error Protocol efficiency E E= Sdata _

[R(Sdata+Sprot+Sack)]

Sdata= data size; Sprot=protocol overhead; Sack = ack size R = expected number of transmissions per packet Or R=1+packet error rate e.g 1.001 if 1 in 1000 errors

Typical bottlenecks

Shared services (centralised servers etc)Multi-user applications and databasesLow-speed NICsShared LAN segmentsLow-bandwidth WAN linksCore routing and switching componentsFirewalls (particularly public-facing)Inappropriate compression usage

Main types of server

File ServersDatabase ServersTransaction ServersGroupWare ServersWeb Servers

Middleware

Resides between the client and serverGives the single system imageTypically a major component in a NOSProvides: directory services, network security etcContains proprietary elements where required

Scalable Client Server

For the single User Client, middleware and most of the business

services on a single machine

For the SME Use of small LAN Often involves multiple clients talking to a

local server

For the Enterprise Connection of multiple servers across a

network To utilise fully requires low cost, high speed

bandwidth

Features of Server S/W

Wait for client initiated requestsExecute many requests at the same timeAre able to prioritise requestsCan run activities in backgroundAre resilient and keep runningMain contenders; Netware Windows (and NT) Server Unix/Linux

Features of Client S/W

Communicate service requests to a serverNeeds to be robustProvide protection from programs that crashProvide a mechanism for file transferProvide multi taskingAllow background processes to take place

Client/Server bottlenecks

Client and servers are subject to constraints from Memory CPU cycles Network and disc input/output System bus throughput

Client/Server Design Issues

User requirements (applications, response rate, latency etc)NOS (free choice or pre-determined)Topology (technology determined)Server placement (on the network)Thick/thin client (balance of services)Groupware (CSCW) useMaintenance (ability/cost)

Protocol Issues

TCP/IP protocol performance depends on The implementation/stack used The OS and platform Packet size distribution of the application Background traffic characteristics of the

contended paths LAN, MAN, WAN media properties , overheads

and BERs Intermediate device-forwarding characteristics TCPs sliding window behaviour

Typical bottlenecks

The LAN/WAN interface WANs are typically an order of magnitude slower

Routers need to buffer WAN traffic Buffers require sufficient memory Insufficient buffer space leads to more re-

transmissions – lowering efficiency

Queuing/buffering also increases end-to-end latency Some applications may not tolerate high latency,

timeout and re-transmissions will occur increasing the problem

Data modelling

Gather information of the users to derive Application maps

Which are used and where Data flow

How much data flows from machine to machine Traffic types

Terminal/host, Client/Server, Peer-to-peer, Server-to server, Distributed entity traffic

Local:Remote 80:20 50:50 in modern intranets Build user-type and server profiles Traffic matrices

Characterise data in and data out of each site

Hierarchical network design

Three-layer architecture Backbone layer

High-speed switching layer Mesh design for resilience/minimise outages

Distribution layer Link points between campus LANs and core

backbone Access layer

End user interface Typically LAN environment

Advantages of hierarchical network design

Scalability Easier to add to the network

Manageability Easier to identify location of problems

Broadcast traffic segmentation Traffic confined to smaller broadcast

domains Less traffic over expensive links

Ethernets

Generic Ethernet design rules Max. stations in a collision domain =1024

(collision domain is where the time taken to transmit a min. frame is shorter than the time to detect a collision)

Only use repeaters at link-ends Avoid exceeding standard specs No more than 4 repeaters in a collision domain No more than 3 coax segments in a collision domain Inter-repeater links are best implemented by fibre

(10baseFL, 10baseFB) or 10baseT 10base5, 10base2 and 10baseT can be mixed if

wanted

LAN performance considerations

Fixed parameters Bit rate, slot time etc

Variable factors Packet length distribution No.of hosts in a collision domain Arrival rate of frames Average length of cable Distance between nodes Average medium acquisition time

Ethernet design rules

To optimise performance Use shorter cables - Long cables

increase collision detection time Do not attach too many nodes to a

segment Use largest possible packet size – this

reduces collisions Try not to mix real-time and heavy

bulk data traffic in the same collision domain

VLANs

Logical hierarchy imposed on a flat switched network allowing Scalability Formation of workgroups Simplified admin Better security

Wireless LANs

Use Wireless LAN access points(WLAP) Simplest LAN use single WLAP

Effectively a wireless star topology Multiple WLAPs can be used

Can incorporate wired and wireless segments

WLAPS can support 10-50 clients Over a 30-60m radius (depends on radio transmission

environment)

Wireless LANs can simplify installation and reduce costs – especially in smaller and older buildings

Summary

Good design should optimise performanceMany factors affect performance Technology Software tuning Physical environment

The interaction of all network components needs to be considered