Transcript Document
LANS, performance and
Client/Server design issues
CP3397
Network 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
Optimum
100%
Max throughput
Throughput
Max capacity
0%
Actual
Load
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 rate
PPSmax =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 sizes
Overlarge 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 databases
Low-speed NICs
Shared LAN segments
Low-bandwidth WAN links
Core routing and switching components
Firewalls (particularly public-facing)
Inappropriate compression usage
Main types of server
File Servers
Database Servers
Transaction Servers
GroupWare Servers
Web Servers
Middleware
Resides between the client and server
Gives the single system image
Typically a major component in a NOS
Provides: directory services, network
security etc
Contains 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 requests
Execute many requests at the same time
Are able to prioritise requests
Can run activities in background
Are resilient and keep running
Main contenders;
Netware
Windows (and NT) Server
Unix/Linux
Features of Client S/W
Communicate service requests to a server
Needs to be robust
Provide protection from programs that crash
Provide a mechanism for file transfer
Provide multi tasking
Allow 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) use
Maintenance (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 retransmissions – 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
performance
Many factors affect performance
Technology
Software tuning
Physical environment
The interaction of all network
components needs to be considered