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LANS, performance and Client/Server design issues 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