Operating Systems and Computer Networks L10 Network ... V10... · Operating Systems and Computer Networks L10 –Network topology and efficiency Prof. Dr.-Ing. Axel Hunger Alexander

Operating Systems and Computer Networks

L10 – Network topology and efficiency

Prof. Dr.-Ing. Axel HungerAlexander Maxeiner, M.Sc.

Institute of Computer EngineeringFaculty of Engineering

University Duisburg-Essen

Alexander Maxeiner M.Sc.

[email protected]

Alexander Maxeiner, M.Sc.University Duisburg-Essen

2Prof. Dr.-Ing. Axel HungerInstitute of Computer Engineering OSCN – Network Topology

Bandwidth

Effective Bandwidth

CRC Bytes

Network Topologies

Realization of Topologies

Internet of things

Overview of content



The bandwidth of a network describes the maximum data transfer on a physical medium -> data transfer rate.

The unit of bandwidth is usually bits per second (bps, bits/s).

Its maximum value is determined by the physical and technical capabilities of a network.

Those capabilities define the maximum frequency a cable or wireless channel can transmit as well as the coding technology on the channels.

Pure bandwidth is assumed to be fault-free, without interference or other influencing factors. In reality ‘effective bandwidth’ or ‘net bit rate’ are the facaors that determine the speed of a data connection.

Bandwidth



Example Network Max. data transfer rate

DCF77 1 bit/s Radio-clock signal

GSM 9,6 kbit/s Mobile

EDGE Down: 260 kbits/s Up: 110 bits/s Mobile

LTE-Advanced 1000 Mbit/s Mobile

Arcnet 2.5 Mbit/s, 20 Mbit/s (Arcnet plus) Cable

Ethernet 10 – 100 Gbit/s Cable

Modem Max 56 kbit/s WAN

ADSL 384 kbit/s – 25 Mbit/s | 64kbit/s – 3.5 Mbit/s WAN

VDSL 25 Mbit/s – 300 Mbit/s WAN

DOCSIS 10 Gbit/s down & 1 Gbit/s up WAN

Example Bandwidths



The effective bandwidth describes the data transfer rate in bit/s of pure payload over a connection with ideal transmission rate 𝑇𝑅.

The factors that limit 𝑇𝑒𝑓𝑓 are the static and dynamic code

efficiency of a transmitted frame.

Payload reduction that occurs with 𝑁𝐸𝑣𝑒𝑛𝑡

𝑠should be averaged

on every frame to calculate the average effective bandwidth / transmission rate 𝑇𝑒𝑓𝑓,𝑎𝑣𝑔.

Faulty transmissions are hard to determine and predict. To include errors during transmission on 𝑇𝑒𝑓𝑓, 𝑇𝑅 is multiplied

with the success rate of data transmission.

Effective bandwidth



CRC stands for ‘Cyclic Redundancy Check’ and is an error detection mechanism within digital networks and memory storage devices.

These bytes contain a check value for the data they are verifying based on the algorithm behind the checksum.

In ideal cases the CRC is capable of detecting and correcting errors during the transmission.

The redundancy check is designed to detect random errors during transmission with a high probability, deliberately induced errors that match the CRC cannot be detected.

The added bytes contain only information that is already present within the frame, thus it is redundant.

CRC



Depending on the implemented CRC method the static and dynamic code efficiency will vary.

Static code efficiency sums up all data containing the results of a CRC computation and either adds them to the data directly or combines them to a CRC table at the trailer of the payload data.

Either way the additional bytes are added as system relevant information, decreasing the amount of payload transported by a frame resulting in a reduced effective bandwidth.

When calculating the optimal block length the average amount of additional CRC-bytes in dependence of the size of the payload data must be known.

CRC and code efficiency



Data transfer rates are highly dependable on the carrier technology used for the transmission.

The slowest part of a network defines the speed for the entire transmission.

CRC bytes incease the amount of trailer data depending on the amount of payload data transmitted by a frame, thus the static code efficiency is a high influencial part on the effective transmission rate.

Though the static code efficiency is decreasing, checking frames and correcting single errors increases the success rate of data transmission, decreases the error rate and therefor allows for larger frames to be transmitted.

Conclusion bandwidth



The structural topology depends on the underlying design of the physical objects necessary for the data transmission as well as the implemented software that regulates the data flow.

Technical limitations within different systems allow only certain topologies for the communication between resources.

Typical topologies are:

– Line/Bus

– Ring

– Star

– Mesh

– Head-end

Network topologies



Bus systems can be found within computer devices on the mainboard.

The name derives from the autobus, who transports people (here: data) from one station to another.

All components connected to the bus can communicate with each other component.

Once the BUS system is used by two connected participants all other components have to wait until the line is free again.

A message send over the bus line can be read by all components connected to it. At the end of each line the message is absorbed to prevent reflection.

BUS



Bus technology is quite stable. A logical high on the line prevents other devices from interacting with it, moderated by the CPU, preventing data crashes.

Most common data bus on the mainboard of a CPU allows the communication between main memory and the CPU.

A DDR4 system uses a 64 bit wide bus with an I/O clock of 1200 MHz leading to a transfer rate up to 2400 MT/s allowing for real time applications to run on the system.

Limitations of this systems are at the physical level of the design. In order to increase the data transfer rate the next step would be a 128 bit bus. This means that 128 copper lines have to be wired on the device, which is considered difficult.

Properties bus



A ring (or loop, daisy chain) is a special variant of the mesh network topology where every participant is connected to its direct neighbor.

Data will always flow in the same direction on the same route if the ring is completely closed. An interrupted ring works more like a bus system.

This structure offers a reduction in protocols and flow instructions since data is always send one way, passing each possible recipient until the destination gathers the information.

Once a line is occupied it is not possible to send other data streams until the line is cleared.

Ring



Ring networks are easy to build and need no additional routing hardware. Early networking devices of home computers used a token ring for data transmission.

Compared to the token ring a daisy chain network needs its devices for the transport of data. Once a station fails the entire network fails.

Devices for the token ring are not necessary for the transport of data, thus the network is more stable for failures. Nevertheless once a device fails the network usually needs to be restarted to add the failed component.

Problematic are errors on the line. If data is not accepted by the recipient due to a fault, the line is blocked.

Ring properties



Star networks use a centralized station to which several devices are connected.

The central system moderates the flow of data, prevents crashes and assigns transfer speed rates.

Modern local are networks are a prime example for a star network. A central router directs the data to the next routing station until it reaches a modem and is ultimately connected to the ISP access points.

Wireless networks are also a hybrid of a star and a mesh network. If a person is within range of several antennas the device can choose to which it connects and sends its data. This helps in times of high transmission demands.

Star



Problems of star network is the bottleneck at the upload link.

No alternate paths can be taken for transmission, should the router fail, the entire network is offline.

Advantages are the observability of devices and checking access rights, as well as modular connectivity.

Since no user-device is necessary for data transmission a drop out of one system is no thread for the network.

In wireless data transmission being connected to a single device may result in problems during times of high access demands. Prime example: New years eve.

Star properties



Mesh networks are used to connect several central stations with each other in order to create backup paths and alternative routes for long range communication.

Also multipath data transmission is possible, where the data is send through several routes within the network and pieced together at the receiving station. The data transfer rate is then the sum of ever transfer rate along the individual paths.

The internet is designed as a mesh (in ideal cases) to ensure connectivity to each routing station.

Mesh networks are more difficult to implement, since every station has to monitor each adjacent neighbor and manage every path going in/out of the transmitting device.

Mesh



The mesh network has a higher cost demand than other networks. The hardware needs to be more advanced, more cables are set in place and all of this increases the maintenance of the network.

The core of the network during a transmission is still a simple point to point data exchange. But since data can be spread over several paths transmission rates can improve.

Keeping the network online during maintenance work very easy, since alternate paths are available.

The network itself has a high power consumption, since all connected devices need to stay online all the time.

Mesh properties



The head-end topology is build from 2 bus connections, the “up-link” and the “down-link”.

The down-links are connected to the same end (-device), the so called “head-end”. The up-links are connected to the same device and send their data once the line is free.

Data that is send through the down-link can be received by each connected device. Data on the up-link is usually only addressed at the head-end and thus invisible for the other connected systems.

Prime example for a head-end network is the satellite-network. One device sends the data up in space and the satellite sends its data back to all stations that can listen.

Head-end



Example Network Max. data transfer rate

Telephone 64kbit/s

Serial ATA 1.5 Gbit/s Hard Drive - Mainboard

PCI-Express 16 (4.0) 31.508 ∗ 109 𝐵𝑦𝑡𝑒𝑠/𝑠 Graphic accelerator bus

USB 3.1 Gen 2 10 Gbit/s

Token ring 4 Mbit/s, 16 Mbit/s Spec. for higher speed possible

Ethernet 10 – 100 Gbit/s Cable

Modem Max 56 kbit/s WAN

Optical fiber 1 Tbit/s

LTE-Advanced 1000 Mbit/s Mobile

Example Transfer rates



IoT is the short form of ‚Internet of things‘.

In its broader description IoT means that everyday objects are equipped with electronics to create an entire network of remotely controllable devices.

But IoT also means creating a network independent of fixed routing devices providing connected systems access to the network.

The devices itself build an ad-hoc network, meaning new devices within range and correct configuration can join, others who leave the network will be dropped out without effort.

IoT



While a commonly used wireless network uses the star-topology, the IoT ad-hoc network is structured like a mesh. As long as they are within range, all devices are technically connected to each other.

Every device also functions as a router of its own. Should a device be ordered to send a transmission to another one not directly, but indirectly connected to it the data is carried on a predetermined path through the network to its destination.

The idea of building an IoT network, expanding to the WAN itself is thus based on the structure of the internet, realized on a microscopically level.

Structure of IOT



On the institute of computer technologies the capability of ad-hoc networks is currently researched. Access times, range and disturbing environmental factors are addressed.

Building an ad-hoc network happens with the new mobile data transmission standard 5G.

Possible fields of application include an intelligent traffic control system, that automatically detects incoming cars, connects to it and makes decisions on routing the traffic according to the load on each lane.

Specifications regarding the concrete definition of IoT are still under development, but it is likely that this will expand within the foreseeable future.

Research regarding IoT



Questions?



Tanenbaum, Andrew S., “Computer Networks”, 4th edition, Pearson Education Inc, Amsterdam, Netherlands, 2003.

Tanenbaum, Andrew S., “Computernetzwerke”, 4th edition, Pearson Education Inc, Amsterdam, Netherlands, 2003.

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Operating Systems and Computer Networks L10 Network ... V10... · Operating Systems and Computer Networks L10 –Network topology and efficiency Prof. Dr.-Ing. Axel Hunger Alexander