Agilent Measurement Journal - Issue 4 - 2008 - (Page 30)

Switching to packets and routers Enter the Internet in the 1980s. Through a combination of suitability and chance, it has become pervasive. Catalyzed by the content of the World Wide Web and mass access to highbandwidth data connections via cable or digital subscriber line (DSL), huge investments have been made in networks based on routed IP datagrams or packets. These IP networks — and the use of IP — offers a presumption of convergence driven by the apparent ease of using a common packet infrastructure to carry all service types. It’s also straightforward to demonstrate that, in and of itself, the adaptation of “tricky” real-time services such as voice and video into and out of IP packets causes no perceptible quality problems. In some ways, the business models that propelled some recent infrastructure investments seem to assume that the shift to a converged-services IP network has a smooth evolution path. The shift can be smooth, but not without some work. The snag is that IP networks were never intended for real-time services such as phone calls and broadcast or on-demand TV, nor to isolate service quality for individual users. Rather, they were designed to be used as a base for ﬁle transfer in conjunction with a number of other signiﬁcant protocols — in particular Transmission Control Protocol (TCP), which provides the necessary reliability. Unfortunately, TCP is unsuitable for use with voice or real-time (streaming) video because it would introduce large and indeterminate delays. To allow for convergence with a mixture of TCP and real-time trafﬁc, almost every aspect of IP networks has had to be hardened and extended to make them work with these diverse services at a huge scale. Today, however, the reality of the technology still lags content and service providers’ commercial aspirations. Reviewing IP networking The principal element in IP networks is the IP router, a device that forwards IP packets from incoming physical ports onto outgoing ports based on simple numerical addresses (e.g., the URL www.agilent.com is just the IPv4 address 157.238.197.58) and other service tags, all carried within the administrative area of the packet called the header. Packets pass from one router to the next in their journey through the network. One fundamental characteristic of IP networks is that there is no inherent determinism about how long a packet should take to traverse the network. These times can vary depending on what types and quantities of trafﬁc are present. In extreme conditions (or during faults), packets can be discarded altogether. Until recently, routers based prioritization on only header information, with no regard to the data content of the packet. Such an egalitarian approach essentially leaves IP packets to ﬁght it out for priority. To resolve this, routers increasingly are designed to look inside packets and make forwarding-priority decisions based on content as well as header information. While the actual policies may vary and evolve, they will always include elements such as service type and user identity. The networks of the future must intervene actively in the journey of every packet from end-to-end in order to provide the required QoS to each service and user. Testing with extreme realism To accurately test the performance of these devices, it’s necessary to construct test trafﬁc that represents huge numbers of users and all plausible service types. Each type has its own challenges: Phone and TV services have signiﬁcant user-signaling aspects that impact QoE as much as content quality. What’s more, integration of these signaling (or control-plane) aspects into the common IP structure means that forwarding devices must be tested using a trafﬁc mix that also contains these packet types. 10010010001110101110100100011010111010110011100010101001110101011010100101101001110100101101101001 10100100100011101011101001000110101110101100111000101010011101010110101001011010011101001011011010 30 Agilent Measurement Journal http://www.agilent.com

Table of Contents Feed for the Digital Edition of Agilent Measurement Journal - Issue 4 - 2008

Agilent Measurement Journal - Issue 4 - 2008 Delivering Confidence through Compliance with Standards Contents Emerging Innovations Trying Early Device Implementations at the IEEE 1588 PlugFest Achieving Greater Confidence in Measurement Accuracy through Consistency in Calibration Services Overcoming the Challenges of RFID Component Testing 3GPP LTE: Introducing Single- Carrier FDMA Ensuring Reliable Operation and Performance in Converged IP Networks Testing Storage Area Networks and Devices at 8.5-Gb/s Fibre Channel Rates Choosing an Appropriate Calibration Method for Vector Network Analysis Making Traceable EVM Measurements with Digital Oscilloscopes Exploring Terahertz Measurement, Imaging and Spectroscopy: The Electromagnetic Spectrum’s Final Frontier Interpreting Quoted Specifications when Selecting Digitizers

Agilent Measurement Journal - Issue 4 - 2008

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