Testing brings home telco TV
5/01/2007, by Martin Rowe, Senior Technical Editor -- Test & Measurement World
WALTHAM, MA�In 2005, Verizon began offering voice, video, and data services over its new fiber-to-the-home (FTTH) network. The move into broadcast and on-demand video thrust engineers at Verizon Labs into video-network integration and testing.
Verizon Labs has locations in Massachusetts, Virginia, New York, and Texas, all of which play some role in video testing. The Waltham, MA, facility, once GTE Labs, now houses test facilities that replicate a complete video-distribution network, from gathering content to delivering it to your home. Alex Laparidis, director of video systems testing, oversees a team of about 40 engineers in the video-test labs. Manager of video systems testing Andrew Marquis manages video lab infrastructure and ensures that test engineers have the equipment they need.
The division�s engineers use several interconnected test beds to verify that video headends, core and edge-network routers, line terminators, home routers, and set-top boxes (STBs) will deliver a video experience worth paying for. For example, engineer Stan Lee operates an end-to-end long-haul core network and metro network to test routers and content distribution. Bill Downey, who manages the test beds� day-to-day operations, uses the long-haul network to test equipment at the ends of the video-distribution network.
In the Customer Premises Equipment (CPE) lab, Rick Halstead manages engineers who test the network components installed in homes. Although CPE is at the end of the network, the CPE lab is fully integrated into the rest of the test network. Earl Vanderhoff uses the Video Distribution Lab to run automated tests on set-top boxes.
Not all test beds are interconnected. Engineers in the RF lab, for example, test network components separately from the rest of the network.
The complete network (Figure 1) consists of interconnected equipment located on two floors and on the building�s roof. Satellite antennas receive analog and digital content from providers such as CNN and ESPN. Aggregators in the super headend (SHE) combine the programs into Ethernet streams for long-haul transport over a SONET network.
Video hub offices (VHOs) combine the national TV content with local content such as broadcasts from local stations and local commercials. The combined video feed travels over a �super trunk� to a video service office (VSO) located in a Verizon central office (CO). Depending on the distance to a VSO, the TV signals may travel as 6-MHz analog and digital channels or remain in IP form. Longer distances require that the video streams remain in IP form.
At the CO, an optical-line terminal (OLT) distributes voice, data, and video-on-demand (VoD) content to home and small businesses through optical splitters. The network optically combines broadcast video signals with the output of the OLT using a wavelength-division multiplexer (WDM). Each splitter can support up to 32 subscribers. (See �Verizon�s last mile,� for a description of the FTTH optical network.)
The network terminates at a subscriber�s home where an optical-network terminal (ONT)�usually located on an outer wall�converts the optical signals into electrical signals. Coax and twisted pair cables carry signals throughout the home. A broadband premises gateway distributes services to VoIP phones, computers, and STBs. In the Waltham facility, a small theatre contains a complete home network, starting with the ONT, that�s connected to an OLT and to the video infrastructure in the other labs.
In the video lab, two SHEs capture video analog and digital video broadcasts from five satellite dishes on the roof. (In the actual network, the SHEs are located in Illinois and Florida to prevent a single natural disaster from taking down the network feed.) Encoders convert all incoming analog video content to MPEG-2 format. The MPEG stream is then formatted into IP packets for transport over the SONET OC-192 (10-Gbps) long-haul network. All content travels over the long-haul network encapsulated in IP packets.
Before sending IP video to the long-haul network, the SHEs add program information and content rules in XML format. XML data identifies each program within the MPEG stream so customers can get the program they want.
�The XML data must comply with CableLabs specifications for content routing,� said test engineer Scot Arena. He runs functional tests on the network, its components, and on vendor software releases by forcing errors into the XML data. �We need to see how the network responds to errors,� he said. (The CableLabs consortium represents the cable telecom industry and pursues new cable telecom technologies, www.cablelabs.com.)
Arena also verifies the quality of the VoD MPEG video before it enters the transport network. He uses video testers from IneoQuest, Pixelmetrix, and Triveni Digital to measure video quality. He compares video quality before and after the IP streams containing the MPEG streams travel through the network.
The video lab also contains an RF test system (Figure 2) that engineers use to test the optical transmitters and ONTs over the 55.25-MHz to 870-MHz broadcast TV band. Downey explained that this tester simulates the analog and digital video signals from which engineers can measure frequency response, distortion, and channel crosstalk.
The test system, built by Matrix Test Equipment, contains several RF signal generators that re-create different combinations of analog and digital signals. Typically, the system produces thirty to forty 6-MHz analog channels. The remainder of the broadcast channels are in digital form. Amplifiers convert the electrical RF signals into optical signals for testing.
The Verizon engineers use a Rohde & Schwarz quadrature amplitude modulation (QAM) tester and an Agilent Technologies spectrum analyzer to characterize the signals. For the QAM (digital) channels, they measure modulation-error ratio. They measure bit-error rate (BER) both before and after forward-error correction (FEC). The engineers make measurements on the RF signals before they enter the amplifiers and then repeat the measurements as the signals exit the ONT, comparing the results to see how the network degrades the signals.
On the analog channels, engineers measure signal-to-noise ratio (SNR) as well as carrier second-order (CSO), carrier triple-beat (CTB), and carrier-noise ratio (CNR) distortion as the system turns adjacent channels on and off. �ONTs have automatic gain control,� Downey noted, �and we test at incoming optical power that simulates an end-of-life laser.� The engineers perform this test with several ONTs to check for interoperability problems. A typical ONT test takes several weeks to complete.
The long haul
Downstairs from the video lab, Verizon Labs keeps its long-haul and medium-haul portion of the video network�the dense-wavelength-division multiplexing (DWDM) lab. Here, Stan Lee tests how all of Verizon�s services travel between cities on long-haul networks (up to 4000 km) and around cities on metro networks (up to 75 km). Rolls of fiber connect the optical transport equipment. He runs tests whenever a supplier makes a software change or when Verizon adds a new service.
Programs from the SHEs in the video lab travel through Ethernet-over-SONET rings, then through the DWDM network, then back to SONET rings (Figure 3). All traffic, including video, travels through this portion of the network as IP streams. Broadcast video will later return to its original form before reaching the access (FTTH) network.
Each SONET ring has its own protection. In case of failure, subscribers will see at most a momentary lapse in TV service. Eight copies of video streams (four from each SHE) travel though the test bed so Lee can test the network by breaking the streams. The network should recover from a failure within 50 ms.
Lee used four optical cables from one SHE to demonstrate the network�s ability to recover from a fiber cut. A TV, connected to the FTTH access network, which is connected to the long-haul network, let me see the effects of line breaks, one at a time. With each break, I saw a momentary freeze in the TV picture, but it was hardly an annoyance. When Lee broke the fourth optical connection, the TV picture remained frozen.
Lee does more quantifiable testing than simply looking at a TV screen. He monitors the network�s transport layer with protocol testers from Agilent Technologies, Anritsu, and Exfo. He measures BER with a tester from Digital Lightwave, and he performs field-level tests with optical-network testers from Sunrise Telecom.
To perform a test, Lee uses real and in-house generated network traffic. In-house voice traffic comes from Spirent generators, and data comes from Spirent and Ixia data generators. The network also has several T1 links to Class 5 switches in Silver Spring, MD, where Lee gets voice traffic to add to the network. He uses the voice traffic to load the network with voice calls to check interference with data and video.
Lee also decodes and analyzes MPEG streams and measures video quality with testers from IneoQuest, Pixelmetrix, and Triveni Digital. �We look at the SONET level, the MPEG level, and the Ethernet level,� he said.
Voice, VoD video, and data traffic from the metro network routers connect to OLTs located in Verizon COs. In the lab, the OLTs reside near the metro routers and connect to ONTs located upstairs. The FTTH network, which uses broadband passive optical network (BPON) and gigabit passive optical network (GPON) technology, connects homes to Verizon�s network. Upstairs, engineers in the video lab can monitor the video after it travels over the FTTH network.
Into the home
Verizon�s network ends inside subscribers� homes where it must work with TVs or STBs, computers, POTS phones, and VoIP phones. That�s where Rick Halstead and his staff in the CPE lab test CPE products for a quality experience.
In the CPE lab, engineers use six test benches, each of which simulates in-home networks. (Verizon has 20 CPE test benches in Reston, VA.) One engineer works at each bench.
A typical test bench consists of a rack with an ONT, six or seven STBs and TV screens, a stereo receiver, and a VCR. A home router connects the STBs to several laptop PCs and to the network. The lab has about 25 TVs that engineers use to test for compatibility with the STBs.
Test cases�scripts that run on an automated tester by TestQuest�include home network integration, an interactive program guide, VoD, and emergency alerting. A PC in the tester runs Mercury Quality Center software that tests the STBs under stressful conditions. For example, a test might consist of two STBs running VoD programs while a PC downloads a large file and several VoIP calls come over the line. �We check the quality of service for each TV, PC, and phone,� said Halstead.
The TestQuest system plays STB commands sent from a remote control. �It receives infrared signals from a remote, stores them, and plays them to an STB through an infrared transmitter,� Halstead explained. Engineers run a sequence of remote buttons and the system records them, thus producing consistent commands for the STBs.
A server PC in the system contains a video-capture card, which lets the system check that the correct menu screens appear at the proper time. Audio and video switches let engineers test programs on each TV screen.
Engineers also check the system for virus vulnerabilities and for denial-of-service attacks. In a denial-of-service attack test, a PC sends large amounts of data traffic to the STB and engineers look for operating-system abnormalities.
While Halstead and others test all of the CPE products that Verizon provides to homes, test engineer Earl Vanderhoff specializes in stressing STBs from a user perspective. He has developed an automated system that exercises the boxes under the load of 120 simulated users. The automated system checks VoD orders for the correct program by maintaining a database of movie clips.
When a simulated user orders a movie through a remote control, the system captures a segment of the delivered movie and compares it to the reference clip. If the wrong movie plays, the system captures 25 s of the program in DVD quality, and it records the failure in a log. �We can run as many as 3000 VoD sessions over a weekend,� said Vanderhoff. �We can also use the video capture to compare video quality if there�s a change at the SHE.�
Vanderhoff also performs real-time picture-quality analysis on STBs, which have several audio and video outputs. They include a combination of left and right audio outputs, a composite video output, a component video output, a digital video output, an S-video output, a Sony/Philips Digital Interface (SPDI) audio port, and an RF video output.
Video changed everything
When expanding into the video-transmission business, Verizon required its engineers to create test plans, execute test cases, and create test reports. Rigorous procedures ensure that engineers test all network elements to uncover potential problems. The testing at Verizon Labs lets the company make informed decisions about products before deploying them.
�The skills required to test video are different from those needed to test traditional telephone equipment,� said Laparidis, the director of video systems testing. �To support video testing, we brought in engineers with video expertise that complement the skills of other engineers. We constructed or redesigned the architecture to support end-to-end system integration. This gave us the ability to test a network system�s integration through the customer experience.�