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Home , Videotelephony

MOBILE Protocol testers

Protocol Tester R&S®CRTU-W Validated tests for in networks

3G mobile radio standards such as Circuit-switched connections The 3G-324M standard

UMTS allow the use of mobile - The standardization bodies‘ first Based on the H.324 standard, which approach was to base all mobile video defines audiovisual communications to simultaneously transmit applications on the protocol (IP). in public analog networks, However, this packet-switched trans- the 3G-324M protocol was developed audio and video signals. For this mission was dropped because latency in for mobile radio networks, which have transmission is still too high in most cur- a comparatively high susceptibility to purpose, standardization bodies have rent mobile IP networks. Moreover, qual- transmission errors. The protocol was ity of service (QoS) is insufficient dur- enhanced by special audio and video defined guidelines specifying that ing the entire time of the connection and for mobile radio. It can thus orga- IP networks require a that nize the exchange of audio, video, data video phone calls be made via circuit- is several times higher than with cir- and signaling information via separate cuit-switched connections. Considering logical channels on a common transpar- switched connections on the basis these drawbacks, only circuit-switched ent 64 kbit/s channel in the transmit and realtime connections are currently suit- receive direction (FIG 1). of the 3G-324M protocol. Whether able for video applications.

a functions in confor- mance with these guidelines can now FIG 1 Schematic of a 3G-324M system, derived from the H.324 standard. be verified and, if necessary, certified Control Video I/O Audio I/O Data

for the first time by performing vali-

dated tests with the R&S®CRTU-W

protocol tester. Command & Video codecs Audio codecs Data control H.263 G.723.1 sharing H.245 (MPEG-4) AMR protocols

Logical channel LC 0 LC 1 LC 2 LC 3

Multiplexer H.223 with annexes A&B

3G modem

13 News from Rohde&Schwarz Number 190 (2006/II) MOBILE RADIO Protocol testers

Different logical channels converge in tion, the terminals inform each other Milliseconds are key to the multiplexers, which play a key role in about their individual performance fea- ­acceptance 3G-324M systems. After a call has been tures, i.e. about the available audio and set up between two mobile phones, the video codecs, the multiplexer capacity The duration of the call setup, the length multiplexers – in accordance with the and the scope of the supported radio of the time delay and the synchroniza- 3G-324M protocol – define the start options. To avoid potential conflicts dur- tion accuracy of voice and pictures sig- level, which is the highest mobile level ing the opening procedures of bidirec- nificantly contribute to the acceptance supported by both terminals. By select- tional channels, the protocol defines and success of videotelephony. Setting ing the start level, the format and struc- which of the terminals is to act as mas- up a videotelephony call takes about ture of the information to be transmit- ter and which as slave, and which is to four seconds, which is twice as long as ted is defined. Based on this informa- assume control if necessary. with pure voice telephony, but is still within the generally accepted limits.

FIG 2 Complete decoding of an ASN.1 (abstract syntax notation) message of the H.245 protocol.

Sequence

Message selected from sequence

Evaluation

14 News from Rohde&Schwarz Number 190 (2006/II) Latency and synchronization of audio and video applications strongly influ- ence how subscribers perceive a mobile videotelephony call. End-to-end latency should therefore be no more than 150 ms; however, modern mobile video services exhibit latencies up to 300 ms. A suitably implemented multiplexer pro- vides considerable room for improve- ment because the entire data flow is handled via the multiplexer, which makes it a main cause of latencies.

For subscribers to experience video con- versations as being natural, the picture / sound skew (lip synchronicity) must be 50 ms or less. However, as it takes much longer to process a video than a voice signal, the audio data must be adequately delayed in the multiplexer to achieve a satisfying lip synchronicity result.

Indispensable: the test solution from Rohde&Schwarz

The R&S®CRTU-WF01 option for the FIG 3 Extract from a message log file. R&S®CRTU-W protocol tester from Rohde&Schwarz allows the videote- lephony functions of WCDMA mobile phones to be verified and certified by using validated test cases. These test cases, which are part of the GCF WI-19 rithm facilitates and visualizes the track- tocol and suitable test methods. The test (Global Certification Forum Work Item) ing of the via the individual cases provided by Rohde&Schwarz are document, help to prove whether sig- protocol layers. In addition to signaling indispensable for certification purposes naling, i.e. the exchange of command information, audio and video information – in addition to being a powerful devel- and control information, is carried out is recorded in separate files and is avail- opment tool. in accordance with the specification. able for postprocessing and analysis. Thomas A. Kneidel By using specification-conforming use- ful data, a videotelephony sequence on mobile phones can be tested for proper Future prospects audiovisual quality. Limited video services such as multime- While a test case is running, the pro- dia messaging service (MMS) for pic- tocol tester records the exchanged sig- tures, video clips and sound are already naling information in a message log file being exchanged via IP in the mobile (FIGs 2 and 3). By activating several fil- . Until IP-based realtime More information and data sheet on the R&S®CRTU at ters, the data volume can be processed video communications is feasible, man- www.rohde-schwarz.com for possible subsequent analysis from ufacturers of mobile phones for videote- (search term: CRTU) different angles. A sophisticated algo- lephony must rely on the 3G-324M pro-

15 News from Rohde&Schwarz Number 190 (2006/II)