1、OR-05-12-1 Thermal Road Map for Telecom Equipment Gamal Refai Ahmed, PhD ABSTRACT The objective ofthis paper is to establish a thermal road mupfor telecom equipment. The intention ofthe authors is not to define a comprehensive thermal road map for telco equip- ment but rather to introduce the basis
2、for a thermal road map that needs to be investigated und examined by the thermal community in this arena. Thepresentpaper also establishes the air cooling limitfrom the component, board, and system level in telecom equipment. Finally, it proposes a growing trend of the bandwidth verses power dissipa
3、tion. INTRODUCTION Until recently, telco providers, as all other electronic system providers, favored the use of air cooling as a means to thermally manage their equipment. A recent worthwhile book on this is by Yeh and Chu (2002). In the early 1980s indirect liquid cooling in the form of a TCM was
4、introduced to cool IBMs multi-chip modules. The cooling package utilizes solid pistons contacting chips and directs the heat to a hat and onto a detachable water-cooled cold plate. One of the big chal- lenges of introducing liquid cooling is the lack of size. Agonafer et al (1994, 1996) introduced p
5、atents on “convert- ible heat exchanger for air or water cooling of electronic circuit components and the like” and “convertible cooling module for air or water cooling of electronic circuit compo- nents” to address some of these issues. The service providers still have concerns about using either l
6、iquid cooling or refrig- eration cooling since a reliable track record has not been estab- lished using liquid cooling. Furthermore, introducing a new cooling approach in the next generation of telco equipment can be associated with a dramatic change in the thermal management ofthe central offices a
7、nd their setups. Therefore, Dereje Agonafer, PhD it is essential to define the air-cooling limits oftelco equipment in the central offices, since it is pertinent in the decision of management to make a change. In addition, the limitations of air-cooling capacity eventually will lead the service prov
8、iders to adapt more aggressive cooling technologies. These types of aggressive technologies should have the capability to support the growing power density in the long term in order to give stability to the infrastructure of the hosted offices. A growing number of papers address electronic cooling.
9、Three levels of cooling need to be addressed in the tele- communication sector: local cooling for critical modules in the system, centralized cooling for the telecom frame (rack cooling), and cooling of the central office. The first item is more controlled by the component and module vendors for amp
10、lifier modules, Raman modules (a specific pump laser), optical switching pump, power supplies, and ASIC devices (e.g., Corning, Furukawa, JDS Uniphase, AMCC, and Agere). The second item is owned by the system vendors, SV (such as Alcatel, Cisco, Sienna, Nortel, and Corvis). However, the SVs will hav
11、e a lot of influence and are really the main drivers, even at the module level of cooling, since the SVs are the main drivers for the launching of tech- nology by the module/component vendors. Indeed, this rela- tion is the opposite of the computer industry, where INTEL and AMD are enforcing the coo
12、ling approach to their custom- ers (computer manufacturers). This is because the technology vendors simply do not know how the technology will be implemented in a cabinet or rack. It is also common to find that Gamal Refai Ahmed is staff thermal engineer at AT1 Technologies, Inc., Canada. Dereje Ago
13、nafer is professor and director of electronics, MEMS, Nano-Electronics and System Packaging Center, University of Texas, Arlington. 02005 ASHRAE. 91 3 the thermal architecture from the frame point of view is quite different from one SV to another. In addition, the centralized cooling of the cabineth
14、ack must be easily integrated in the present central offices as well as easily maintained. It is essential to remember that telecom services are very critical services compared to other indus- tries, including the computer industry. Losing service can be very costly to the SV if the fault is due to
15、the SV. A system vendor can easily be charged hundreds of thousands of dollars for losing one minute of service. For example, it is critical that there be no down time in teleco services for 91 1 emergency calls. Therefore, the availability and reliability of the cooling system for teleco equipment
16、must be very high. In the present paper, local cooling for critical modules in the system and centralized cooling for the telecom cabinet or rack cooling will be addressed. It is important to note that this study will only focus on the telco industry and will not address the thermal challenges in ei
17、ther the military or the computer industry, which is indeed a very critical and broad market that will be addressed in an upcoming paper. In the next three sections, the following will be discussed: Defining the operating environment for indoor telco equip- ment Present cooling strategies in a telco
18、 system fiom a compo- nent point of view Present cooling strategy of a telco cabinet (rack cooling) 1. 2. 3. DEFINING THE OPERATING ENVIRONMENT FOR INDOOR TELCO EQUIPMENT The Telcordia Generic Requirements (GR) documents are published by Telcordia to inform the industry of Telcor- dias view of gener
19、ic requirements for a variety of conditions affecting the telecommunications industry and the equipment it uses. One can find the imposing criteria in the thermal management of telco equipment from GR-63 and GR-3028 (Telcordia 2002,2001). These criteria can be summarized as follows: 1. 2. 3. 4. 5. T
20、he surrounding ambient temperature can be varied between 5C and 40C. For short-tem operations, the ambient temperature range can be -5C to 50C; however, for a less than half populated frame, the maximum temper- ature is 55C. The nominal ambient temperature is 24v (*2“C). The specific humidity can be
21、 between 5% and 85%, and for short-term operation the range can be extended from 5% to The high altitude range is -60 m below sea level to 1800 m above sea level. However, it is recommended that the equip- ment should be operated up to 4000 m above sea level. The air intake to the equipment can be f
22、rom the front or the bottom of the equipment. However, the exhaust air can either come from the top or the back of the equipment. 90%. PRESENT COOLING STRATEGIES FROM COMPONENT POINT OF VIEW IN A TELCO SYSTEM The primary cooling technology in telecom equipment is air cooling. Forced convection air a
23、nd free convection air cool- ing were used as the main technology for many years. In the last 15 years, we have started to approach forced convection air cooling limits at localized spots in the system at the module level. Therefore, forced air convection was enhanced with the use of heat pipes and
24、more exotic heat sinks to spread the heat to areas where appropriate cooling can be implemented. For example, a heat pipe can be an extremely effective heat spreader as its thermal conductivity can be on the order of 1 O0 times that of copper (Nnanna et al. 2001). In addition, in the last 1 O years,
25、 especially for active optical components, ther- moelectric cooling was introduced in conjunction with forced convection. This type of enhancement allowed the system to provide local cooling and maintain the temperatures of some of the active optical components at 30C below the operating ambient tem
26、perature. The pump laser is a very important device in optical tele- communications. It is commonly used in ampliing the opti- cal signal mostly in the transponder systems. Two types of pump lasers are commonly used: uncooled laser and cooled laser. The first type is usually used for 980 nm waveleng
27、th application. In this type of application, the laser diode can function at any case temperature in the range of -5C to 80C. The second type, the cooled laser, can support either 980 nm or 1420 nm optical wavelength. In this type of application, the case temperature of the laser diode must be maint
28、ained at a fixed temperature. In general, this temperature is 25C. This case temperature can only allow variation within k0.2“C. Ifthe temperature variation exceeds this tolerance, it can vary the optical wave length. Therefore, a thermoelectrical cooler is always used to provide this thermal manage
29、ment feature at any ambient temperature in the range of -5C to 60C. Figure 1 shows the active laser power density trends. One can note that the 400 mW pump laser is approaching 13.9 W/cm2 when the hot side of the thermoelectric cooler is maintained at 70C. The present authors concluded through their
30、 recent research and development of the air-cooling limits of telecom equip- ment that the air-cooling limit for this type of device in a telco system will be around 13 W/cm2 at a case temperature of 75C. It is also noted in Figure 1 that the power density is significantly lower (9.5 W/cm2) when the
31、 thermoelectric cooler is maintained at 65C. This leads us to believe that any telecom system that uses 400 mW lasers will face a tremendous thermal challenge to allow this type of device to function at an ambicnt of 60C and 1800 rn above sea level (the thermal budget cant exceed 15C; otherwise, the
32、 pump laser thermal electric cooler case cant withstand temperatures greater than 75C-if greater, there will be a thermal runaway). Another local cooling problem in a telecom system is the ASIC device (Figure 2), which needs to be maintained at a case temperature of 105C at an ambient of 60C and 180
33、0 m 91 4 ASHRAE Transactions: Symposia %id power dissipafion for example, in the year 2010 the POP will have 76 frames to support 102 TbsIPoP. On the other hand, Figure 13 shows the number of frames if we use the power densitylframe from the road map of the Uptime Institute (2001). It is very import
34、ant to remember that the road map of the Uptime Insti- tute did not indicate the maximum limit of the air cooling; rather, it defined the expected power density per frame. It was assumed that the service providerskarriers will have the responsibility of dealing with this growing power density. The T
35、elecordia document GR-63 indicates that the center office can accommodate equipment with a power density of approx- imately 2 kW/m2. This power density was based on the foot- print of the equipment and the surrounding space in the front Using air to thermally manage the fiam where the max. air cooli
36、ng limit per frame is 20 kWlm 30 20 1 10 Predication 98 02 04 08 Figure 13 Number offramesiPol? and rear aisles. This means that this power density needs to be corrected by a factor to reflect the power density of the frame. For a 0.6 m by 0.6 m frame, the correction factor is 2.5. So based on the T
37、elecordia standard, the air-cooling limit of a unit of equipment can be up to 5 kWlm2. In other words, equipment with a power density of 20 kWlm2 will not be able to receive the requested airflow rate from the central office ventilation system unless the service providerlcarrier reduces the number o
38、f frames in the central office or provides a new way of venti- lation that is capable of removing more than 8 kW/m2 (foot- print of the central office). SUMMARY AND CONCLUSIONS This paper discussed the thermal challenges in local cool- ing for critical modules in a telecom system and the thermal cha
39、llenges for centralized cooling for the telecom frame. It is very clear that the local thermal problem, from a component- level point of view, is approaching the air-cooling limit. A good example of that is the 400 mW laser and ASIC devices, which will eventually be used in future optical transport
40、and switching systems. This type of active device will need to be thermally managed by a cooling technology different from air cooling. It is also noted that the air mover technology in tele- com applications needs to be improved significantly in the next generation of equipment. In addition, the ai
41、r-cooling architecture in central offices will not be able to keep up with the growing increase in power density of the telecom racks (system level cooling). This leads one to believe that the next generation of telecom equipment will need to have either liquid cooling, refrigeration cooling, or liq
42、uid immersion cooling. However, to avoid these methods ASHRAE Transactions: Symposia 91 9 of packaging, the air-cooling architecture in the central offices should be revised and improved. A good example is if the central offices can be maintained at 4OoC during an air-condi- tioning failure, this ca
43、n improve the air-cooling limit in the frame by approximately 25%. NOMENCLATURE D = depth, m EH = effective height, m E W = effective width, m H = height, m h = altitude, m W = width, m T,. = room temperature, K ATa = air temperature rise, K = ratio of airflow rate to power density, CFM/$W/m2) REFER
44、ENCES Agonafer, D., et al, 1996. US patent 5,482,113, Convertible heat exchanger for air or water cooling of electronic cir- cuit components and the like. Agonafer, D., et al. 1994. US patent 5,370,178, Convertible cooling module for air or water cooling of electronic cir- cuit components. Ding, Y.,
45、 D. Agonafer, and R. Schmidt. 2000. Mathematical model for thermostatic expansion valve. IMECE 2000, EEP Vol. 28, Orlando. Ellsworth, M., D. Agonafer, and R. Schmidt. 2002. Design and analysis of a scheme to mitigate condensation on an assembly used to cool a processor module. BMJournal of Research
46、and Development 46(6):753-761. Kulkarni, A., V. Mulay, and D. Agonafer. 2002. Effect of the thermostatic expansion valve characteristics on the sta- bility of a refrigeration system-Part I. ITHERM, May 29-June 1,2002, San Diego, CA. Nnanna, A.G., M. North, and D. Agonafer. 2001. Optimiza- tion of he
47、at pipe spacing in heat sinks: A computational fluid dynamics study. InterPACK 2001, Paper 15758, Kauai. Telcordia. 2002, GR-63-COF2, NEBS Requirements: Physi- cal Protection. Issue 2 (April). Telcordia. 200 1. GR-3028-CORE. Thermal Management in Telecommunications, Issue 1. Uptime Institute. 2000.
48、Heat-density trends in data process- ing, computer systems, and telecommunications equip- ment. White paper, The Uptime Institute, Inc., New Mexico, US. Yeh, L.-T., and R.C. Chu. 2002. Thermal Management of Microelectronic Equipment: Heat Transfer Theory, Analysis Methods, and Design Practices. ASME
49、 Press Book Series on Electronic Packaging, D. Agonafer, edi- tor-in-chief. New York: ASME. DISCUSSION Chris Kurkjian, Mechanical Engineer, EYP Mission Crit- ical Facilities, Albany, N.Y.: The papedpresentation indi- cated an RH range of 5% to 85%. This implies that the equipment can operate without humidification. Does the author believe the telco center can be designed without humid- ification? Gama1 Refai Ahmed: The authors would like to thank Mr. Kurkjian for his question. The relative humidity range is spec- ified by the GR-63 Teleocordia document for indoor equip
