Wireless Metro WiMAX Backhaul : Licensed vs Unlicensed
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| There are typically a number of layers to be considered in any metropolitan network design. In many cases, these networks will often rely on existing metro fiber for the central metro-core layer, building out new [wireless] layers for the access and metro backhaul segments. Within the backhaul layer, perceived spectrum scarcity can make the use of unlicensed technology an attractive alternative to licensed technology. Typical Metropolitan WiMax Network Topologies (1) WiMax Multi-point Access + Point-to-Point (or daisy-chained) Backhaul The typical configurations of this topology are illustrated below in Figure 1 and Figure 2. The key difference in these topologies are that daisy-chaining reduces overall path availability performance and increased delay and delay variability. The daisy chain, however, allows the effective reach of the metro fiber PoP to be considerably extended.
Figure 1 � WiMax MpT Access + PtP Backhaul Figure 2 � WiMax MpT Access + Daisy-Chained PtP Backhaul
In this topology, an Ethernet mesh is used to aggregate and backhaul WiMax hub sites, delivering the traffic to the metro fibered PoP location (see figure 3). In this case, the network delivers superior path availability due to the inherent angle diversity and location diversity within the meshed backhaul layer. These gains can provide 5 � 10 X improvements in availability. Additionally, compared to the single layer PtP topology of Figure 1, the meshed solution often results reduced [average] path lengths, further enhancing availability performance.
Usually, low delay Ethernet performance network design requires ultra-low delay network elements, careful traffic segregation (normally using VLANs), flow prioritization and cut-through processing. Low delay performance tends to deliver low delay variability (packet jitter), which in-turn facilities proper operation of mobile hand-offs within access cells (sector-to-sector) or between access cells. This functionality is one of the cardinal/fundamental enablers for the support of converged fixed-mobile services. The Wireless Backhaul Layer: Licensed vs Unlicensed Backhaul Technologies
Although these attributes are different, they are effectively co-dependent upon the underlying over-air radio layer. To illustrate this, consider the use of 5.8 GHz unlicensed technology for backhauling WiMax access hubs/cells. In the event that there is interference on critical network links, a number of inter-related things happen in the backhaul network, for example:
When the network is focused on the transport of delay-sensitive, high value traffic (i.e. VoIP, VIDoIP, TDMoIP) there is little/no elasticity in the traffic to accommodate the above scenario � so there are usually direct, measurable/noticeable resulting impacts on service quality (i.e. dropped calls, severely-errored seconds, etc). Interestingly when the traffic is highly elastic (i.e. best effort in nature) one might think that the above network conditions might be acceptable. However, higher layers in the applications, when confronted with unsuccessful communications resulting from packet discard (buffer overruns) and inappropriate delays often begin retransmitting �. Potentially further aggravating the network bandwidth problem. 5.8 GHz1 Interference problems are seen by some as non-issues which can be ignored. However, when interference in the unlicensed bands is present it can be very difficult and costly to debug and/or remedy. In an effort to determine the extent of interference as it applies to directional backhaul links that would normally be deployed in elevated scenarios running between metro roof-tops (i.e. over the normal low lying building clutter), field studies2 have been undertaken to record activity in typical large (pop> 5M), mid (pop ~ 1M) and small (pop <> Data sampled3 from hundreds of arbitrary distributed sites within the respective general metropolitan areas indicated that close to 100% were exposed to interference which would be deemed harmful to directional PtP radio link performance4 (see sample in Figure 4 and Figure 5 below). The longer range and high bandwidth demands of PtP backhaul make this system highly susceptible to interference effects as compared to their short range, Multi-Point (MpT) counterparts using the same spectrum.
In contrast, licensed technology employed in the backhaul layer provides more predictable available RF conditions since [unpredictable] interference conditions are largely non-existent. Using licensed technology, normally at higher frequencies where larger bandwidths are available and where coordinated link congestion is avoided drives the network design to consider rain-induced availability impacts. The network design benefit with this is that the statistical rain databases are available and have been used to successfully predict high frequency radio link availability for many, many years. Today there are literally millions of these links in service globally. The use of licensed backhaul technology provides the foundation for attaining network performance attributes consistent with the basic goals outlined throughout this paper, namely;
Notes 1 -- even more so in the 2.4 GHz unlicensed band. 2. Field testing and data analysis conducted by the Canadian Federal Government, Industry Canada, Communications Research Center (CRC). Funded by DragonWave. 3. Data gathered at fixed sites using calibrated, elevated, directional receiver system. 4. Typically levels of 20 � 40 dB of noise floor escalation was measured across the entire 5.8 GHz UNI band, resulting from unknown 5.8 GHz transmission signals received at the test locations. | |
華爾街日報報導，市場傳聞已久的Google手機計畫將在兩周內發表。Google將推出整套軟體與開發工具，使廠商能在明年中推出整合網路搜尋、地圖等服務的Google手機，有顛覆行動通信市場的潛力。宏達電、樂金電子被點名是可能的合作夥伴。
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Google的手機計畫或所謂的「Gphone」，自2004年就開始有消息傳出，但Google至今不證實也不否認。Google執行長史密特（Eric Schmidt）在本月稍早的Web 2.0研討會上，還拿出蘋果iPhone當擋箭牌，閃避記者追問。
但Google顯然想進軍行動通訊市場，特別是在手機通訊進入3G與4G世代後，行動上網的潛力十足，使手機的廣告市場有機會出現爆炸性成長。
Google稱霸個人電腦的搜尋廣告市場，但在手機通訊的搜尋市場中，落後競爭對手雅虎。
行動通訊市場仍牢牢掌握在電信商手中，各式鈴聲、音樂與資訊的下載是電信商的金雞母，Google就和Skype一樣，很可能是電信市場的破壞者。
因此，Google一方面表態競標美國將釋出的700MHz行動通訊波段，一方面可能找市場弱勢的電信商合作。
