nó không phải là đáng chú ý cho đến khi sự chậm trễ chuyến đi vòng trở nên đủ dài. Tất cả những thiếu sót của một kênh truyền dẫn có tác dụng làm giảm công suất kênh, thông qua một liên kết truyền tải. Kênh công suất dùng để chỉ giới hạn trên lý thuyết bao nhiêu thông tin, về các bit trong một giây, có thể được truyền qua một kênh | 750 Web Search Technology document selector is to utilize the fact that most search engines return retrieved results in groups. Usually only the top 10 to 20 results are returned in the first result page but the user can make additional requests for more result pages and more results. Hence a document selector may ask each search engine to return the first few result pages. This method tends to return the same number of pages from each selected search engine. Since different search engines may contain different numbers of useful pages for a given query retrieving the same number of pages from each search engine is likely to cause over-retrieval from less useful databases and under-retrieval from highly useful databases. More elaborate document selection methods try to tie the number of pages to retrieve from a search engine to the ranking score or the rank of the search engine relative to the ranking scores or ranks of other search engines. This can lead to proportionally more pages to be retrieved from search engines that are ranked higher or have higher ranking scores. This type of approach is referred to as a weighted allocation approach in Meng et al. 2002 . For each user query the database selector of the metasearch engine computes a rank . 1st 2nd . and a ranking score for each local search engine. Both the rank information and the ranking score information can be used to determine the number of pages to retrieve from different local search engines. For example in the D-WISE system Yuwono Lee 1997 the ranking score information is used. Suppose for a given query q ri denotes the ranking score of the local database Di i 1 . k where k is the number of selected local databases for the query and a k 1 rj denotes the total ranking score for all selected local databases. D-WISE uses the ratio ri a to determine how many pages should be retrieved from Di. More precisely if m pages across these k databases are to be retrieved then D-WISE retrieves m ri a pages from
