Unit 1: Market structure; more on the broadband market

From my post the other day, the current broadband market is broken down as follows:

Market share June 2006
NTL 27% BT 24% AOL/Carphone Wharehouse 13% Tiscali 11% Wanadoo 9% All other providers 16%

Today comes news that Vodafone is to enter this crowded market

http://business.guardian.co.uk/story/0,,1945365,00.html

a) Which market structure does the broadband market best represent?
b) What barriers to entry are there to the market?
c) How will Vodafone be able to overcome these barriers?
d) Evaluate the impact Vodafone's entry is likely to have on the market