Margins in the residential mortgage-backed securities market, which had its first issue of the year last week, have risen sharply from last year's levels.Commonwealth Bank raised A$1.6 billion with an issue of mortgage-backed securities, paying 140 basis points over the one-month bank bill swap rate on the senior notes. Pricing on the B and C notes was not disclosed.CBA's last RMBS issue was in September last year, when it raised $2 billion and paid 90 bps over one-month BBSW on the senior notes. And in February last year it raised $2 billion through an RMBS issue, paying 80 bps on the senior notes.In both of last year's issues the senior notes had a weighted average life of 2.8 years, compared with 3.5 years in the latest issue. However, a longer term would not account for much, if any, of the higher margin.Small lenders will have to pay a premium above CBA's margin to get their RMBS issues away. This is likely to put pressure on mortgage rates.