[itsm] SLA and ConfigItems

Hi everybody, following with my questions on ITMS.....one more question: I have a service with several data lines: some DSL, some fiber, etc..... As you can imagine, a DSL line has different "quality" features than a fiber line..... differents MTBF, differents recovery times....etc So, if the SLA is linked to a service and not to a ConfigItem, how would you define an SLA for these kind of services which have such different CI's?.... Thx!!