To answer your first question, H.248 is a VoIP protocol that refers to an enhanced version of the Media Gateway Control Protocol (MGCP). WhatIs.com defines MGCP as follows:
MGCP is a standard protocol for handling the signaling and session management needed during a multimedia conference. The protocol defines a means of communication between a media gateway, which converts data from the format required for a circuit-switched network to that required for a packet-switched network and the media gateway controller. MGCP can be used to set up, maintain, and terminate calls between multiple endpoints.
To answer your second question, the IT Knowledge Exchange has an excellent resource on how Asynchronous Transfer Mode (ATM) and frame relay (FR) technology differ.
This was first published in May 2004