What is the earliest time that Shema can be recited? 

A person who will not be able to say Kriyas Shema in the period between misheyakir and the end of the third hour may say it at alos hashachar. For example, one who will be traveling and will be unable to stop and focus on the first parsha may recite Shema before he leaves. The poskim disagree about whether the brocha of Yotzer Ohr can be recited at that time.

סעיף ג וס"ק יג, יד ו־יז; ביאורים ומוספים דרשו, 15

Is it better to say Kriyas Shema without tefillin at the proper time or with tefillin at alos?

If one can say Shema at the proper time but will be unable to lay tefillin then, some poskim say that he is better off saying Kriyas Shema with tefillin at alos. Saying Shema with tefillin supersedes saying Shema without tefillin at the proper time.

ביאורים ומוספים דרשו, 16

Is davening late permissible without a valid reason?

Even if a person does not say Shema just before sunrise, he must be sure to say it at the first possible time. This is because of the general rule of doing mitzvos at the earliest opportunity, without delay. One may wait for the purpose of reciting it with the tzibur or reciting it while wearing tefillin.

סעיף ב; ביאורים ומוספים דרשו, 12 ו־14

• A person may begin saying Birchos Kriyas Shema and Shema when it is light enough outside that he would be able to recognize a casual acquaintance at a distance of four amos. The poskim offer various definitions of this time (known as misheyakir), ranging from six minutes after alos hashachar until a half an hour before hanetz hachama.

• The most appropriate time to say Kriyas Shema is immediately prior to hanetz hachama (sunrise), so that one will be able to say Shemoneh Esrei at sunrise. If one did not recite it at this time, he should say it at the first possible opportunity.

• Chazon Ish explains that vasikin means davening close to sunrise but not necessarily exactly at sunrise. He adds that people who daven according to an exact clock are included among those who make their tefilla cumbersome. 

• Can the evening and morning Kriyas Shema be read one after another?

• What is the deadline for Birchos Kriyas Shema?

• Can one make up for a missed Kriyas Shema?