BALTIMORE -- The Denver Broncos are unbeaten because they have the top-ranked defense in the NFL and a quarterback who is very protective of the football.

It`s a formula that for years worked quite well for the Baltimore Ravens, who have lost three straight.

One team is undefeated, the other desperate. It should make for a very interesting matchup today for a couple of well-rested AFC powerhouses coming off a bye.

"It doesn`t get any better for football," Ravens linebacker Ray Lewis said. "There are a couple teams left undefeated. I`m OK with not being undefeated, because I want to do some other things. Those guys are in a great position right now. The bottom line is, the Broncos are playing very sound football right now. They have a reason to be undefeated."

Reason No. 1 is a defense that has allowed an NFL-low 66 points, including 10 in the second half. Denver has given up only five touchdowns in 66 possessions and leads the league in third-down defense (27 percent conversion rate).

All of this has come under the direction of coordinator Mike Nolan, who held the same job with Baltimore earlier this decade.

"Mike`s a very good communicator," Denver`s rookie head coach Josh McDaniels said. "I think that`s very important for every coach, and Mike certainly has the great ability to get his players to understand what he`s asking them to do and then get it out of them. He`s very intelligent."

Nolan`s defense has been aided by an offense run by quarterback Kyle Orton, who has thrown nine touchdown passes and only one interception -- on a why-not heave at the end of a half. Orton`s careful play has enabled the Broncos to compile a plus-7 turnover differential and, of course, a 6-0 record.

Denver hopes the same philosophy can work against the Ravens (3-3).

"They`re very good. I don`t think we`re going to be intimidated by anybody," former Baltimore receiver Brandon Stokley said. "We will have to be very good on offense, and be smart, to be successful against that type of defense."

The Ravens used to depend on a stingy defense headed by Lewis and a conservative offense that preached running the ball and passing prudently. As recently as last year, Baltimore led the NFL in yards rushing and time of possession, which enabled the defense to stay off the field and resulted in a trip to the AFC championship game.

This year, the offense has stepped it up behind two second-year players. Joe Flacco has thrown at least one touchdown pass in six straight games and running back Ray Rice leads the team with 33 catches. But the pass-happy attack has not been backed by the defense as effectively as in the past.

Baltimore gave up 33 points in a loss to Minnesota before the bye, and the Ravens rank an uncharacteristic eighth in yardage allowed. Thus, Baltimore is 1½ games out of first place in the AFC North after getting off to a 3-0 start.

If it comes down to performance in the second half, then something`s got to give. Denver has been spectacular after halftime, but the Ravens have scored 66 points alone in the fourth quarter -- including 21 against Minnesota on the road.