The Oakland Raiders signed former Atlanta defensive tackle Vance Walker to a free-agent contract on Monday in the latest move to upgrade a depleted defense.Walker is the sixth defensive free agent signed by Oakland in the past week. He joins tackle Pat Sims and Jason Hunter on the line with linebackers Nick Roach, Kaluka Maiava and Kevin Burnett also added to the mix.The Raiders are expected to bring back only three starters from the defense that gave up the fifth most points in the NFL last season. After addressing the front seven in the first week of free agency, the Raiders now need to work on the secondary, where the only starter from last year under contract is safety Tyvon Branch.The Raiders have also met with free agent cornerback Terrence Newman. Newman said on Twitter that he expects to make a decision about whether to remain with Cincinnati or sign with Oakland sometime this week.Walker is considered a run stopper who did not play in obvious passing situations last year with the Falcons. Walker started nine games for Atlanta last season, making 32 tackles with three sacks and a forced fumble.He played 58 career games with the Falcons over four seasons after being drafted in the seventh round out of Georgia Tech in 2009. He is versatile enough to also play nose tackle or defensive end on a three-man line.Oakland has already lost two defensive linemen to free agency with Matt Shaughnessy signing with Arizona and Desmond Bryant reaching a deal with Cleveland. Richard Seymour is a free agent and is not expected to return. Backup Dave Tollefson has already been released and longtime starter Tommy Kelly is expected to be cut this offseason.