Which of the following switch or router capabilities can help improve VoIP call quality on a LAN?

Which of the following switch or router capabilities can help improve VoIP call quality on a LAN?

• QoS policies that give preference to VoIP packets
• Pre-emptive packet drop to limit packet delay
• MPLS to speed general traffic handling
• DiffServ to give preference to VOIP packets 

EXPLANATION

While all of these approaches can produce faster throughput for VoIP packets, only QoS is widely enough implemented to make sense on a LAN. MPLS is primarily a WAN technology; DiffServ is not widely implemented, and pre-emptive packet drop techniques won't solve all VoIP delay issues, nor guarantee call quality (or even packet delivery).