VoIP in IEEE 802.11

1
VoIP in IEEE 802.11

• Andrea G. Forte
• Sangho Shin
• Henning Schulzrinne

2
Handoff - Overview

• L2 handoff
• Connectivity (Scanning, Auth., Assoc.)
• Fast MAC Layer Handoff
• Authentication (802.1x, 802.11i)
• Work in progress (!)
• L3 handoff
• Subnet detection
• IP address acquisition (DAD)
• Fast L3 handoff
• Passive DAD
• Multimedia Session update (SIP)

3
Fast MAC Layer Handoff (1/2)

• Overview
• Selective scanningWe do not need to scan all the
  channels. Some heuristics can be used to improve
  the scanning procedure.
• CacheThe APs information is saved on the client
  in a cache so to avoid unnecessary scans in the
  future handoffs.

Changes in the client ONLY.
4
Fast MAC Layer Handoff (2/2)
5
DHCP - Overview

• DHCP ServerAssigns IP addresses to clients that
  request them via the DHCP protocol. It directly
  serve clients in its subnet while it needs the
  Relay Agent in order to server clients in a
  different subnet than its own.
• Relay Agent (RA)We usually have one RA per
  subnet and usually the RA is located on the
  router/gateway of that subnet. The RA needs to
  relay DHCP packets between its network and the
  DHCP server. The server will know to which subnet
  a client belongs to (and which IP address to
  assign) according to which RA the packets came
  from.

6
Fast Layer 3 Handoff (1/4)

• Spatial locality ? Cache
• We use an extension of the L2 cache

Current AP (KEY) Best AP Second best AP
MAC A MAC B MAC C
Channel 1 Channel 11 Channel 6
Gateway D Gateway E Gateway F
LEASE FILE
7
Fast Layer 3 Handoff (2/4)

• Subnet detectionSend a bogus DHCP REQUEST packet
  so to acquire the gateway/router IP address.We
  then compare the new gateway IP address with the
  one contained in our L2 cache. If they match the
  subnet is the same and no other action is needed
  if they do not match, we have a subnet change and
  a L3 handoff has to be performed.

8
Fast Layer 3 Handoff (3/4)

• IP address acquisitionThis is the most time
  consuming part of the L3 handoff process. DAD
  takes most of the time.We optimize the IP
  address acquisition time as follows
• Checking Lease file for a valid IP.
• Temporary IP (Lease miss) ? The client picks
  a candidate IP using particular heuristics.
• SIP re-invite ? The CN will update its session
  with the TEMP_IP.
• Normal DHCP procedure to acquire the final IP.
• SIP re-invite ? The CN will update its session
  with the final IP.

While acquiring a new IP address via DHCP, we do
not have now any disruption regardless of how
long the DHCP procedure will take. We can use the
TEMP_IP as a valid IP for that subnet until the
DHCP procedure completes.
9
Fast Layer 3 Handoff (4/4)

• Multimedia session update (SIP)After a change in
  IP address, we have to inform the Correspondent
  Node (CN) about it. This is usually done with a
  re-Invite. The data stream will be resumed right
  after the 200 OK has been received.

10
Fast Layer 3 - Implementation

• MCA SIP client for PDAs by SIPquest Inc.
• DHCP client by Internet System Consortium (ISC)
• HostAP wireless driver

11
Fast Layer 3 Handoff - Results
12
Passive DAD

• Duplicate Address Detection (DAD)Before the DHCP
  server decides to assign an IP address, it has to
  be sure that such address is not already in use.
  In order to do this, the DHCP server sends ICMP
  Echo requests and waits for ICMP Echo replies.

The delay introduced by DAD is on the order of
seconds!

• Passive DAD (p-DAD)We introduce a new agent,
  namely Address Usage Collector (AUC), which
  collects information about the IP addresses in
  use in its subnet. The AUC will then inform the
  DHCP server about IP addresses already in use in
  a particular subnet.

13
Thank You!

• For more information
• Web
• http//www.cs.columbia.edu/andreaf
• http//www.cs.columbia.edu/IRT
• E-mail
• andreaf_at_cs.columbia.edu

14
Related Work

• IEEE 802.11k
• IEEE 802.11f (Dead)
• IEEE 802.11r
• SyncScan.

Requirements

• Change in the protocol.
• Change in the infrastructure.

15
Handoff delay
16
Handoff Delay
17
Experiment Result Packet Delay
18
Experiment Result Packet Loss
19
WEP Shared Secret

• Auth Ass 7msec

Cache Handoff Delay 7msec
20
IEEE 802.11 MAC Protocols

• Distributed Coordination Function (DCF)
• No QoS supported
• The de-facto standard MAC protocol
• Point Coordination Function (PCF)
• Suitable for real-time media
• Optional and generally not adopted

21
Dynamic PCF (1/2)

• Classification of traffic
• Real-time traffic (VoIP) ? Pollable
• Uses CFP and CP
• Best effort traffic ? Not Pollable
• Uses CP only
• Give higher priority to real-time traffic also in
  CP
• Dynamic Polling List
• Store active nodes only

22
Dynamic PCF (2/2)

• Dynamic CFP Interval and More data field
• Nodes set more data field when they have more
  than two packets to send.
• Solution to the various packetization interval
  problem
• The AP uses the biggest packetization interval as
  a CFP interval.
• Solution to the synchronization problem
• We send in CP only if there is more than one
  packet in queue.

23
Dynamic PCF - Model

• Speech model (ITU-T P.59)

Parameter Duration (s) Rate ()
Talk-spurt 1.004 38.53
Pause 1.587 61.47
Double-Talk 0.228 6.59
Mutual Silence 0.508 22.48

• Ethernet-to-Wireless Topology

24
Dynamic PCF - Results