1. Andrea G. Forte Sangho Shin Henning Schulzrinne
VoIP in IEEE
Andrea G. Forte
Sangho Shin
Henning Schulzrinne

2. Handoff - Overview L2 handoff L3 handoff
Connectivity (Scanning, Auth., Assoc.)
Fast MAC Layer Handoff
Authentication (802.1x, i)
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 scanning We do not need to scan all the channels. Some heuristics can be used to improve the scanning procedure.
Cache The 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 Server Assigns 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 detection Send 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 acquisition This 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. MN
SIP Re-INVITE
RTP Data
SIP ACK
New IP CN
SIP OK

10. Fast Layer 3 - Implementation
SIP client
(mca)
Wireless card driver
(HostAP driver)
DHCP client
User Space
Kernel Space
kernel
Red Hat 9.0
MCA: SIP client for PDAs by SIPquest Inc.
DHCP client by Internet System Consortium (ISC)
HostAP wireless driver

11. Fast Layer 3 Handoff - Results22
138 8 29 20 40 60 80
100
120
140
160
180
200
L3 handoff time (ms)
Scenario 1
Scenario 2
Scenario 3
SIP Signaling
Client processing
IP acquisition
Detection of subnet
change

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: E-mail:

14. Related Work Requirements IEEE 802.11k IEEE 802.11f (Dead)
IEEE r
SyncScan.
Requirements
Change in the protocol.
Change in the infrastructure.

15. Handoff delay Mobile station All APs Probe delay New AP
Probe request (broadcast)
Probe response
New AP
Authentication request
Authentication response
Association request
Association response
Probe delay
Authentication delay
Association 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 AP
MN1
MN2
MN3
MN4
Router
CN1
CN2
CN3
CN4

24. Dynamic PCF - Results Capacity for VoIP in IEEE 802.11b 32% 37
Transmission Rate (M b/s) 5 10 15 20 25 30 35 40 45 1 2 3 4 6 7 8 9 11 12
Number of VoIP Flows
DCF
PCF
DPCF 13 23 28 24 14 37
Capacity for VoIP in IEEE b
32%