A Brief Overview on IEEE 802.11s

A Brief Overview on IEEE 802.11s Young-Bae Ko, Ph.D., Associate Professor Dept of Info & Computer Engineering, Ajou Univ. Korea (Visiting Professor, CSL, UIUC)

Contents • Introduction and Backgrounds • Architectural and Usage Models in the 802.11s Draft • Framework of the 802.11s Mesh Network • Topology creation • MAC layer forwarding • MAC functionality enhancements • Potential Issues of the current 802.11s • References

Introduction & Backgrounds

Introduction (1/2) • Wireless Mesh Networks • Wireless multi-hop infra networks, where a few nodes provide a connection to the external world (e.g., Internet) through a cable • Alternative wireless access technology, which can replace the traditional sets of IEEE 802.11 wireless LANs • Commercialized and managed ad hoc networks, which Introduce a hierarchy in the network architecture with fixed, special routers and mobile, general clients

Introduction (2/2) • Many vendors have developed their own WMN solutions and put them on the market, because it is flexible and more cost effective than the typical wired APs. • Motorola • Tropos • Belair • PacketHop • However, though most of them are based on the common 802.11 MAC, these products are not interoperable. • Need for defining a standard architecture for WMNs!

WMN Standardization Efforts in IEEE 802 • IEEE has been playing a key role in the development of wireless mesh standards [1]. • IEEE 802.11s – WLAN Mesh • IEEE 802.15.5 – WPAN Mesh • IEEE 802.16a/d/j – WMAN Mesh • Motivation of WLAN Mesh standards • Current 802.11 ad hoc mode is not sufficient for multi-hop mesh. • Recent efforts for the advance of 802.11 standards, such as 11e for QoS support[2] or 11n for high data rates (>100 Mbps), are still limiteddue to their inherent dependency upon the wired infrastructure backbones and the last, single-hop wireless communication.

WLAN (Layer 2) Mesh Networks • The 802.11 Task Group “s” (TGs) • Formed in May 2004 to design mesh networks consisting of different WLAN devices performing routing at link layer (layer 2) • To be based on extensions to the current IEEE 802.11 architecture and protocols:

IEEE 802.11s: Meshed WLAN Networks It will provide an IEEE 802.11 Wireless DS that supports both broadcast/multicast and unicast delivery at the MAC layer using radio-aware metrics over self-configuring multihop topologies. • The objectives: • Increased range/coverage & flexibility in use • Possibility of increased throughput • Reliable performance • Seamless security • Power efficient operation • Multimedia transport between devices • Backward compatibility and interoperability for interworking

Current Status of IEEE 802.11s • 802.11 TGs has defined the following: • Scale: up to 50 mesh nodes • Architectural model • Usage models: 4 usage scenarios • Functional requirements • Several standard drafts released since early 2006: • The initial proposal was released at 2006 March meeting [3]. • Draft 1.0 (released in Nov. 2006) was failed to be approved in the first trial of a letter ballot. • Draft 2.0 (May 2008) again failed in their 2nd letter ballot because many issues still remain. • Draft 3.0 (March 2009) is the most recent one [4]! • The target releasing date of an official 802.11s standard is 2010.

Architectural & Usage Models in 802.11s

802.11s WLAN Mesh - Network Architecture • Mesh Portal: Acting as a gateway/bridge to external networks • Mesh STA (station): Relay frames in a router-like hop-by-hop fashion • Mesh AP (Access Point): Mesh relaying functions + AP service for clients

Mesh Basic Service Set (MBSS) • Internal L2 behavior of WLAN Mesh is transparent to higher layers • An MBSS (Mesh Basic Service Set) appears as a single access domain.

Usage Models • Residential • Inside home or a residential building • High bandwidth application, such as multimedia content distribution • Office • Small to medium sized enterprise buildings • Campus/Community/Public access • Out-door deployment environment • Seamless connectivity • Public Safety • Emergency sites

Functional Framework in 802.11s

LAN metaphor, 802.1 bridging support Single-hop/multi-hop neighbor discovery, Extensible path selection & forwarding MAC enhancements 802.11i link security based Unmanaged, autonomic management Legacy 802.11 a/b/g/n Functional Requirements • The set of services provided by the WLAN Mesh that support the control, management, and other operation, including the transport of MSDUs between MPs within the WLAN Mesh.

Key Functionality of 802.11s Networks • Mesh Topology Creation • Self-configuring neighbor discovery , named as “Mesh Peering” • Channel selection • L2 Routing • MAC address based mesh path selection and forwarding • Radio-aware metrics for routing • MAC Enhancement • For supporting QoS, and increasing the network throughput • Power management, Multi-channel operation, and so on • Security • IEEE 802.11i as basis

Mesh Peering Mechanism • To discover peer Mesh STA devices and their properties: • MSTA performs passive scanning (via periodic beacons) or activescanning (via probe messages) • The received beacon or probe response frame contains mesh related information • Mesh ID: name of the mesh (SSID like string) • Mesh configuration element (including version and support functions) • A discovered MSTA will become a peer MSTA after peering processes by 4-way handshaking. • 2-way handshaking with peering-open-frame/peering-confirm-frame exchange in each direction

Example Unified Channel Graphs Channel Selection • Support for single & multiple channels/interfaces • Each logical interface on one RF channel, belongs to one “Unified Channel Graph (UCG)” • MP specifies one of the two channel selection modes for each interface: • Simple Unification mode – enables the formation of a fully connected UCG • Advanced mode – not fully defined in the proposal (opened to the venders)

Mesh Path Selection and Forwarding • To select single/multi-hop path(s) and to forward data frames across these paths between MPs at the link layer. • Extensible path selection framework • A WLAN Mesh may include multiple path selection metrics and protocols for flexibility. • A mandatory protocol and metric for all implementations are specified. • Hybrid Wireless Mesh Protocol (HWMP) • Airtime link metric function • Only one protocol/metric will be active on a particular link at a time. • A particular mesh will have only one active protocol at a time.

Airtime Link Metric Function • A default link metric to be used by a path selection protocol to select the best paths. • Other metrics can also be used. • Its cost function is based on airtime cost (Ca), which reflects the amount of channel resources consumed by transmitting the frame over a particular link.

Example • Unicast Cost Function based on Airtime Link Metrics This path having the minimum airtime cost is the Best!

Hybrid Wireless Mesh Protocol (HWMP) • A default path selection protocol for interoperability. • To combine the flexibility of on-demand route discovery with extensions to enable efficient proactive routing to mesh portals. • On-demand mode • Used in intra-mesh routing for the route optimization • When a root portal is not configured or it can provide a better path even if root is configured. • Proactive, Tree based mode • If a root portal is present, a distance vector routing tree is built. • Tree based routing avoids unnecessary discovery flooding during discovery and recovery

HWMP: On-demand Path Selection Mode • Source broadcasts PREQ (path request) with the destination and metric initialized. • Upon receiving PREQ, MPs update the path to source if sequence number is greater and offers a better metric • If a new path is created or the existing one is modified, PREQ is forwarded further. • PREQ provides “Target only” (TO) and “Reply and Forward” (RF) flags. • If TO=1: Only destination sends PREP (path reply) after selecting best path. • If TO=0 and RF =0: Intermediate node with path sends a unicast PREP to the source MP and does not forward PREQ • If TO=0 and RF =1: The first intermediate node with the path to the destination sends a PREP and forwards PREQ setting TO =1 to avoid other intermediate nodes to send back PREP. • 5. When source receives the PREP, it creates a path to the destination.

HWMP – Proactive tree building mode • Proactive PREQ mechanism • Root MP periodically broadcast PREQ • To create paths between the root mesh and all mesh nodes in the network proactively (2-way handshaking) • Proactive RANN mechanism • Root MP periodically broadcast RANN • Distribute path information for reaching the root mesh but the actual paths to the root mesh can be built on-demand (3-way handshaking)

Example – Proactive PREQ mechanism • The root MP periodically propagates a PREQ into the network • - Destination Address set to all ones • - The TO flag set to 1 and the RF flag set to 1 PREP R PREQ • Upon reception of a PREQ, each MP has to create or refresh a path to the root MP • The recipient MP’s action • - If “Proactive PREP” bit set to 0, MP may send a proactive PREP if required. • - If “Proactive PREP” bit set to 1, MP shall send a proactive PREP. • Tree path construction is completed

Example – Proactive RANN mechanism • The root MP periodically propagates a RANN into the network. PREP R • Upon reception of a RANN, each MP has to create or refresh a path to the root through sending a unicast PREQ to the root MP. RANN • The root MP sends a PREP in response to each PREQ. PREQ • Tree path construction is completed

802.11s MAC Enhancements • The existing 802.11 MAC layer is being enhanced for • Supporting QoS: • EDCA(Enhanced Distributed Channel Access) specified in 802.11e, as the 802.11s’ basic operation mechanism • Other features of 802.11e, like HCCA, are not considered. • Improving the network capacity: • The usage of multiple channels and multiple radios • Efficient handling of the two different kinds of traffic (BSS traffic & Forwarding mesh traffic) • Intra-mesh congestion control • Mesh coordinated channel access (optional)

MAC Enhancements– More Details • Handling BSS and mesh traffic by Mesh AP • Giving priority to mesh traffic may starve STAs • Giving priority to STAs might waste resource utilized by mesh traffic • Advanced solutions: separate radio for mesh and BSS traffic • Intra-mesh congestion control • A simple hop-by-hop congestion control implemented at each MP • Local congestion monitoring  Congestion control signaling  Local rate control • Mesh Coordinated Channel Access (MCCA) • Optional scheme based on the reservation of contention free time slots • Lower contention (more deterministic) mechanism for improved QoS for periodic flows

IEEE 802.11s MAC – Remaining Issues [5,6] • Mobility is of little concern (do not support seamless handover). • No mechanism for multi-channel operation • One proposal called “CCF (Common Channel Framework) was adopted in the early version of the draft (before draft 1.0), but removed from the draft. • Limitations caused by the EDCA • Performance limitations in multi-hop environments • End-to-end QoS limitations • And many more • More reliable and stable metric for link quality measurement and routing? • Better solutions for power management? • More robust approaches than its current securitysolution inherited from 802.11i, in terms of routing security or end-to-end security?

References [1] “Emerging standards for wireless mesh technology”, by M.-J. Lee, J. Zheng, Y.-B. Ko, and D.M. Shrestha, IEEE Wireless Comm., Apr. 2006. [2] “Wireless LAN MAC and PHY specifications, Amendment 8: Medium Access Control (MAC) Quality of service Enhancement”, IEEE std 802.11e-2005., Nov. 2005. [3] Joint SEE-Mesh/Wi-Mesh Proposal to IEEE 802.11 TGs, Feb. 2006. [4] “Draft Amendment to Standard for Information Technology - Telecommunications and Information Exchange Between Systems - LAN/MAN Specific Requirements - Part 11: Wireless Medium Access Control (MAC) and physical layer (PHY) specifications: Amendment: ESS Mesh Networking”, IEEE P802.11s/D3.0, March 2009 [5] “IEEE 802.11s wireless mesh networks: Framework and challenges”, by X. Wang and A.O. Lim, Ad Hoc Networks, vol. 6, 2008. [6] “IEEE 802.11s: WLAN Mesh Standardization and High Performance Extensions”, by G.R. Hiertz, et. al., IEEE Network, May/June 2008.

Thanks ! Q&A – youngko@illinois.edu http://uns.ajou.ac.kr

A Brief Overview on IEEE 802.11s