IEEE 802.11s ESS Mesh Networking Prof. Young-Bae Ko (youngko@ajou.ac.kr) Ubiquitous Networked Systems (UbiNeS) Lab (http://uns.ajou.ac.kr) KRnet 2006
Contents Introduction - Wireless Mesh Networks IEEE 802.11s - IEEE Standard for Wireless LAN Mesh Network architecture Usage models Functional requirements Mesh topology discovery Layer 2 routing & forwarding 802.11s MAC enhancements Mesh security MAC data transport over WLAN Mesh References #2
Wireless Mesh Networks (WMN) It is emerging as a new class of multi-hop wireless networks. One of the main constraints of mobile ad hoc networks (MANETs), Infrastructureless, is relaxed. For MANETs, lack of infrastructure is required, cost is not an issue, and Internet access is not a must! Internet WMN introduces a hierarchy in the network architecture, consisting of mesh routers and mobile clients. Mesh Router with Gateway Mesh Router with Gateway Wireless Mesh Backbone Mesh Router Mesh Router with Gateway Mesh Router with Gateway/Bridge Wired Client Mesh Router Mesh Router Mesh Router Wireless Clients Wired Client Access Point Wi-Fi Networks Wireless Clients Wireless Mesh Clients Sink Node Sensor Sensor Networks #3
Mesh Networking in IEEE Standardizations Several IEEE working groups are actively working to provide wireless mesh networking extensions to their standards. IEEE 802.15.5 WPAN Mesh IEEE 802.16j WMAN Mesh/Relay IEEE 802.11s WLAN Mesh IEEE 802.11 is the most successful WLAN standard, and continues to advance with various amendments. 802.11e for providing QoS 802.11n for providing high data rates in excess of 100 Mbps However, multi-hop connections are not regarded by any amendments. #4
IEEE 802.11 WLAN - Network Architecture Terminologies AP (Access Point) Internet STA (Station) IBSS BSS (Basic Service Set) IBSS (Independent BSS) AP DS AP DS (Distribution System) WDS (Wireless DS) ESS (Extended Service Set) STA STA BSS STA ESS STA BSS STA #5
IEEE 802.11 WLAN Mesh 802.11s 802.11s ESS Mesh Networking Task Group To extend the current IEEE 802.11 architecture and protocols for providing the ESS mesh functionality. To define MAC and PHY layers for creating an IEEE 802.11-based WDS. 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 #6
802.11s s WLAN Mesh - Network Architecture New Terminologies MP (Mesh Point): Relay frames each other in a router-like hop-by-hop fashion MAP (Mesh Access Point): Mesh relaying + AP service for clients MPP (Mesh Portal): Acting as a bridge to other networks 802.11s Mesh links STAs MPP ` ` MP MP ` MP MAP Legacy 802.11s links #7
IEEE 802.11s - Architectural Model Targeted at unmanaged WLAN Mesh networks and at enabling interoperability with low complexity. Internet IBSS AP DS Mesh Portal MP STA STA BSS STA MP MAP STA BSS ESS STA #8
Current Status of 802.11 TG s 802.11 TGs has defined the following: Scale: Target ~32 active mesh APs Architectural model Usage models: 4 usage scenarios Functional requirements Two major proposals have been emerged: The one from Wi-Mesh Alliance, lead by Nortel Networks, Philips, Another from SEEMesh, lead by Intel, TI, Samsung, Nokia, Motorola, Joint SEEMesh/Wi-Mesh proposal was presented in 2006 March meeting. It is expected to have an initial draft by July 2006, and a ratified 802.11s standards by early 2008. #9
Usage Models Residential Usage Model To be deployed inside home or a residential building High bandwidth application, such as multimedia content distribution Office Usage Model Small to medium sized enterprise buildings Campus/Community/Public access networking Model Out-door deployment environment Seamless connectivity Public Safety Model Emergency sites #10
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 Mesh Points within the WLAN Mesh. LAN metaphor, 802.1 bridging support Single-hop/multihop neighbor discovery, Extensible path selection & forwarding Mesh Interworking with other 802 networks Mesh Topology Learning, Routing & Forwarding Mesh Security Medium Access Coordination Discovery & Association Mesh Measurement MAC enhancements 802.11i link security based 802.11 service integration Mesh Configuration & Management PHYs Unmanaged, autonomic management Legacy 802.11 a/b/g/n #11
Mesh Point (MP) Boot Sequence Neighbor discovery Active and Passive scanning Channel selection Simple channel unification mode Link establishment with neighbor MPs Authentication Association Local link state measurement Radio aware metrics AP service available Path Forwarding Tables Initialized Measured Associated & authenticated Discovered neighbors Common function Profile-specific path initialization Optional function Path selection and forwarding Extensible path selection framework with more than one protocol AP initialization (optional - if MAP) Neighbor discovery, Channel selection Link establishment Local link state discovery Path Selection (Unicast, Mcast, Bcast) Access point initialization #12
MP Neighbor Discovery Mechanism To discover neighbor MP devices and their properties: A configured MP has at least one Mesh ID. A MP performs passive scanning (via periodic beacons) or active scanning (via probe messages) The MP attempts to maintain the discovered neighbor MP information in a table, named MP Neighbor Table. Neighbor MAC address Operating channel number The most recently observed link status and quality information If no neighbors are detected, MP adopts the Mesh ID for its highest priority profile and remain active. #13
Channel Selection Support for single & multiple channels/interfaces Each logical interface on one RF channel, belongs to one Unified Channel Graph (UCG) Example Unified Channel Graphs MP specifies one of the two channel selection modes for each interface: Simple Channel Unification mode -- enables the formation of a fully connected UCG Advanced mode not fully defined in the proposal #14
Mesh Path Selection and Forwarding To select single-hop or multi-hop paths 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. #15
Airtime Link Metric Function A default radio-aware metric to be used by a path selection protocol to identify an efficient radio-aware path. 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. c a = O ca + O p + B r t 1 e 1 Parameter Description pt O ca O p Channel access overhead (Constant) Protocol overhead (Constant) B t Number of bits in test frame (Constant) r Transmission bit rate for Bt e pt Error rate for Bt #16
Example: Unicast Cost Function based on Airtime Link Metrics 48Mb/s, 10% PER 54Mb/s, 8% PER 12Mb/s, 10% PER 54Mb/s, 2% PER 54Mb/s, 2% PER 48Mb/s, 10% PER This path having the minimum airtime cost is the Best! #17
Hybrid Wireless Mesh Protocol (HWMP) A default mandatory path selection protocol for interoperability. It combines the flexibility of on-demand route discovery with extensions to enable efficient proactive routing to mesh portals. Radio Metric AODV (RM-AODV) for on-demand routing service Used in intra-mesh routing for the route optimization When a root portal is not configured, RM-AODV is used to discover routes to destinations in the mesh on-demand. Tree based routing for pro-active routing service If a Root portal is present, a distance vector routing tree is built and maintained. Tree based routing avoids unnecessary discovery flooding during discovery and recovery #18
HWMP Example Case 1: No Root Portals, Destination inside Mesh Scenario: MP A (Source) wants to communicate with MP C (Destination). 1. MP A first checks its local layer-2 forwarding table for route entry of C. 2. If entry does not exist, A broadcasts a RREQ to discover the best path to C. 3. C replies back to A with a RREP forming bidirectional link for data forwarding. A D E B 4. A starts data communication with C. AF AG C #19
HWMP Example Case 2: No Root Portals, Destination outside Mesh Scenario: MP A (Source) wants to communicate with MP X (Destination). 1. MP A checks its local forwarding table for an active forwarding entry to X. 2. If entry does not exist, A broadcasts a RREQ for finding the best path to X. X 3. If no RREP is received, A assumes X is outside the mesh and sends messages destined to X to mesh portal B for interworking. A D E B 4. Mesh portal B forwards messages to other LAN segments according to locally implemented interworking protocol. AF AG C #20
HWMP Example Case 3: With Root Portals, Destination inside Mesh Scenario: MP A (Source) wants to communicate with MP C (Destination). 1. MP A checks its local forwarding table for route entry of C. 2. If no entry exists, A may directly send the message on the proactive path towards the root portal B. 3. When B receives the message, it flags message as intra-mesh and forwards it to C using proactive route. A D E B 4. When C receives the message, it may issue RREQ to A for finding the best on-demand intra-mesh MP-to-MP path. AF AG C 5. A and C may use the best on-demand path for data delivery. #21
HWMP Example Case 4: With Root portals, Destination outside Mesh Scenario: MP A (Source) wants to communicate with MP X (Destination). 1. MP A checks its local forwarding table for route entry of X. 2. If no entry exists, A may directly send the message on the proactive path towards the root portal B. A D E B X 3. When B receives the message and it does not have an active forwarding entry to X, it may assume the destination is outside the mesh and forward the message to other LAN segments according to locally implemented interworking. AF AG C #22
Radio Aware OLSR (RA-OLSR) An optional path selection protocol A unified, extensible proactive routing framework Based on the two link-state routing protocols: OLSR (Optimal Link State Routing) FSR (Fish eye state routing) -- optional Utilization of radio-aware metrics in forwarding path calculation RA-OLSR, proactively maintains link-state for routing Suitable for usage models with low mobility and multimedia services #23
802.11s s MAC Enhancements The existing 802.11 MAC layer is being enhanced to support mesh services. EDCA (Enhanced Distributed Channel Access) as a basis for the 802.11s media access mechanism: Re-use of latest MAC enhancement in 802.11e Compatible with legacy WLAN devices Other MAC Enhancements for Mesh Common Channel Framework (optional) Mesh deterministic access (optional) #24
Traffic Management -- Congestion Control Engineering traffic to avoid congestion within a multi-hop wireless mesh network is a challenge. Heterogeneous link capacities along the path of a flow Traffic aggregation: Multi-hop flows sharing intermediate links Extensions to the QoS mechanisms defined in 802.11e are being considered to support hop-by-hop congestion control. Intra-mesh congestion control mechanism Local congestion monitoring Congestion control signaling Local rate control 1 2 4 5 3 6 7 #25 High capacity link Low capacity link Flow
Mesh Security Basically, the 802.11s group intends to take advantage of security mechanisms specified in 802.11i (completed in 2004). However, extensions will be necessary because 802.11i provides only one-hop link security. Multi-hop or end-to-end security is required. Association/authentication among neighboring MPs/MAPs is needed. #26
MAC Data Transport over a WLAN Mesh MSDU source may be: Endpoint application Higher-layer protocol (802.1D, IP, etc.), e.g. at Mesh Portal Etc. MSDU Source MSDU (e.g. ARP, DHCP, IP, etc) MPDU MSDU Dest MAC SAP Mesh Point Mesh Point Mesh Point Mesh Point Mesh Point WLAN Mesh is transparent to higher layers. Internal layer 2 behavior of WLAN Mesh is hidden from higher-layer protocols under the MAC-SAP. #27
References 1) R. Bruno, M. Conti and E. Gregori, Mesh Networks: Commodity Multihop Ad Hoc Networks, in IEEE Communications Magazine, March 2005. 2) I. F. Akyildiz, X. Wang and W. Wang, Wireless mesh networks: a survey, in Computer Networks Journal (Elsevier), March 2005. 3) Joint SEE-Mesh/Wi-Mesh Proposal to IEEE 802.11 TGs, Feb. 2006. #28
Thank you! KRnet 2006