Overview of TDMA Like Protocol v2 protocol

Transcription

1 Overview of TDMA Like Protocol v2 protocol TLPv2 protocol is proprietary wireless protocol developed by HauteSpot Networks for use with Atheros wireless chips TLPv2 is based on TDMA (Time Division Multiple Access) media access technology instead of CSMA (Carrier Sense Multiple Access) media access technology used in regular devices. TDMA media access technology solves hidden node problem and improves media usage, thus improving throughput and latency, especially in PtMP networks. Media access in TLPv2 network is controlled by TLPv2 Access Point. TLPv2 AP divides time in fixed size "periods" which are dynamically divided in downlink (data sent from AP to clients) and uplink (data sent from clients to AP) portions, based on queue state on AP and clients. Uplink time is further divided between connected clients based on their requirements for bandwidth. At the beginning of each period AP broadcasts schedule that tells clients when they should transmit and the amount of time they can use. In order to allow new clients to connect, TLPv2 AP periodically assigns uplink time for "unspecified" client - this time interval is then used by fresh client to initiate registration to AP. Then AP estimates propagation delay between AP and client and starts periodically scheduling uplink time for this client in order to complete registration and receive data from client. TLPv2 implements dynamic rate selection on per-client basis and ARQ for data transmissions. This enables reliable communications across TLPv2 links. For QoS TLPv2 implements variable number of priority queues with built-in default QoS scheduler that can be accompanied with fine grained QoS policy based on firewall rules or priority information propagated across network using VLAN priority or MPLS EXP bits. Compatibility and coexistence with other wireless protocols TLPv2 protocol is not compatible to or based on any other available wireless protocols or implementations, either TDMA based or any other kind, including Motorola Canopy, Ubiquiti Airmax and FreeBSD TDMA implementation. This implies that only TLPv2 supporting and enabled devices can participate in TLPv2 network. Regular devices will not recognize and will not be able to connect to TLPv2 AP. HauteRouterOS devices that have TLPv2 support (that is - have HauteRouterOS version 4.17 or P.O. Box 4016, San Luis Obispo, California Phone: (800) (805) 541-WISP (9477) (310) 598-WISP (9477) Fax: (805) sales@hautespot.net Web:

2 higher) will see TLPv2 APs when issuing scan command, but will only connect to TLPv2 AP if properly configured. As TLPv2 does not use CSMA technology it may disturb any other network in the same channel. In the same way other networks may disturb TLPv2 network, because every other signal is considered noise. The key points regarding compatibility and coexistence: only HauteRouterOS devices will be able to participate in TLPv2 network only HauteRouterOS devices will see TLPv2 AP when scanning TLPv2 network will disturb other networks in the same channel TLPv2 network may be affected by any (TLPv2 or not) other networks in the same channel TLPv2 enabled device will not connect to any other TDMA based network TLPv2 vs The key differences between TLPv2 and : Media access is scheduled by AP - this eliminates hidden node problem and allows to implement centralized media access policy - AP controls how much time is used by every client and can assign time to clients according to some policy instead of every device contending for media access. Reduced propagation delay overhead - There are no per-frame ACKs in TLPv2 - this significantly improves throughput, especially on long distance links where data frame and following ACK frame propagation delay significantly reduces the effectiveness of media usage. Reduced per frame overhead - TLPv2 implements frame aggregation and fragmentation to maximize assigned media usage and reduce per-frame overhead (interframe spaces, preambles). TLPv2 vs TLPv1 The key differences between TLPv2 and TLPv1: Reduced polling overhead - instead of polling each client, TLPv2 AP broadcasts uplink schedule that assigns time to multiple clients, this can be considered "group polling" - no time is wasted for polling each client individually, leaving more time for actual data transmission. This improves throughput, especially in PtMP configurations. Reduced propagation delay overhead - TLPv2 must not poll each client individually, this allows to create uplink schedule based on estimated distance (propagation delay) to clients such that media usage is most effective. This improves throughput, especially in PtMP configurations. Overview of TDMA Like Protocol v2 protocol.docxpage 2 of 7 3/22/2011

3 More control over latency - reduced overhead, adjustable period size and QoS features allows for more control over latency in the network. Configuring TLPv2 As of version 4.17 new wireless interface setting wireless-protocol has been introduced. This setting controls which wireless protocol selects and uses. Note that meaning of this setting depends on interface role (either it is AP or client) that depends on interface mode setting. Find possible values of wireless-protocol and their meaning in table below. Note that in the user interface TLPv1 is referred to as nstreme and TLPv2 is referred to as nv2. This is due the OEM nature of the interface versus the HauteSpot built driver. Wherever you see nv2 think TLPv2. value AP client unspecified establish TLPv1 or network based on old TLPv1 setting connect to TLPv1 or network based on old TLPv1 setting any same as unspecified scan for all matching networks, no matter what protocol, connect using protocol of chosen network establish network connect to networks only nstreme establish TLPv1 network connect to TLPv1 networks only Nv2 establish Nv2 network connect to Nv2 networks only Nv2-nstreme establish Nv2 network scan for Nv2 networks, if suitable network found - connect, otherwise scan for TLPv1 networks, if suitable network found - connect, otherwise scan for network and if suitable network found - connect. Nv2-nstreme establish Nv2 network scan for Nv2 networks, if suitable network found - connect, otherwise scan for TLPv1 networks and if suitable network found - connect Note that wireless-protocol values Nv2-nstreme and Nv2-nstreme DO NOT specify some hybrid or special kind of protocol - these values are implemented to simplify client configuration when protocol of network that client must connect to can change. Using these values can help in migrating network to Nv2 protocol. Overview of TDMA Like Protocol v2 protocol.docxpage 3 of 7 3/22/2011

4 Most of Nv2 settings are significant only to Nv2 AP - Nv2 client automatically adapts necessary settings from AP. The following settings are relevant to Nv2 AP: Nv2-queue-count - specifies how many priority queues are used in Nv2 network. Nv2-qos - controls frame to priority queue mapping policy. Nv2-cell-radius - specifies distance to farthest client in Nv2 network in km. This setting affects the size of contention time slot that AP allocates for clients to initiate connection and also size of time slots used for estimating distance to client. If this setting is too small, clients that are farther away may have trouble connecting and/or disconnect with "ranging timeout" error. Although during normal operation the effect of this setting should be negligible, in order to maintain maximum performance, it is advised to not increase this setting if not necessary, so AP is not reserving time that is actually never used, but instead allocates it for actual data transfer. tdma-period-size - specifies size in ms of time periods that TLPv2 AP for media access scheduling. Smaller period can potentially decrease latency (because AP can assign time for client sooner), but will increase protocol overhead and therefore decrease throughput. On the other hand - increasing period will increase throughput but also increase latency. It may be required to increase this value for especially long links to get acceptable throughput. This necessity can be caused by the fact that there is "propagation gap" between downlink (from AP to clients) and uplink (from clients to AP) data during which no data transfer is happening. This gap is necessary because client must receive last frame from AP - this happens after propagation delay after AP's transmission, and only then client can transmit - as a result frame from client arrives at AP after propagation delay after client's transmission (so the gap is propagation delay times two). The longer the distance, the bigger is necessary propagation gap in every period. If propagation gap takes significant portion of period, actual throughput may become unacceptable and period size should get increased at the expense of increased latency. Basically value of this setting must be carefully chosen to maximize throughput but also to keep latency at acceptable levels. The following settings are significant on both - TLPv2 AP and TLPv2 client: Nv2-security - specifies TLPv2 security mode Nv2-preshared-key - specifies preshared key to be used Migrating to TLPv2 Using wireless-protocol setting aids in migration or evaluating TLPv2 protocol in existing networks really simple and reduce downtime as much as possible. These are the recommended steps: upgrade AP to version that supports TLPv2, but do not enable TLPv2 on AP yet. upgrade clients to version that supports TLPv2 configure all clients with wireless-protocol=nv2-nstreme Clients will still connect to AP using protocol that was used previously, because AP is not changed over to TLPv2 yet configure TLPv2 related settings on AP if it is necessary to use data encryption and secure authentication, configure TLPv2 security related settings on AP and clients. Overview of TDMA Like Protocol v2 protocol.docxpage 4 of 7 3/22/2011

5 set wireless-protocol=nv2 on AP. This will make AP to change to TLPv2 protocol. Clients should now connect using TLPv2 protocol. in case of some trouble you can easily switch back to previous protocol by simply changing it back to whatever was used before on AP. fine tune TLPv2 related settings to get acceptable latency and throughput implement QoS policy for maximum performance. The basic troubleshooting guide: clients have trouble connecting or disconnect with "ranging timeout" error - check that TLPv2-cell-radius setting is set appropriately unexpectedly low throughput on long distance links although signal and rate is fine - try to increase tdma-period-size setting QoS in TLPv2 network QoS in TLPv2 is implemented by means of variable number of priority queues. Queue is considered for transmission based on rule recommended by 802.1D only if all higher priority queues are empty. In practice this means that at first all frames from queue with higher priority will be sent, and only then next queue is considered. Therefore QoS policy must be designed with care so that higher priority queues do not make lower priority queues starve. QoS policy in TLPv2 network is controlled by AP, clients adapt policy from AP. On AP QoS policy is configured with Nv2-queue-count and Nv2-qos parameters. Nv2-queue-count parameter specifies number of priority queues used. Mapping of frames to queues is controlled by Nv2-qos parameter. Nv2-qos=default In this mode outgoing frame at first is inspected by built-in QoS policy algorithm that selects queue based on packet type and size. If built-in rules do not match, queue is selected based on frame priority field, as in Nv2-qos=frame-priority mode. Nv2-qos=frame-priority In this mode QoS queue is selected based on frame priority field. Note that frame priority field is not some field in headers and therefore it is valid only while packet is processed by given device. Frame priority field must be set either explicitly by firewall rules or implicitly from ingress priority by frame forwarding process, for example, from MPLS EXP bits. Queue is selected based on frame priority according to 802.1D recommended user priority to traffic class mapping. Mapping depends on number of available queues (Nv2-queue-count parameter). For example, if number of queues is 4, mapping is as follows (pay attention how this mapping resembles mapping used by WMM): priority 0,3 -> queue 0 Overview of TDMA Like Protocol v2 protocol.docxpage 5 of 7 3/22/2011

6 priority 1,2 -> queue 1 priority 4,5 -> queue 2 priority 6,7 -> queue 3 If number of queues is 2 (default), mapping is as follows: priority 0,1,2,3 -> queue 0 priority 4,5,6,7 -> queue 1 If number of queues is 8 (maximum possible), mapping is as follows: priority 0 -> queue 2 priority 1 -> queue 0 priority 2 -> queue 1 priority 3 -> queue 3 priority 4 -> queue 4 priority 5 -> queue 5 priority 6 -> queue 6 priority 7 -> queue 7 For other mappings, discussion on rationale for these mappings and recommended practices please see 802.1D Security in TLPv2 network TLPv2 security implementation has the following features: hardware accelerated data encryption using AES-CCM with 128 bit keys; 4-way handshake for key management (similar to that of i); preshared key authentication method (similar to that of i); periodically updated group keys (used for broadcast and multicast data). Being proprietary protocol TLPv2 does not use security mechanisms of , therefore security configuration is different. Interface using TLPv2 protocol ignores security-profile setting. Instead, security is configured by the following interface settings: Nv2-security - this setting enables/disables use of security in TLPv2 network. Note that when security is enabled on AP, it will not accept clients with disabled security. In the same way clients with enabled security will not connect to unsecure APs. Nv2-preshared-key - preshared key to use for authentication. Data encryption keys are derived from preshared key during 4-way handshake. Preshared key must be the same in order for 2 Overview of TDMA Like Protocol v2 protocol.docxpage 6 of 7 3/22/2011

7 devices to establish connection. If preshared key will differ, connection will time out because remote party will not be able to correctly interpret key exchange messages. Overview of TDMA Like Protocol v2 protocol.docxpage 7 of 7 3/22/2011