User's Manual

Initial Link Optimization

The SkyPilot Synchronous Mesh Protocol (see separate section) implements an Automatic Repeat reQuest (ARQ) mechanism that

allows for the rapid retransmission of errored packets. This feature operates transparently to the all protocols (such as TCP/IP)

transmitted over a SkyPilot system. When a link is first established, typically when a new SkyExtender or SkyConnector is added to

a network or after a network reset, a series of test packets are exchanged in order to select the optimal modulation type and rate.

The ARQ protocol is disabled and the two nodes constituting the link transmit 90 packets at each of the modulation levels in turn.

The number of packets received at each modulation rate is then returned to the transmitting node. Using these results, the nodes

select the highest modulation and coding level that supports a > 95% successful reception rate.

Continuous Link Maintenance

Every active link used to transfer subscriber traffic (see the Mesh Routing section for details about link selection) is monitored for

basic connectivity and to ensure the optimal modulation levels.

The general link connectivity is continuously tested through the use of “keep-alive” messages. These messages are exchanged

every 1.5 seconds. This message transfer tests not only the RF connectivity, but also management control level connectivity. After

three consecutive message failures, the link is considered to be down, and a reroute or Frequency Hunt is initiated (see the

Spectral Management section for details).

The modulation level of each link is continuously optimized. This is achieved by monitoring the packet retransmission rates over

the link. A sliding window of 576 packet transactions is monitored. This means that at any moment in time the previous 576 packet

transmissions/receptions are analyzed. The following actions are taken based on the failure rate experienced:

• If the failure rate exceeds 50 packets (8.68%), then the modulation is reduced by one level. Note that a packet failure is

defined as a single packet not being received successfully.

• If the packet failure rate is less than 10 (1.74%) then the modulation is increased one level.

SkyPilot Synchronous Mesh Protocol

The SkyPilot Carrier-Class Broadband Wireless System implements a synchronized variant of the asynchronous IEEE 802.11

protocol. This SkyPilot protocol enhancement utilizes the baseband capabilities of 802.11, including all modulation levels and

OFDM. In addition, the SkyPilot protocol utilizes the 802.11 frame format.

The SkyPilot enhanced synchronous protocol, called Time Division Duplex (TTD), provides the following advantages when

compared to the asynchronous 802.11 protocol:

• Predictable, low latency

• Scalability

• Accurate, granular QoS control

• Spectral efficiency

IEEE 802.11

The 802.11 standard was created to provide a low cost, easily implemented Wireless Local Area Network (WLAN). The standard

defines a Carrier Sense Multiple Access (CSMA) protocol that is used to control access to the channel. The CSMA mechanism

requires that all (or at least the vast majority of) nodes within the network are able to hear the transmission of all other nodes.

When standard 802.11 devices are unable to hear their peers, as is very often the case in large-scale outdoor deployments,

multiple nodes attempt to transmit at the same time, causing unwanted collisions and a loss of capacity. Testing has shown that

such a network, with as little as four active 802.11 nodes, can experience heavy degradation of system throughput. This issue is

often referred to as the “hidden node problem.”

To address the “hidden node problem,” a small number of vendors have implemented polling schemes based on the standard

802.11 Request to Send/Clear to Send (RTS/CTS) mechanisms to allow a central base station to control CPE traffic. This method

successfully avoids the high level of data traffic collisions, but provides only a crude level of bandwidth and latency control. SkyPilot

has developed this approach further, providing a fully scheduled MAC layer protocol with a very fine granularity of bandwidth

control.