20. Architecture and Evaluation of an Unplanned 802.11b Mesh Network

This paper describes an experimental 37-nodes wireless network in Cambridge, Massachusetts. Users volunteer to install an antenna on their roofs and either connect it to a home broadband connection or not. Average throughput of the Rooftop network is 627 Kbps with averaging 3 hops.

Roofnet uses a unique addressing system on top of IP addresses. A special routing protocol, Srcr, was also introduced to automatically update the routing table among different wireless access points. Since most of the links in between are rather weak, the routing must find an optimum throughput itself. This is called Estimated Transmission Time (ETT).

Roofnet also uses its own algorithm, SampleRate, to determine the bit rates of 1, 2, 5.5, 11Mbps in 802.11b network. The decision is based on actual data speed rather than periodic probes.

Comments:

• The beauty of this system is that, you don't need elaborate planning ahead where to put the wireless hops. The antenna is omni-directional, and software is self-configured.
• I'm not sure if this is ever popular in the States, but there's this thing called FON, which basically gives you an almost-free wireless AP if you agree to share your existing Internet access, whether charging your users or not. And when you are out of your own FON AP, you can access others' depending on your sharing mode. This is/was very, very, very popular in Spain. Would it be possible to apply similar techniques to the FON firmware and give it a try?

19. Modeling Wireless Links for Transport Protocols

This paper provides a lot of background knowledge of wireless links and reviews several protocol solutions. As stated in previous papers, wireless networks have different bandwidth, lossy link, channel allocation delays, and asymmetrical data flows. Ad-hoc and sensor wireless networks are not discussed here, and the focus is mainly in wireless unicast transport protocols.

The main issues why TCP doesn't work well in wireless links are listed as follows,

• packet loss is considered as network congestion, lossy link isn't implemented in a separate packet header
• high round-trip time (RTT)

But in order to achieve interoperability between wireless and LANs, it is still beneficial to use the same TCP stack. Some error assumptions can be made on wireless links. For example, in poor GSM connections, timeouts should be rather common if running on TCP, but real world simulations show otherwise because the delay pattern is more of jitter in packet arrivals, resulting in higher values of TCP retransmit timer.

Several models are suggested so that researchers can simulate more accurately,

• Error losses and corruption: dropping packets probabilistically on a time-based loss, or by per-packet, or per-bit
• Delay variation: by suspending data transmission.
• Packet reordering: swapping two packets in the queue at a given time
• On-demand resource allocation: giving and additional delay when packets arrive at queue
• Bandwidth variation: changing bandwidth. e.g. CDMA2000 0.02-20s / 9.6~163Kbps.
• Asymmetry in bandwidth and latency: configuring uplink/downlink with different parameters for bandwidth and latency.
• Queue: WLAN, drop-tail; satellite, RED. (large buffer causes high queuing delay and poor loss recovery; small low link utilization and frequent packet drops)

Comment: This paper provides abundant background knowledge of wireless network and ways of simulating the iffy link environment. It is also very interesting to know that handovers happen once per minute while driving in a urban area.

18. A Comparison of Mechanisms for Improving TCP Performance over Wireless Links

In TCP protocol, packet drops are mostly considered to be network congestion. But in wireless links, they could result from iffy link conditions. TCP doesn't tell the difference between these, and drops transmission window size before retransmitting packets, and the link speed will be going from bad to worse.

Two approaches:

• hide non-congestion losses from TCP sender, making lossy link appears as high quality link with a reduced bandwidth. AIRMAIL, indirect-TCP, snoop protocols
• make sender aware of wireless hops and packet losses are not because of congestion, good bandwidth

Several schemes were proposed and are compared in this paper, including

• end-to-end retransmissions: selective ACKs (SACKs), Explicit Loss Notifications (ELN)
• split-TCP connections: hide wireless link from sender
• link-layer proposals: hide link losses from TCP sender by local retransmissions and forward error correction on wireless links

Benchmark was measured by throughput and goodput for wired and wireless link.

Comment: Would it be possible to apply XCP on wireless link because it has an extra field of congestion level, so that packet drops of lossy link can be distinguished from congestion?

17. MACAW: A Media Access Protocol for Wireless LAN's

Wireless network is a shared environment and resource is limited. WLAN are either token-based or multiple access. The protocols discussed in this paper focus on multiple access, including Carrier Sense Media Access (CSMA), Multiple Access Collision Avoidance (MACA), and their own modification of MACA, called MACAW.

CSMA:

• avoid collisions by testing the signal strength nearby
• collisions occur at receiver side, not sender

MACA:

• A Request-to-Send RTS) packet is sent before the transmission starts, and receiver respondes with Clear-to-Send (CTS) packet.
• Upon receiving a(n) RTS/CTS, all stations defer according to the proposed data transmission length included in the packet.
• avoids collisions at receiver, not sender
• Binary exponential backoff (BEB): backoff time is doubled after every collision, and reduced to min after every successful RTS-CTS exchange (unfair)

MACAW:

• add a backoff counter in the packet header to achieve fairness
• gentle adjustment when there's a collision (1.5x and -1, Multiplicative Increase and Linear Decrease, MILD)
• multiple stream model to be fair among uplinks and downlinks
• basic message exchange (802.11)

Comment: The idea of adding an ACK in RTS-CTS is great, and their high throughput / fair allocation lives up to the trend of today's network. I just wished they had more real world experiments to proove that MACAW is, indeed, better than MACA.