Modeling Wireless Links for Transport Protoocols

New transport protocol should be designed for good performance over both fixed and wireless links. However, Variable bandwidth, corruption, channel allocation delay and asymmetry of wireless links can affect negatively on transport protocol performance. This paper presents a model for cellular, WLAN and satellite links wireless links that help to minimize these impacts on transport protocols. It considers single or multiple Wireless links located at one end or in the middle of the communication path

The goal is to optimized realistic (set up parameters that accurately portrays real world behavior), easy, exploring wide space of the parameters, and detailed traffic model to evaluate the effect of link-level mechanisms on transport protocol's performance for two ways transfers. Performance metric of wireless link is the goodput that reflects the energy and spectrum efficiencies, interference and cost.

The authors discussed some characteristics of wireless link, their effects on transport protocol and how they can be modeled, like

• Error loss can reduce the sending rate by triggering lengthy retransmission timeout. At the same time the bursty loss can increase the throughput than in case of equivalent number of non-bursty losses. Such impact can be modeled at the end of the link by dropping packets
• Abrupt increase in link delay can trigger spurious timeout, retransmission and reduce the sending rate. The delay can be modeled by deferring the data transmission on the link.
• Packet reordering due to link-level retransmission can incorrectly trigger congestion control responses for TCP while such delivery is helpful to decrease the delay for unreliable transport protocol. It can be modeled by swapping the packets in a queue, or delaying one packet at a time and letting other to pass
• Resource allocation delay translates to increased delay to the user and it can be modeled by introducing additional delay when a packet arrives to a queue that has been empty for a time longer than the channel hold time
• Decrease in link bandwidth results in triggering spurious timeouts in TCP while increase in the bandwidth could result in inefficient utilization of link bandwidth. This oscillation also leads to packet reordering. It can be modeled by changing link bandwidth
• Asymmetric bandwidth and latency in up/downlink directions of a wireless link could result in congestion and limit the throughput. It requires configuration different parameters for up/downlink's bandwidth and latency.
  
  The paper also discusses
  1) the buffering model and its management and size impacts on transport protocol. While large size of buffer leads to poor loss recovery, small buffer size results in low link utilization and packet drop. Drop-Tail queue management can lead to bursty loss of packet followed by triggering retransmission timeout.
  2) Mobility model and its effects, where the mobility can result in packet loss, introduce delay and can also lead to change in link bandwidth and latency.
  
  Finally, the paper suggests that cross-layer communication (between link layer and transport layer) could be helpful to optimize the performance over wireless link. It could be investigated for transport protocol to inform the link layer that it is tolerant to packet reordering, or the degree of delay or bit errors –tolerance. It can also be investigated for link-layers to inform transport end-nodes of bandwidth or delay changes from handovers