Asymmetry in Wireless Networks: Definition & Potential Issues

Instructor: Vignesh Sivabalan
This lesson explores asymmetry in computer networks. You'll learn how asymmetry affects network operation and find out how to recognize and test for asymmetries.

What Is an Asymmetric Network?

In computer networks, asymmetry refers to varying data quantity or data speed highly in one direction when compared to another direction in an average period of time. In the case of symmetric network communications, the data speed varies similarly in both the directions over an average time period.

A Practical Example for Asymmetric Networks

In Internet connections, data from the servers to end user systems flow through high-speed broadband links (downstream), whereas data from end user systems to servers are sent through low-speed twisted-pair cables. The downstream data can be more than 1 Mbps, and the upstream data can be less than 54 Kbps. This is a good example of an asymmetric communication. The downstream data consists of large multimedia, graphics, HTML files, and sound. The upstream data from end users are of fewer bytes. Therefore, the asymmetric communication improves the performance of the internet.

Asymmetry in Wireless Networks

Asymmetric wireless networks send and receive data through different nodes with varying parameters. The asymmetry in wireless networks can be of various types like latency, packet error rate and bandwidth.

Bandwidth Asymmetry

The bandwidth in the forward direction (from server to host) is greater than the backward direction (from host to server). The bandwidth asymmetry in wireless networks is the root cause of other kinds of asymmetries like latency.

Packet Error Rates Asymmetry

In wireless networks, the packet error rates are higher in the wireless parts than the wired parts of the network, resulting in packet error rate asymmetry.

Routing Asymmetry

Routing asymmetry occurs when upstream packets take different routes compared to downstream packets.

Media Access Symmetry

In mobile wireless networks, the centralized base station faces lower MAC overhead in distributing data to mobile hosts than data transfer from mobile hosts to base stations.

Symmetric vs. Asymmetric Wireless Networking

While the symmetric network offers equal resource access to all the networking devices, the asymmetric network provides unequal access of resources. Though encryption schemes are used in both symmetric and asymmetric networks for confidentiality, symmetric networks use a common key for both encryption and decryption, whereas asymmetric networks use different keys at both ends (sender and receiver).

The encryption key used by the asymmetric network at the sender end is different from that of the decryption key used at the receiver end. There exists a naive assumption among network users that the internet traffic is always symmetric, but that is not true. Internet traffic is symmetric only at network edges; otherwise it is asymmetric.

Major causes of asymmetry are hot-potato routing and link redundancy. Hot-potato routing is implemented in settlement-free peer networks, where the receiving side of the network bears a much higher cost per packet (received packet) than the sending end. The major advantage of settlement-free peering network is to provide an even sharing of traffic in the network. Link redundancy means choosing alternative paths during dynamic routing. The load balancing algorithms along with routing algorithms play a vital role in forwarding packets at run time. This stimulates asymmetry as each route handles asymmetric data load, but alternative paths increase network performance.

Advantages of Asymmetric Wireless Networks

With increasing streaming of television shows and online movies, home routers need to download larger volumes of data like multimedia videos for a family than the number of videos they tend to upload online. These home routers must handle data discrepancy between downloaded and uploaded information. In such cases, the satellite or cable company itself provides higher data download rates than the upload speeds.

For example, DSL (Digital Subscriber Line) is helpful in both symmetric and asymmetric communications. ADSL (Asymmetric DSL) is used for asymmetric communications and offers higher download bandwidth than upload bandwidth.

How Asymmetry Affects Network Operations

  • Network asymmetry affects the performance of transport protocols like TCP. This is because even if the network traffic in one direction is free of congestion, the data flow from opposite direction can make network congestion and decrease the performance of the network.
  • In packet radio networks, the MAC asymmetry causes overhead to switch constantly the direction of transmission.
  • Network loss may be too high in a particular direction than its opposite direction.

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