Using a Logical Token

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If you’re planning on taking the Network+ exam, you should have a basic understanding of several common network topologies such as a topology that uses a logical token.

For example, can you answer this question?

Q. Computers within a network are only able to transfer data when they have access to a logical token. What type of topology does this describe?

A. Star

B. Ring

C. Bus

D. MPLS

More, do you know why the correct answer is correct and the incorrect answers are incorrect? The answer and explanation is available at the end of this post.

Token Ring Topologies

A ring topology has devices logically connected in a circle or ring. Data travels around the ring in a single direction through each device. Ring topologies use a logical token and are often referred to as token ring topologies. A token is special three-byte data unit passed around the ring. Devices only transmit data when they have the token.

You may have heard of talking sticks used by some Native American cultures. A group of people sits in a circle and they pass a stick around the circle to each other. Members of the group can only talk when they have the stick, and they listen when they don’t have the stick. As long as the group doesn’t get too large, this is an effective method of ensuring that everyone’s voice is heard. The three-byte token is similar to an electronic file, but it works just like the talking stick.

Ring topologies have two significant weaknesses: scalability and fault tolerance. Traditional token ring topologies have been replaced with newer technologies that don’t have these weakness, but that doesn’t stop CompTIA from listing them on the objectives.

First, ring topologies do not scale well. Performance degrades with each computer added to the ring. With five computers on the network, each computer has the token about 20 percent of the time (1/5). If you increase the number of computers on the network to 100, each computer only has the token about 1 percent of the time (1/100).

Second, a single break in the circle takes down the entire network. The break can be in the ring itself or a failure from any single device in the ring. Ring topologies commonly add a multistation access unit (MAU) to compensate for this. The devices are still connected in a logical circle and data travels one way around the circle. However, each device connects to the MAU instead of adjoining devices. Devices then transmit and receive data to and from the MAU. If one of the devices in the ring fails, the MAU senses the failure logically removes it from the ring.


Learn more about  several common network topologies.

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The following figure shows the logical layout of a token ring network topology. On the right, you can see how devices connect to each other via central MAU. The ring with the MAU looks similar to a star topology but the difference is that data travels in a circle around the ring. In a star network, data can travel to and from any device without regard to any direction.

Token

Ring topology

Some fiber optic networks use Fiber Distributed Data Interface (FDDI). These are similar to token ring networks with one significant difference. Instead of the network using a single closed circle network for data transmissions, the FDDI network has two rings. Each ring transmits data in the opposite direction providing redundancy for the network.

Synchronous Optical Networking (SONET) is a group of protocols using fiber optic cables, and some SONET implementations use ring topologies.

Remember This

A ring network connects devices in a circle and transmits a token one way around the circle. Devices in the ring only transmit data when they have the token. FDDI networks use two rings and each ring transmits data in the opposite direction providing redundancy for the network.


 

Q. Computers within a network are only able to transfer data when they have access to a logical token. What type of topology does this describe?

A. Star

B. Ring

C. Bus

D. MPLS

Answer: B is correct. Ring topologies (often called token ring) use a logical token and devices can only communicate when they have access to the token. A ring topology is configured in a circle or ring.

Star topologies use a central device such as an Ethernet switch or a hub, which do not use tokens.

Bus topologies do not use tokens. They are rarely used on networks because a failure in one connection results in a failure for all devices on the bus.

Multiprotocol Label Switching (MPLS) does not use tokens. MPLS is a wide area network (WAN) protocol that uses labels to identify addressing paths.

Practice Test Questions To Help You Pass the Network+ Exam (N10-006) The First Time You Take It.

 

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