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In 1985, the IBM Token Ring became an American National Standards Institute
(ANSI)/IEEE standard. (ANSI is an organization that was formed in 1918
for the development and adoption of trade and communication standards in
the United States; ANSI is the American representative of ISO.)
The architecture of a typical Token Ring network begins with a physical ring. However, in its IBM implementation, a star-wired ring, computers on the network are connected to a central hub. Figure 3.23 shows a logical ring and a physical star topology. The logical ring represents the token's path between computers. The actual physical ring of cable is in the hub. Users are part of a ring, but they connect to it through a hub.
Figure 3.23 Logical ring, in which the physical ring is in the hub
Token Ring Basics
A Token Ring network includes the following features:
Figure 3.24 Token Ring data frame
Table 3.9 Components of a Token Ring Frame
|Start delimiter||Indicates start of the frame|
|Access control||Indicates the frame's priority and whether it is a token or a data frame|
|Frame control||Contains either Media Access Control information for all computers or "end station" information for only one computer|
|Destination address||Indicates the address of the computer to receive the frame|
|Source address||Indicates the computer that sent the frame|
|Information, or data||Contains the data being sent|
|Frame check sequence||Contains CRC error-checking information|
|End delimiter||Indicates the end of the frame|
|Frame status||Tells whether the frame was recognized, copied, or whether the destination address was available|
After the computer captures the token, it sends a data frame (such as the one shown in Figure 3.25) out on the network. The frame proceeds around the ring until it reaches the computer with the address that matches the destination address in the frame. The destination computer copies the frame into its receive buffer and marks the frame in the frame status field to indicate that the information was received.
The frame continues around the ring until it arrives at the sending computer, where the transmission is acknowledged as successful. The sending computer then removes the frame from the ring and transmits a new token back on the ring.
Figure 3.25 Clockwise flow of the token around the logical ring
Only one token at a time can be active on the network, and the token can travel in only one direction around the ring.
Does the token flow clockwise or counterclockwise? The answer is that it doesn't really matter. The direction taken depends on hardware connections. Logically, you can make the token travel in any direction or order you wish. The designers of hubs determine the order in which each port is addressed, and you determine the order in which computers are connected to the hub. The IEEE 802.5 standard says clockwise, and the IBM publication SC30-3374, section 3, says counterclockwise.
Monitoring the System
The first computer to come online is assigned by the Token Ring system to monitor network activity. The monitoring computer makes sure that frames are being delivered and received correctly. It does this by checking for frames that have circulated the ring more than once and ensuring that only one token is on the network at a time.
The process of monitoring is called beaconing. The active monitor sends out a beacon announcement every seven seconds. The beacon is passed from computer to computer throughout the entire ring. If a station does not receive an expected announcement from its upstream neighbor, it attempts to notify the network of the lack of contact. It sends a message that includes its address, the address of the neighbor that did not announce, and the type of beacon. From this information, the ring attempts to diagnose the problem and make a repair without disrupting the entire network. If it is unable to complete the reconfiguration automatically, manual intervention is required.
Recognizing a Computer
When a new computer comes online on the network, the Token Ring system initializes it so that it can become part of the ring. This initialization includes:
Figure 3.26 Hub showing the internal ring and clockwise token path
An IBM MSAU has 10 connection ports. It can connect up to eight computers. However, a Token Ring network is not limited to one ring (hub). Each ring can have up to 33 hubs.
Each MSAU-based network can support as many as 72 computers that use unshielded wire or up to 260 computers that use shielded wire.
Other vendors offer hubs with more capacity; the capacity depends on the vendor and the hub model.
When one Token Ring is full—that is, when every port on an MSAU has a computer connected to it—adding another ring (MSAU) can enlarge the network.
The only rule that must be followed is that each MSAU must be connected in such a way so that it becomes part of the ring. Figure 3.27 shows 1, 2, and 3 MSAU connected and maintaining a logical ring. An MSAU's ring-in and ring-out connection points make use of patch cables to connect many MSAUs on top of each other while still forming a continuous ring inside the MSAUs.
Figure 3.27 Adding hubs while maintaining the logical ring
Built-in Fault Tolerance
In a pure token-passing network, a computer that fails stops the token from continuing. This in turn brings down the network. MSAUs were designed to detect when a NIC fails, and to disconnect from it. This procedure bypasses the failed computer so that the token can continue on.
In IBM's MSAUs, bad MSAU connections or computers are automatically
by-passed and disconnected from the ring. Therefore, a faulty computer
or connection will not affect the rest of the Token Ring network.
Each computer can be no more than 101 meters (330 feet) from an MSAU when connected with Type 1 cable. Each computer can be up to 100 meters (about 328 feet) from the MSAU when STP cabling is used, or 45 meters (about 148 feet) when UTP cabling is used. The minimum length for shielded or unshielded cable is 2.5 meters (about 8 feet).
Figure 3.28 Maximum hub to computer distances on Type 1, STP, and UTP cables
According to IBM, the maximum cabling distance from an MSAU to a computer or a file server is 46 meters (150 feet) when Type 3 cabling is used. Some vendors, however, claim that data transmission can be reliable for up to 152 meters (500 feet) between an MSAU and a computer.
The maximum distance from one MSAU to another is limited to 152 meters (500 feet). Each single Token Ring can accommodate only 260 computers with STP cable and 72 computers with UTP cable.
Patch cables extend the connection between a computer and an MSAU. They can also join two MSAUs together. In the IBM cabling system, these are Type 6 cables and can be any length up to 46 meters (150 feet). Patch cable will allow only 46 meters between a computer and an MSAU.
The IBM cabling system also specifies a Type 6 patch cable for:
Token Ring networks usually join cables to components with the following types of connectors:
Implementing Token Ring cards requires caution because a Token Ring network will run at only one of two possible speeds: 4 Mbps or 16 Mbps. If the network is a 4-Mbps network, the 16-Mbps cards can be used because they will revert back to 4-Mbps mode. A 16-Mbps network, however, will not accept the slower 4-Mbps cards because they cannot increase speed.
Although several manufacturers make Token Ring NICs and other Token
Ring components, IBM currently sells the majority of them.
Table 3.10 Token Ring Specifications
|IEEE specification||Token Ring|
|Cable type||Shielded or unshielded twisted-pair cable|
|Terminator resistance, ? (ohms)||Not applicable|
|Impedance, ?||100_120 UTP, 150 STP|
|Maximum cable segment length||From 45 to 200 meters (about 148 to 656 feet), depends on cable type|
|Minimum length between computers||2.5 meters (about 8 feet)|
|Maximum connected segments||33 multistation access units (MSAUs)|
|Maximum computers per segment||Unshielded: 72 computers per hub; Shielded: 260 computers per hub|