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Micro House PC Hardware Library Volume II: Network Interface Cards And Modems Micro House PC Hardware Library Volume II: Network Interface Cards And Modems
by Micro House International, Inc. and Scott Mueller
Que, Macmillan Computer Publishing
ISBN: 078971664x   Pub Date: 06/17/98
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Token-Ring Adapters

Except for fiber optic and some of the new high-speed technologies, Token Ring is the most expensive type of LAN. Token Ring can use STP or UTP cable. Token Ring’s cost is justified, however, when you have a great deal of traffic generated by workstations because under normal conditions, collisions are all but eliminated. You often find Token Ring in large corporations with large LANs, especially if the LANs are attached to mainframe computers. Token Ring can operate at 4 or 16Mbps.

Workstations on a Token-Ring LAN continuously pass an electronic token among themselves. The token is just a short message indicating that the workstation or server possessing it is allowed to transmit. If a workstation has nothing to send, as soon as it receives the token, it passes it on to the next downstream workstation. Only when a workstation receives the token can it transmit data onto the LAN. After transmitting, the token is again passed down the line. If the LAN is busy, and you want your workstation to send a message to another workstation or server, you must wait patiently for the token to come around. Only then can your workstation send its message. The message circulates through all the workstations and file servers on the LAN, and eventually winds its way back to you, the sender. The sender then generates a new token, releasing control of the network to the next workstation. During the circulation of the message around the ring, the workstations or server that is the designated recipient recognizes that the message is addressed to it and begins processing that message, but still passes it on to the next workstation.

Token Ring is not as wasteful of LAN resources as this description makes it sound. An unclaimed token takes almost no time at all to circulate through a LAN, even with 100 or 200 workstations. It is also possible to assign priorities to certain workstations and file servers so that they get more frequent access to the LAN. And, of course, the token-passing scheme is much more tolerant of high traffic levels on the LAN than the collision-prone Ethernet.

Early Token Release

On a momentarily idle Token-Ring LAN, workstations circulate a token. The LAN becomes busy (carries information) when a workstation receives a token and turns it into a data frame targeted at another computer on the network. After receipt by the target node, the data frame continues circulating around the LAN until it is returned to its source node. The source node turns the data frame back into a token that circulates until a downstream node needs it. So far, so good—these are just standard Token Ring concepts.

When a workstation sends a file request to a server, it consists of only a few bytes, far fewer than the transmission that actually returns the file to the workstation. If the request packet must go into and out of many workstations to circulate the ring, and if the data frame is small, latency occurs. Latency is the unproductive delay that occurs while the source node waits for its upstream neighbor to return its data frame.

During the latency period, the source node appends idle characters onto the LAN following the data frame until the frame circulates the entire LAN and arrives back at the source node. The typical latency period of a 4Mbps ring will result in the transmission of about 50 to 100 idle characters. On a 16Mbps ring, latency may reach 400 or more bytes worth of LAN time.

Early Token Release, available only on 16Mbps networks, is a feature that allows the originating workstation to transmit a new token immediately after sending its data frame. Downstream nodes pass along the data frame and then receive an opportunity to transmit data themselves—the new token. If you were to perform a protocol analysis of a network using Early Token Release, you would see tokens and other data frames immediately following the file request, instead of a long trail of idle characters.

Sometimes a station fumbles and “drops” the token. LAN stations monitor each other and use a complex procedure called beaconing to detect the location of the problem and regenerate a lost token. Token Ring is quite a bit more complicated than Ethernet, and the hardware is correspondingly more expensive.

ARCnet and Token Ring are not compatible with one another, but ARCnet uses a similar token-passing scheme to control workstation and server access to the LAN.

Adapter Functions

As mentioned in the “Network Interface Card (NIC)” section earlier, network adapters generally are collision-sensing or token-passing. A network adapter’s design ties it to one of the low-level protocols—Ethernet, Token Ring, FDDI, ARCnet, or some other protocol.

Collision-sensing and token-passing adapters contain sufficient on-board logic to know when it is permissible to send a frame and to recognize frames intended for the adapters. With the adapter support software, both types of cards perform seven major steps during the process of sending or receiving a frame. When sending data out from the card, the steps are performed in the order presented in the following list. When receiving data in however, the steps are reversed. Here are the steps:

1.  Data transfer. Data is transferred from PC memory (RAM) to the adapter card or from the adapter card to PC memory via DMA, shared memory, or programmed I/O.
2.  Buffering. While being processed by the network adapter card, data is held in a buffer. The buffer gives the card access to an entire frame at once, and the buffer enables the card to manage the difference between the data rate of the network and the rate at which the PC can process data.
3.  Frame formation. The network adapter has to break up the data into manageable chunks (or, on reception, reassemble it). On an Ethernet network, these chunks are about 1,500 bytes. Token-Ring networks generally use a frame size of about 4K. The adapter prefixes the data packet with a frame header and appends a frame trailer to it. The header and trailer are the Physical layer’s envelope. At this point, a complete, ready-for-transmission frame exists. (Inbound, on reception, the adapter removes the header and trailer at this stage.)
4.  Cable access. In a CSMA/CD network such as Ethernet, the network adapter ensures that the line is quiet before sending its data (or retransmits its data if a collision occurs). In a token-passing network, the adapter waits until it gets a token it can claim. (These steps are not significant to receiving a message, of course.)
5.  Parallel/serial conversion. The bytes of data in the buffer are sent or received through the cables in serial fashion, with one bit following the next. The adapter card does this conversion in the split second before transmission (or after reception).
6.  Encoding/decoding. The electrical signals that represent the data being sent or received are formed. Ethernet adapters use a technique called Manchester encoding, while Token Ring adapters use a slightly different scheme called Differential Manchester. These techniques have the advantage of incorporating timing information into the data through the use of bit periods. Instead of representing a 0 as the absence of electricity and a 1 as its presence, the 0s and 1s are represented by changes in polarity as they occur in relation to very small time periods.
7.  Sending/receiving impulses. The electrically encoded impulses making up the data (frame) are amplified and sent through the wire. (On reception, the impulses are handed up to the decoding step.)


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