Exit Zen
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packet
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`C` sends its distance vector `[C=0]` to `B` and `E`
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The token is composed of three fields. First, the `Starting Delimiter` is the marker that indicates the beginning of a frame. The first Token Ring networks used Manchester coding and the `Starting Delimiter` contained both symbols representing `0` and symbols that do not represent bits. The last field is the `Ending Delimiter` which marks the end of the token. The `Access Control` field is present in all frames, and contains several flags. The most important is the `Token` bit that is set in token frames and reset in other frames.
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a packet is the unit of information transfer in the network layer
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`E` sends its distance vector `[E=0,D=1,A=2,C=1]` to `D`, `B` and `C`. `B` can now reach `A`, `C`, `D` and `E`
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Let us consider the five station network depicted in figure :ref:`fig-tokenring` above and assume that station `S1` sends a token. If we neglect the propagation delay on the inter-station links, as each station introduces a one bit delay, the first bit of the frame would return to `S1` while it sends the fifth bit of the token. If station `S1` is powered off at that time, only the first five bits of the token will travel on the ring. To avoid this problem, there is a special station called the `Monitor` on each Token Ring. To ensure that the token can travel forever on the ring, this `Monitor` inserts a delay that is equal to at least 24 bit transmission times. If station `S3` was the `Monitor` in figure :ref:`fig-tokenring`, `S1` would have been able to transmit the entire token before receiving the first bit of the token from its upstream neighbor.
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PBL
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`B` sends its distance vector `[B=0,A=1,C=1,D=2,E=1]` to `A`, `C` and `E`. `A`, `B`, `C` and `E` can now reach all five routers of this network.
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011
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Using sequence numbers to count bytes has also one advantage when the transport layer needs to fragment SDUs in several segments. The figure below shows the fragmentation of a large SDU in two segments. Upon reception of the segments, the receiver will use the sequence numbers to correctly reorder the data.
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Problem-based learning is a teaching approach that relies on problems.
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`A` sends its distance vector `[A=0,B=1,C=2,D=1,E=2]` to `B` and `D`.
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The parity bit allows a receiver to detect transmission errors that have affected a single bit among the transmitted `N+r` bits. If there are two or more bits in error, the receiver may not necessarily be able to detect the transmission error. More powerful error detection schemes have been defined. The Cyclical Redundancy Checks (CRC) are widely used in datalink layer protocols. An N-bits CRC can detect all transmission errors affecting a burst of less than N bits in the transmitted frame and all transmission errors that affect an odd number of bits. Additional details about CRCs may be found in [Williams1993]_.
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802.5 data frame format
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POP
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At this point, all routers can reach all other routers in the network thanks to the routing tables shown in the figure below.
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It is also possible to design a code that allows the receiver to correct transmission errors. The simplest `error correction code` is the triple modular redundancy (TMR). To transmit a bit set to `1` (resp. `0`), the sender transmits `111` (resp. `000`). When there are no transmission errors, the receiver can decode `111` as `1`. If transmission errors have affected a single bit, the receiver performs majority voting as shown in the table below. This scheme allows the receiver to correct all transmission errors that affect a single bit.
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To capture a token, a station must operate in `Listen` mode. In this mode, the station receives bits from its upstream neighbor. If the bits correspond to a data frame, they must be forwarded to the downstream neighbor. If they correspond to a token, the station can capture it and transmit its data frame. Both the data frame and the token are encoded as a bit string beginning with the `Starting Delimiter` followed by the `Access Control` field. When the station receives the first bit of a `Starting Delimiter`, it cannot know whether this is a data frame or a token and must forward the entire delimiter to its downstream neighbor. It is only when it receives the fourth bit of the `Access Control` field (i.e. the `Token` bit) that the station knows whether the frame is a data frame or a token. If the `Token` bit is reset, it indicates a data frame and the remaining bits of the data frame must be forwarded to the downstream station. Otherwise (`Token` bit is set), this is a token and the station can capture it by resetting the bit that is currently in its buffer. Thanks to this modification, the beginning of the token is now the beginning of a data frame and the station can switch to `Transmit` mode and send its data frame starting at the fifth bit of the `Access Control` field. Thus, the one-bit delay introduced by each Token Ring station plays a key role in enabling the stations to efficiently capture the token.
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