Multiplex communications – Communication techniques for information carried in plural... – Combining or distributing information via time channels
Reexamination Certificate
1998-06-24
2002-07-30
Ton, Dang (Department: 2734)
Multiplex communications
Communication techniques for information carried in plural...
Combining or distributing information via time channels
Reexamination Certificate
active
06426962
ABSTRACT:
FIELD OF THE INVENTION
Present invention relates to local area network (LAN) testers in particular, and in particular to software to detect phase jitter intolerant stations on a token ring local area network. BACKGROUND OF THE INVENTION
In general, most computer networks have the goal of moving data among the computers connected to the network. Each network must have some mechanism for regulating access to the transmission of this data. Several approaches exist to regulate access to the transmission of data in local area networks. These include contention, token passing, demand priority, and direct switching. The present invention relates to token passing networks, and more specifically, token ring networks as defined by the ISO/IEC 8802-5 token ring standard.
A token ring network uses a logical ring topology, shown in
FIG. 1
, that is physically wired as a star with workstations connected to a wiring concentrator. Wiring concentrators can be cabled together to expand the capacity of the LAN.
FIG. 2
illustrates the wiring of workstations to each wiring concentrator. Each workstation uses a twisted pair connection for input and a second twisted pair for output with the data flowing from station to station in one direction to implement a logical ring. The wiring concentrator also controls access of each work station to the LAN.
Token ring adapter cards or Network Interface Cards (NICs) are installed on each workstation to allow it to be connected to the wiring concentrator. Each NIC has several capabilities. First, it has input and output buffers which allow data files of various lengths communicated from the workstation to be stored and then serialized before being clocked onto the network. Second, each NIC is also equipped with a phase lock loop circuit which is capable of extracting a clock signal which is transmitted with the data. Small variations in the clock rate (or phase) may occur as data is transmitted between workstations. If the rate of clock phase change is excessive, the phase lock loop circuits in a typical NIC cannot recover the clock signal and will generate an error. A third capability of the NIC is to report clock signal and other errors automatically by generating Error Report MAC frames which are transmitted onto the network.
ISO/IEC 8802-5 networks may operate at 4 or 16 megabits per second (Mbps). Each station on the network must operate at the same speed.
A “token” is a special data “frame” which is passed from workstation to workstation in sequence along the ring. The token frame is only a few bytes in length and contains a bit pattern which can be changed into a start of frame delimiter (SOF) by changing a series of bits into a start sequence. Immediately following the SOF is a sequence of source and destination address pairs that identify the sending and transmitting workstations, respectively.
The token is recognized by all workstations as being the arbiter of access for the transmission of the data. A workstation must have the token to place data frames on the network. If it does not have the token it must wait until it gets the token from its upstream neighbor in order to transmit data. If the token is passed to a workstation that needs to transmit data, the workstation converts the token to an SOF, appends the source and destination address. and then adds data to be transmitted. If the token is passed to a workstation that doesn't need to transmit data, the workstation sends the token to the next downstream workstation.
The SOF and the data frame travel around the ring from workstation to workstation. Each workstation reads the start sequence and the destination address. If the workstation's address does not match the destination address, the workstation merely retransmits it on downstream. If the workstation address matches the destination address, the workstation copies the frame into memory and modifies a bit pattern to signify it copied the transmitted data. If a data frame is transmitted in broadcast mode, each workstation receives and copies the frame without regard to the destination address. The frame travels around the ring until it eventually returns to the sending workstation. Latency is a measure of the delay it takes one byte to travel completely around the network ring. The sending workstation then strips the data frame from the ring and transmits a token in its place.
The ISO/IEC 8802-5 Token Ring data frame structure is the standard data frame structure that has been adopted by most token ring topologies. The minimum length of a token ring data frame is 21 bytes. The maximum data frame size under this standard is determined by the ring's signaling speed. A frame that is just large enough to fill the ring at any given time can be called the latency limit of the network.
FIG. 3
shows a diagram of the ISO/IEC 8802-5 data frame structure. The first field is that Starting Delimiter, which identifies the beginning of the frame. Next is the Access Control field. This field stores the “type” bits that identify the transport protocol. This field is also used to differentiate between data frames and control frames. The first two bits identify the type of frame: data or Media Access Control (MAC) management. MAC frames are used to collect network performance data that can be analyzed by standards-compliant network management products. Each MAC frame performs a specific network management function, such as a claim token used to select an active monitor for the network. The next six bits are used to inform the recipient of the priority of the data Protocol Data Unit (PDU) and/or MAC PDU.
The next two fields are the destination and source MAC addresses. Each one is a 6-byte field. The data field follows. It varies in size, based upon the signaling speed of the ring. 4Mbps Token Rings can support data fields of maximum size 4,332 bytes. 16 Mbps Token Rings can support data field that are of maximum size 17,832.
The last three fields in the data frame are the 2 byte Frame Check Sequence (FCS) and a 1 byte ending delimiter. The FCS contains a mathematically derived value that is calculated based upon the length and content of the frame. The recipient and transmitter each apply the same algorithm to the frame. If the recipient computes the same value stored in the FCS field (which was calculated by the transmitter), then it assumes that the contents were transmitted correctly.
The ISO/IEC 8802-5 Token Ring standard establishes several Network Management Agents (NMAs). These agents may reside at each Token Ring workstation and are used in the normal management operation of the ring. These agents include active or standby monitors (AM or SM), ring error monitor (REM), Configuration Report Server (CRS) and Ring Parameter Server (RPS).
Token Ring network protocol is monitored by the Active Monitor (AM). The AM theoretically can be any station in the ring. Usually, it is the first station to become active but, after more stations activate, the responsibility can be contested and assumed by another station. The AM monitors all traffic, ensures that the network's protocol rules are adhered to and initiates action that may be necessary to overcome protocol violations or failures. All the network workstations have the capability to be the AM, but only one can be the AM at a time. The other stations are known as Standby Monitors (SM).
The AM's duties includes the following: 1) initializing the ring by issuing a Ring Purge MAC Frame upon start-up, 2) creating tokens, 3) providing the master clock for the network and 4) ensuring that tokens and frames do not traverse the ring more than once.
The ISO/IEC 8802-5 standard provides for an AM contention process called token claiming. This process is initiated by any of the SMs whenever they detect a possible failure of the AM. Numerous possible symptoms can trigger this process. They include the failure of the AM to transmit a good token frame at least once every 2.6 seconds, the failure of the AM to transmit an AM Present MAC Frame at least once every 15 seconds, unsuccessful ring purge
Cabezas Rafael Graniello
Gregory Randy Lynn
Ross Tamleigh Jonathan
De Frank Edmond A.
Emile Volel
International Business Machines - Corporation
Ton Dang
LandOfFree
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