Handbook of Local Area Networks, 1998 Edition:LAN Basics Click Here! Search the site:   ITLibrary ITKnowledge EXPERT SEARCH Programming Languages Databases Security Web Services Network Services Middleware Components Operating Systems User Interfaces Groupware & Collaboration Content Management Productivity Applications Hardware Fun & Games EarthWeb sites Crossnodes Datamation Developer.com DICE EarthWeb.com EarthWeb Direct ERP Hub Gamelan GoCertify.com HTMLGoodies Intranet Journal IT Knowledge IT Library JavaGoodies JARS JavaScripts.com open source IT RoadCoders Y2K Info Previous Table of Contents Next Token and Frame Format The 802.5 standard specifies two data formats, token and frames. The token, which is three octets long, is the means by which the right to use the medium is passed from one station to another. The frame carries the message between stations. Token and frame structures are shown in Exhibit 1-1-26. The starting delimiter must contain the following bit pattern in order for a frame or token to be valid: JK0JK000, where J is nondata J, K is nondata K, and 0 is binary zero. Exhibit 1-1-26.  Frame Format for IEEE 802.5 Standard The access control assigns special meaning to the token. Its bi pattern is PPPTMRRR, where PPP are priority bits, T is the token bit, M is the monitor bit, and RRR are reservation bits. The priority bits, PPP, indicate the priority of the token. These bits identify eight priority levels, from 000 (the lowest) through 111 (the highest). In a multipriority system, stations use different priorities based on the priority of the protocol data unit to be transmitted. The token bit, T, setting distinguishes a token from a frame. (T set to 1 is a frame; T set to 0 is a token.) The monitor bit, M, is used to prevent a frame or token from continuously circulating on the ring and is normally set to 0 for all tokens or frames. Only the monitoring station can modify this bit; all other stations merely repeat it. The reservation bits, RRR, allow stations with higher-priority protocol data units to request that the next token be issued at a given priority. As with the priority bits, eight levels of reservations are supported, from 000 (lowest) through 111 (highest). The frame control field defines the type of the frame and includes certain media access control and information frame functions. The bits are FFZZZZZZ, where FF are frame-type bits and ZZZZZZ are control bits. The frame-type bits identify the type of frames as: •  00: A media access control frame (contains a media access control protocol data unit). •  01: A logical link control frame (contains a logical link control protocol data unit). •  1X: Undefined format. When the frame-type bits are 00 (i.e., the frame contains a media access control protocol data unit), the control bits ZZZZZZ are interpreted and acted on by all stations on the ring. In a logical link control frame (FF bits 01), the control bits are designated rrrYYY, where rrr has the value 000 during transmitted frames and is ignored on reception and YYY indicates the priority of the protocol data unit from the source logical link control entity to the destination logical link control entity or entities. The end delimiter byte marks the end of transmission. The format is JK1JK11E, where J is nondata J, K is nondata K, 1 is binary 1, I is intermediate frame bit, and E is error-detection bit. The receiving station will interpret a valid ending delimiter if the first six symbols it receives are JK1JK1. The intermediate bit (I), if set to 1, indicates that this is a continuation frame of a multiple frame transmission. If I is set to 0, it is the last frame. The error-detection bit (E) is used as an indicator by the stations during receive and repeating mode. Each station checks the passing frame for errors and if an error is detected, sets E to 1. The frame status contains the address recognition and frame-copied bits. The bit pattern is ACrrACrr, where A is address-recognized bits, C is frame-copied bits, and r is received bits. The originating station transmits A and C as 0. The destination station sets A to 1 and if the receiving station is able to copy the frame then it sets C to 1. These bits help the originating station to identify the following three conditions after the transmission attempt: •  There is a nonexistent or nonactive station on the ring (A set to 0, C set to 0). •  The station exists but it failed to copy the frame. •  The frame was copied by the destination station. A frame’s information field may be empty or may contain one or more bytes of data. The values of this field are called vectors. The vector contains a length value, a function identifier, and zero or more subvectors as follows: VL V1 SVL SVI SVV . . . SVL SVI SVV Subvector 1 Subvector m The vector length (VL) is a 16-bit number that specifies the length of the vector in octets and includes its subfield. The vector length range falls between 4 and 65,535 octets. The vector identifier (VI) is a 16-bit field that identifies the vector itself. The valid VI codes (shown in hexadecimal form) and their meanings are: •  0002: Beacon. •  0003: Claim token. •  0004: Purge MAC frame. •  0005: Active monitor present. •  0006: Standby monitor present. •  0007: Duplicate address set. Subvectors contain all data or modifiers. One subvector is required to contain each piece of data or modifier that is transported. A subvector is not position dependent within a vector; rather, each subvector must be identified by its subvector identifier. The subvector length (SVL) is an 8-bit number that specifies the subvector length in octets, including the length of the field itself. A value of FF (hexadecimal) means that the length exceeds 254 octets and that the actual length value follows in the next two octets. The subvector identifier (SVI), is also 8 bits long. An SVI value of FF (hexadecimal) indicates an expanded identifier, which is included in the next two octets. Two types of subvectors exist. Subvectors having values between 00 and 7F (hexadecimal) designate specific, common, standardized strings of data. Subvector values (SVVs) from 80 through FE (hexadecimal) are used for specific definition within a particular vector by vector identifier. The subvector values, variable in length, contain the actual data the station needs to transmit. It is important to understand that subvectors themselves may contain other subvectors and other types of vectors. The frame check sequence is a 32-bit field calculated and appended to the frame as specified in the IEEE 802.3 and 802.4 standards. 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