On the first pass of a round trip down the length of the tape and back, 32 tracks at LTO Ultrium  8 format are concurrently read or written. See the Read/write channels column in Table 2-18  on page 137. At the end of the tape, the second pass of the round trip starts. The read/write  heads are indexed and positioned over 32 new tracks, and the tape reverses direction back  toward the beginning of the tape to complete the round trip. For the next round trip, the heads  again are indexed to a new position over a new group of 32 tracks. Because track densities (as shown in Table 2-18 on page 137) are high and because the tape  is subject to lateral movement as it is moved, for performance and data integrity, the  read/write heads must always be positioned precisely over the correct tracks. This positioning  is accomplished by using the  timing-based servo  technique. This technique makes it possible  to use high track densities (now and in the future) without changing the format of the media.  This technique allows data with media imperfections to be read. In the LTO system, electronic signals are generated through the real-time reading of servo  data bands that are pre-recorded on the LTO tape. These signals enable the servo system to  dynamically control the positioning of the read/write heads across the width of the tape.  Similar magnetically based, track-following servo systems are used successfully in tens of  thousands of tape drives that are in use today, such as the IBM TS1100 tape drive (IBM  3592). The LTO formats also use advanced error correction codes for data integrity. These systems  automatically correct most cross-track errors and provide data correction even if a full track is  lost. Data is further protected by the demarcation of bad areas of the tape (for example,  where servo signals are unreliable) and through dynamically rewriting bad blocks.  Cartridge memory is embedded in the LTO cartridges to record usage and error information.  A noncontacting radio frequency (RFI) module, with nonvolatile memory capacity of 16,320  bytes for Ultrium 8, 7 and 6, and 8192 bytes for Ultrium 5 and 4, provides storage and  retrieval. Note:  The TS4500 supports three LTO tape drives, the HD2-compatible models of the  LTO-8, LTO-7, LTO-6, and LTO-5. The LTO drive uses an interleaved, serpentine, longitudinal recording format that is similar to  the method that is used in 3592 drives. The first set of 16 or 32 data tracks is written from  near the physical beginning of the tape to near the physical end of the tape. The head then  repositions to the next set of tracks for the return. This process continues until all tracks are  written and the tape is full. Servo tracks  or  bands  enable the accurate positioning of the tape drive head over the data  track, which ensures that the head does not stray onto an adjacent track. They are necessary  to support high-data densities on the tape where the tracks are extremely close together. The  servo bands are written when the cartridge is manufactured before the cartridge is usable for  data storage and retrieval. If the servo bands are erased, the tape becomes unusable. The format of the recording of the data and servo tracks is defined as part of the LTO  specification to meet the requirement for interchange among implementations of different  manufacturers. 138  IBM TS4500 R6 Tape Library Guide Servo tracks Interleaved recording