Baltimore County (P25) System Information

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System Information

Linear Simulcast Modulation

Baltimore County's new Motorola P25 7.x system employs an RF simulcast method known as Linear Simulcast Modulation. This technology supposedly enhances a subscriber radio's ability to overcome digital simulcast multipath distortion by shaping the digital packet waveforms using CQPSK (Compatible Quadrature Phase Shifting Key) modulation. Because of this, if the digital packets arrive at a subscriber radio from multiple transmitters out of sync, the radio is able to organize them and decode them without distortion.

Because scanners are not readily prepared to capture digital packets with these modified shapings, scanners have trouble decoding the packets. This can hold true not only for voice signals, but also control channel signals. This is why you may be in a solid coverage footprint area, but not receiving the system signals in a clear manner.

Baltimore County Fire Department talk group assignments

The Baltimore County Fire Department is divided into three battalions. The first battalion encompass the central and northern portions of the county, and is bordered to the west by Falls Road, and to the east by Harford Road. The second battalion is the west battalion and is all points west of Falls Road. The third battalion is the east battalion and is all points east of Harford Road. Each battalion is assigned two complete sets of fire ground talk groups, allowing each incident to have a primary operations talk group, an incident command talk group, seven expansion talk groups, and an announcement call talk group. As an incident with a battalion chief is dispatched, the operations for that incident are assigned to the TAC x2 talk group for that battalion. If the incident escalates and the need for multiple tactical and IMS talk groups are needed, "command net" is instituted and the incident commander will use the TAC x1 talk group, while the incident functions can expand to the TAC x3 thru TAC x9 talk groups. Typically, operations will remain on the TAC x2 talk group, eliminating the need for the people "in the trenches" to change their radio. The announcement call talk group (AC x0) allows commanders to use regroup logic to make transmissions over all the TAC talk groups assigned to that set.

Here is a sample for the first battalion's first set of fire ground talk groups. The Bxx is the position of the talk group in the portable radio. For example, the first talk group is located in ZONE B, and knob position 12.

  • 9460 B12 AC 10 Announcement Call for TAC 11 thru TAC 19
  • 9461 B11 TAC 11 Command NET talk group
  • 9462 B3 TAC 12 Primary operations talk group
  • 9463 B4 TAC 13 First expansion talk group
  • 9464 B5 TAC 14 Second expansion talk group
  • 9465 B6 TAC 15 Third expansion talk group
  • 9466 B7 TAC 16 Fourth expansion talk group
  • 9467 B8 TAC 17 Fifth expansion talk group
  • 9468 B9 TAC 18 Sixth expansion talk group (typically used for EMS)
  • 9469 B10 TAC 19 Ninth expansion talk group (typically used for HAZMAT)
  • The first battalion is assigned AC 10 thru TAC 19 and AC 60 thru TAC 69.
  • The second battalion is assigned AC 20 thru TAC 29 and AC 50 thru TAC 59.
  • The third battalion is assigned AC 30 thru TAC 39 and AC 40 thru TAC 49. These are non-wide-area talk groups that will only work on the south system.
  • There are two sets of spares, AC 70 thru TAC 79 and AC 80 thru TAC 89.
  • There are two sets of non-wide-area talk groups for the north system, AC 110 thru TAC 119 and AC 120 thru TAC 129. These are currently not regularly assigned, and are reportedly for site trunking and failsoft conditions.

Other Talk groups

  • 9455 A3 ADO is Administrative Duty Officer
  • 9456 A8 ADM is Admin
  • 9457 A4 SUP is Supply and Maintenance
  • 9458 A2 EMS is EMS Supervisor
  • 9459 A5 DFM is the Deputy Fire Marshal


Map of towers

The UPGRADED Baltimore County system is a two-zone system. The reason for this is because you can only effectively simulcast from a certain number of towers before timing sync is degraded.

Your scanner (and the RRDB) refer to these zones as "sites." While typically a "site" is referring to a single tower site/transceiver location, in this case, a "site" is a group of tower sites all transmitting the same EXACT signal (known as simulcasting). With the old system, all 9 tower sites always transmitted the same exact signal -- it was a total simulcast --- therefore, scanners and radios only see the system as one "site."

With the upgraded system, there will be two "sites" or "zones" to cover the county, because, with the addition of other tower site locations, there are too many towers to have a total simulcast. The reason is very technical and beyond the scope of this post.

The main "zone" or "site" covers the majority of the county. The tower sites simulcasting the signal of this zone are as follows:

  1. Towson (Fire Station #1)
  2. Woodlawn (county highways shop on Windsor Mill Rd)
  3. Hunt Valley (on Warren Rd east of I83)
  4. Cub Hill (on the grounds of Hickey School for Boys)
  5. Kingsville (Belair Rd at Harford County line)
  6. Allender (Eastern Sanitary Landfill on Days Cove Rd)
  7. Essex (rooftop site at CCBC Essex mid-rise building)
  8. Northpoint (Precinct 12)
  9. Sparrows Point (Fire Station #57)
  10. Halethorpe (county highways shop on Brady Ave near Washington Bl and I695)
  11. Catonsville (northwest corner of Rt. 40 & Rolling Rd)
  12. Red Run (New Towne High School in Owings Mills)
  13. Arcadia (Arcadia Volunteer Fire Company carnival grounds)
  14. MEMA (Camp Fretterd military reservation in Reisterstown)
  15. Jacksonville (Jacksonville Elementary School)

The following sites will cover the extreme northern end of the county, comprising the second "site" or zone:

  1. Maryland Line (Rt. 439 just east of I83)
  2. Hereford (state highways yard on Mt. Carmel Rd just east of I83)
  3. Spookhill (at the city water facility on Prettyboy Reservoir)

It seems clear at this point that, because of the large footprint covered by the south site, it will carry most traffic on the system. It is unclear exactly how much of the traffic will be carried on the north site. It is likely that there will be a few talk groups "forced" onto the north site at all times, while most talk groups will only be carried on the north site based on registration/affiliation. What this means is that, as a radio enters the footprint of the north site, it will register with the site's controller, identifying the talk group with which is it affiliated, causing the site to begin carrying that talk group. If no radio is registered/affiliated with the north site for a specific talk group, that talk group will not be carried by the site. This is significant for scanner users because a scanner cannot register, therefore, some talk groups may be silent on this zone.

To add to the fun, there are a handful of talk groups that are ONLY for the north site. These talk groups appear to be reserved for a condition called "site trunking." Site trunking is a fallback function that occurs when a remote site (such as the north site) loses contact with the system controller, causing wide area calls (calls over multiple sites) to fail. If this were to occur, users on the "detached" site would revert to talk groups specific to that site, allowing users to continue to communicate within the footprint of the site. Dispatchers may also continue to connect with the site via some sort of RF link or back-up microwave path.

Thanks to ocguard in this thread for this information