The fundamental principle of the half rate function is that two logical half rate TCHs are multiplexed in a timeslot of a physical TDMA frame as two logical channels. The hardware provides a more advanced speech coding/decoding algorithm to make the speech QoS on a half rate channel close to that on a full rate channel.
The BSC deals with the same call signaling procedures as before after the half rate function is performed. However, the contents of some signaling messages may change because a different channel type is selected. For example, that BSC allocates a half rate TCH to MS may be reported to the MSC in an Assignment Complete or Handover Complete message. The BSC shall select a speech version (in the case of speech transmission) or data rate (in the case of data transmission) for the current call after allocating a TCH. Due to the FTC capability limitation, the BSC does not support half rate data services by default. The operator may enable the corresponding software parameter switch (see "Parameter") to enable the BSC to support the half rate data services. The BSC always select the first data rate the MSC allows. For selecting a speech version, a multi-module BSC takes into consideration the speech version the MSC allows, the rate type of the current channel, the speech version supported by the circuit pool of the A interface circuit of the current call and the capability of the FTC bearing the current circuit. Then the BSC fetches the intersection of the above four sets.
It selects the latest one among the speech versions (if so) in the intersection. A
single-module BSC differs from a multi-module BSC in speech version selection. The single-module BSC does not take the capability of the FTC bearing the A interface circuit into consideration. Therefore, the A interface circuit configuration in a single-module BSC should comply with the principle that the speech version set supported by the circuit pool should be a subset of that supported by the FTC.
The dynamic switchover between a full rate TCH and a half rate TCH makes it possible to optimize the TCH configuration according to the current capacity situation on the existing resource basis. This lessens the possibility of TCH congestion and makes it no longer a problem that the initially configured full rate TCHs and half rate TCHs cannot satisfy the actual traffic requirement.
2) MSC channel rate selection policy
When the requested channel type is "Only select full rate channel" or "Only select half rate channel ", only a channel at a fully matched rate can be allocated. When the requested channel type is "Select full rate channel priority", a full rate TCH shall be allocated if other conditions are satisfied and there is a full rate TCH in the cell. When the requested channel type is "Select half rate channel priority ", a half rate TCH shall be allocated if other conditions are satisfied and there is a half rate TCH in the cell. This rigid channel allocation as per MSC's rate assignment is difficult to get the system capacity and speech QoS into the optimum status. To break down this limitation, Huawei introduces a BSC channel rate selection policy. The MSC channel rate selection policy is still provided in order that the A interface interconnection test may prove that channel allocation can be implemented as per MSC's assignment.
3) BSC channel rate selection policy
When the requested channel type is" Only select full rate channel " or " Only select half rate channel ", only a channel at a fully matched rate can be allocated. When the requested channel type is " Select full rate channel priority " or " Select half rate channel priority ", a full rate TCH is preferred to guarantee the speech QoS if there are many idle full rate TCHs and a half rate TCH is preferred to guarantee the system capacity if there are few full rate TCHs. This is the basic principle of BSC channel rate selection. In practice, a full rate TCH is preferred when the number of idle full rate TCHs > Idle Thrsh for TCH/F Priori in the current cell and a half rate TCH is preferred when the number of idle full rate TCHs ≤ Idle Thrsh for TCH/F Priori in the current cell.
III. Parameter
1) Channel Management Parameter
"TCH Rate Adjust Allowed" in [Channel\Radio CH management ctrl. table].
"TCH Rate Adjust Traffic Thrsh" in [Channel\Radio CH management ctrl. table]
2) Networking Parameter
The 34BIE has to be used to support the half rate function. Since the 34BIE applies a different exchange mode from before, the following networking parameters are added or modified.
"Connection mode" in [Local Office\BSC BIE description table].
"BIE networking configuration" in [Local Office\Site BIE config. table].
"Site ID 1 ~ Site ID 30" in [Local Office\BSC BIE active/stby. group table].
"Belong to BIE group No.1 ~ Belong to BIE group No.8" in [Site\Site description table].
"BSC BIE Port No " in [Local Office\Radio channel config. table].
"BSC BIE E1Timeslot No " in [Local Office\Radio channel config. table].
"BSC BIE port No " in [Local Office\LAPD semi-perm. connection table].
"BSC BIE E1Timeslot No "in [Local Office\LAPD semi-perm. connection table].
A [BSC BIE semi-perm. connection table] is added to support semi-perm. connection establishment. It includes the following fields:
"Module No."in [Local Office\BSC BIE semi-perm. connection table]
"Trunk circuit No."in [Local Office\BSC BIE semi-perm. connection table]
"Transfer Rate "in [Local Office\BSC BIE semi-perm. connection table]
"BIE Port No "in [Local Office\BSC BIE semi-perm. connection table]
"E1 Timeslot No "in [Local Office\BSC BIE semi-perm. connection table]
2.2.13 E1 Ring Topology
I. Overview
E1 ring topology is a networking mode in which several sites are connected in a ring. All the sites work in the forward ring normally. In case the transmission ring is broken at one point, the sites before the breakpoint still work in the forward ring while those after the breakpoint are reinitialized and begin to work in the reverse ring. Compared with the normal chain topology, E1 ring topology has an advantage that the transmission ring can be automatically divided into two chains when it is broken at one point so that the sites before and after the breakpoint can both still work normally. This enhances the robustness of the system.