Um Interface

 

Um interface  From Wikipedia, the free encyclopedia

This article needs attention from an expert on the subject. Please add a reason or  a talk  talk parameter parameter to this template to explain the issue with the article. Consider  associating this request request  with a  a WikiProject. WikiProject . (February 2009)  This article includes a listbecause a  of references, references , related or . external links, links, but sources remain unclear it lack  sinlinereading citations Please Please  improve  improve  thisitsarticle  by introducing more precise citations. (February 2009)  The Um interface is the the  air interface  interface for the  the GSM  GSM mobile telephone standard. It is the interface between the  the mobile station  station (MS) and the the  Base transceiver station  station (BTS). It is called Um because it is the mobile analog to the  the U interface  interface of  ISDN ISDN.. Um is defined in the GSM 04.xx and 05.xx series of specifications. Um can also GPRS  packet-oriented communication. support  GPRS support Contents [hide] hide] 

 

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1 Um layers  layers  o

  1.1 Physical Layer (L1)  (L1)   

modem   1.1.1 Radio modem

 

timing   1.1.2 Multiplexing and timing

 

1.1.3 Coding  Coding 

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(L2)     1.2 Data Link Layer (L2)

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  1.3 Network Layer (L3)  (L3) 

2 Um logical channels  channels  o

(TCH)     2.1 Traffic channels (TCH)

 

2.1.1 Full-rate channels (TCH/F) (TCH/F)  

 

(TCH/H)   2.1.2 Half-rate channels (TCH/H)

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  2.2 Dedicated Control Channels (DCCHs) (DCCHs)    

(SDCCH)   2.2.1 Standalone Dedicated Control Channel (SDCCH)

 

2.2.2 Fast Associated Control Channel (FACCH) (FACCH)  

 

(SACCH)  2.2.3 Slow Associated Control Channel (SACCH) 

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  2.3 Common Control Channels (CCCHs)  (CCCHs)    

2.3.1 Broadcast Control Channel (BCCH) (BCCH)  

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(SCH)   2.3.2 Synchronization Channel (SCH)

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2.3.3 Frequency Correction Channel (FCCH) (FCCH)  

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(PCH)   2.3.4 Paging Channel (PCH)

 

(AGCH)  2.3.5 Access Grant Channel (AGCH) 

 

(RACH)   2.3.6 Random Access Channel (RACH)

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combinations     2.4 Allowed channel combinations

transactions  3 Fundamental Um transactions  o

  3.1 Radio channel establishment  establishment 

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  3.2 Location updating  updating 

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establishment    3.3 Mobile-Originating Call (MOC) establishment 

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  3.4 Mobile-Terminating Call (MTC) establishment  establishment  

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clearing     3.5 Call clearing

4 SMS transfer on Um  Um  o

  4.1 Mobile-Originated SMS (MO-SMS)  (MO-SMS) 

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  4.2 Mobile-Terminated SMS (MT-SMS) (MT-SMS)  

5 Um security features  features  o

  5.1 Authentication of subscribers  subscribers 

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  5.2 Um encryption  encryption 

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  5.3 Anonymization of subscribers subscribers  

 

6 See also  also 

 

reading   7 Further reading

 

links  8 External links 

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Um layers[edit edit]]  The layers of GSM are initially defined def ined in GSM 04.01 Section 7 and roughly follow the the  OSI model. model. Um is defined in the lower three layers of the model.

edit]]  Physical Layer (L1)[edit The Um  Um physical layer  is defined in the GSM 05.xx series of specifications, with the introduction and overview in GSM 05.01. For most channels, Um L1 transmits and receives 184-bit control frames or 260-bit vocoder  frames over the radio interface in 148-bit bursts with one burst per timeslot. There are three sublayers:

1. Radiomodem. Th This is is the actual radio transceiver, defined in largely in GSM 05.04 and 05.05. 2. Multiplexing and Timing. GSM uses uses  TDMA  TDMA to subdivide each radio channel into as many as 16 traffic channels or as many as 64 control channels. The multiplexing patterns are defined in GSM 05.02. 3. Coding. Thi This s sublayer is defined on GSM 05.03.

Radio modem[edit edit]] 

 

GSM uses  uses GMSK  GMSK or  8PSK  8PSK modulation with 1 bit per symbol which produces a 13/48 MHz (270.833 kHz or  270.833 K symbols/second) symbol rate and a channel spacing of 200 kHz. Since adjacent channels overlap, the standard does not allow adjacent channels to be used in the same cell. The standard defines  defines  several bands  ranging from 400 MHz to 1990 MHz. Uplink and downlink bands are generally separated by 45 or  bands 50 MHz (at the low-frequency end of the GSM spectrum) and 85 or 90 MHz (at the high-frequency end of the GSM spectrum). Uplink/downlink channel pairs are identified by an index called the  the  ARFCN.  ARFCN. Within the BTS, these ARFCNs are given arbitrary carrier indexes C0..Cn-1, with C0 designated as a  a  Beacon Channel  Channel and always operated at constant power. GSM has physical and logical channels. The logical channel is  time-multiplexed time-multiplexed  into 8 timeslots, with each timeslot lasting for 0.577ms and having 156.25 symbol periods. These 8 timeslots form a frame of 1,250 symbol periods. Channels are defined by the number and position of their corresponding burst period. The capacity associated with a single timeslot on a single ARFCN is called a physical channel (PCH) and referred refer red to as "CnTm" where n is a carrier index and m is a timeslot index (0-7). Each timeslot is occupied by a radio burst with a guard interval, two payload fields, tail bits, and a midamble sequence)). The lengths of these fields vary with the burst type but the total burst length is 156.25 (or  training sequence symbol periods. The most commonly used burst is the Normal Burst (NB). The fields of the NB are:

3

57

1

Tail  bits

Payload

26

1

Stealing Stealing Midamble  bit  bit

57

Payload

3

8.25

Tail Guard  bits  period

Midamble  A 26-bits training sequence that helps in multipath equalisation at the center of the burst "Stealing bits" each side of the midamble, used to distinguish control and traffic payloads Payload two 57-bit fields, symmetric about the burst Tail bits 3-bit field, at each end of the burst Guard period 8.25-symbols at the end of the burst There are several other burst formats, though. Bursts that require higher processing gain for signal acquisition have longer midambles. The random access ac cess burst (RACH)

 

has an extended guard period to allow it to be transmitted with incomplete timing acquisition. Burst formats are described in GSM 05.02 Section 5.2.

Multiplexing and timing[edit] edit]  Each physical channel is time-multiplexed into multiple logical channels according to the rules of GSM 05.02. One logical channel constitute of 8 burst periods (or physical channels) which is called a

Frame.

Traffic channel multiplexing follows a 26-frame (0.12

second) cycle called a "multiframe". Control channels follow f ollow a 51-frame multiframe cycle. The C0T0 physical channel carries the t he SCH, which encodes the timing state of  the BTS to facilitate synchronization to the TDMA pattern. GSM timing is driven by the serving BTS through the SCH and FCCH. All c clocks locks in the handset, including the symbol clock and local oscillator, are slaved to signals received from the BTS, as described in GSM 05.10. BTSs in the GSM network can be asynchronous and all timing requirements in the GSM standard sta ndard can be derived from f rom a stratum-3  OCXO. stratum-3 OCXO. 

Coding[edit] edit]  The coding sublayer provides  provides  forward error correction. correction. As a general rule, each GSM channel uses a  a block parity code  code (usually a Fire code), a rate-1/2, 4thorder  convolutional code codeand and a 4-burst or 8-burst  8-burst  interleaver . Notable exceptions are the synchronization channel (SCH) and random access channel (RACH) that use singleburst transmissions and thus have no interleavers. For speech channels, vocoder bits are sorted into importance classes with different degrees of encoding protection applied to each class (GSM 05.03). Both 260-bit vocoder frames and 184-bit L2 control frames are coded into 456 bit L1 frames. On channels with 4-burst interleaving (BCCH, CCCH, SDCCH, SACCH), these 456 bits are interleaved into 4 radio bursts with 114 payload bits per burst. On channels with 8-burst interleaving (TCH, FACCH), these 456 bits are interleaved over 8 radio bursts so that each radio burst carries 57 bits from the current L1 frame and 57 bits from the previous L1 frame. Interleaving algorithms for the most common traffic and control channels are described in GSM 05.03 Sections 3.1.3, 3.2.3 and 4.1.4.

Data Link Layer (L2)[edit] edit]  The Um  Um data link layer , LAPDm LAPDm,, is defined in GSM 04.05 and 04.06. LAPDm is the mobile analog to ISDN's  ISDN's LAPD LAPD.. 

Network Layer (L3)[edit] edit] 

 

The Um  Um network layer  is defined in GSM 04.07 and 04.08 and has three sublayers. A subscriber terminal must establish a connection in each sublayer before accessing the next higher sublayer.

1.

Resource (RR). This sublayer manages the assignment and release Radio Resource  r elease of 

2.

logical channels on the radio link. It is normally terminated in the  the BSC. BSC.  Management (MM). This sublayer authenticates users and tracks their  Mobility Management  VLR  or  HLR HLR..  movements from cell to cell. It is normally terminated in the  the VLR

3.

Control (CC). This sublayer connects telephone calls and is taken directly Call Control  from  ITU-T Q.931. from Q.931. GSM 04.08 Annex E provides a table of corresponding paragraphs in GSM 04.08 and ITU-T Q.931 along with a summary of  differences between the two. The CC sublayer is terminated in the  the  MSC. MSC. 

The access order is RR, MM, CC. The release order is the reverse of that. Note that none of these sublayers terminate in the BTS itself. The standard GSM BTS operates only in layers 1 and 2.

Um logical channels[edit] edit]  Um logical channel types are outlined in GSM 04.03. Broadly speaking, non non-GPRS -GPRS Um logical channels fall into three categories: traffic channels, dedicated control channels and non-dedicated control channels.

Traffic channels (TCH)[edit edit]]  These point-to-point channels correspond to the ISDN ISDN  B channel  channel and are referred to as Bm channels. channels. Traffic channels use 8-burst(Break) diagonal interleaving with a new block starting on every fourth burst and any given burst containing bits from two different traffic frames. This interleaving pattern makes the TCH robust against single-burst fades since the loss of a single burst destroys only 1/8 of the frame's channel bits. The coding of a traffic channel is dependent on the traffic or vocoder type employed, with most coders capable of overcoming single-burst losses. All traffic channels use a 26multiframe TDMA structure.

Full-rate channels (TCH/F)[edit edit]]   A GSM full rate channel uses 24 frames out of a 26-multiframe. The channel bit rate of a full-rate GSM channel is 22.7 kbit/s, although the actual payload data rate is 9.6-14 kbit/s, depending on the channel coding. This channel is normally used with the GSM

 

Rate,, GSM 06.60  06.60 Enhanced Full Rate  Rate or GSM 06.90  06.90  Adaptive Adaptive Multi06.10  Full Rate 06.10 Rate  speech codec Rate codec.. It can also be used for  fax  fax and  and Circuit Switched Data. Data. 

Half-rate channels (TCH/H)[edit] edit]   A GSM half rate channel uses 12 frames out of a 26-multiframe. The channel bit rate of  a half-rate GSM channel is 11.4 kbit/s, although the actual data capacity is 4.8-7 kbit/s, depending on the channel coding. This channel is normally used with the GSM Rate  or GSM 06.90 Adaptive Multi-Rate speech codec. 06.20  Half Rate 06.20

Dedicated Control Channels (DCCHs)[edit] edit]  These point-to-point channels correspond to the ISDN  ISDN  D channel  channel and are referred to as Dm channels. channels.

Standalone Dedicated Control Channel (SDCCH) [edit] edit]  The SDCCH is used for most short s hort transactions, including initial call setup step, registration and and  SMS  SMS transfer. It has a payload data rate of 0.8 kbit/s. Up to eight SDCCHs can be time-multiplexed onto a single physical channel. The SDCCH uses 4burst block interleaving in a 51-multiframe.

Fast Associated Control Channel (FACCH)[edit] edit]  The FACCH is always paired with a traffic traff ic channel. The FACCH is a a  blank-andburst  channel that operates by stealing bursts from its associated traffic channel. Bursts burst that carry FACCH data are distinguished from traffic bursts by stealing bits at each end of the midamble. The FACCH is used for f or in-call signaling, including call disconnect,  handover  and the later stages of call setup. It has a payload data rate of 9.2 disconnect, kbit/s when paired with a full-rate channel (FACCH/F) and 4.6 kbit/s when paired with a half-rate channel (FACCH/H). The FACCH uses the same interleaving and multiframe structure as its host TCH.