5.1 Handshake Modulation Method

   


All ITU-T compliant DSL modems provide support of the handshake mechanism defined in ITU-T Recommendation G.994.1 prior to the core initialization of the specific DSL. The purpose of the handshake is to identify the type of DSL modem at each end of the loop and to negotiate and agree on a common set of parameters for the communication session. As shown in Figure 5.1, handshake sits "in front" of the core DSL modem to operate in preactivation sessions. During the handshake session, the two modems exchange messages negotiating configuration parameters for the communication session. Once the common set of parameters is identified and agreed upon, the handshake session clears down and the core initialization of the DSL modem begins.

Figure 5.1. System reference model for handshake.

graphics/05fig01.gif

Because handshake is common to all ITU-T compliant modems, it needs to operate under the most extreme conditions, namely, on long loops and under relatively noisy conditions. At a minimum, it must operate at a range that is longer than that which can be supported by the core DSL modem. The modulation method used during handshake needs to be very robust as a top priority, while transmission speed on the link is not as important. Another key requirement is that the modulation method needs to be simple, preferably based on or related to the transmission method of the core DSL modem.

The modulation method chosen for handshake is binary differential phase shift keying (DPSK). The core modulator is modeled as a one-dimension double sideband suppressed carrier amplitude modulator as shown in Figure 5.2. The differential encoding takes place in the transmit sequence A n , where A n takes on the values of +1 or “1. Transmit symbol A n is rotated 180 degrees from the previous symbol if the transmit bit is a logic 1 and A n is the same phase as the previous symbol if the transmit bit is a logic 0. Mathematically, the differential encoding rule is specified as A n = A n -1 if b n = 0, and A n = - A n -1 if b n = 1, where b n is the transmit data bit at time sample index n .

Figure 5.2. DPSK modulator block diagram.

graphics/05fig02.gif

The transmit signal s ( t ) per the DPSK modulator in Figure 5.2 is represented by

graphics/05equ05.gif


where A n is the differentially encoded bit sequence per the rules of

graphics/05equ02.gif


where b n is the transmit data bit sequence, g ( t ) is the impulse response of the transmit shaping filter, T is the data symbol interval, f i is the carrier frequency of index i , and f i is an arbitrary carrier phase for carrier index i .

G.994.1 defines different carrier sets, and a carrier set may contain more than one carrier that transports the same information in each carrier. In the case where more than one carrier is used to transmit the same data sequence in each carrier, the transmit signal may be represented as the sum of all the modulated carriers , namely

graphics/05equ03.gif


Because handshake operates "in front" of the DSL modem, it makes sense to define a set of carriers consistent with the frequency spectrum used by the specific DSL and to utilize the corresponding core signal processing element(s) for generation of the handshake tones. The ITU-T suite of DSL modems contains a family of ADSL modems that uses multicarrier modulation with carrier frequency spacing of 4.3125 kHz; the suite also contains the symmetric high-rate DSL (SHDSL), whose signaling is derived from a fundamental 8 kHz reference clock. Because various DSL modems in the ITU-T suite are based on different fundamental frequencies, G.994.1 defines two signaling families: one family is based on 4.3125 kHz signaling (i.e., the ADSL modems), and the other is based on 4 kHz signaling (i.e., the SHDSL modems).

Table 5.1. Carrier Sets for the 4.3125 kHz Signaling Family
 

Upstream Carriers

 

Downstream Carriers

Carrier Set

 

F u1

 

F u2

 

F u3

   

F d1

 

F d2

 

F d3

N u1

(kHz)

N u2

(kHz)

N u3

(kHz)

 

N d1

(kHz)

N d2

(kHz)

N d3

(kHz)

A43

9

38.8125

17

73.3125

25

107.8125

 

40

172.5000

56

241.5000

64

276.0000

B43

37

159.5625

45

194.0625

53

228.5625

 

72

310.5000

88

379.5000

96

414.0000

C43

-

-

7

30.1875

9

38.8125

 

12

51.7500

14

60.3750

64

267.0000


5.1.1 The 4.3125 kHz Signaling Family

The 4.3125 kHz signaling family has three carrier sets in support of ADSL. Table 5.1 shows the carrier sets for the 4.3125 signaling family; each carrier is identified by an index ( multiplier ) value N and corresponding frequency F. The subscripts u and d identify upstream and downstream, respectively.

The sets are labeled A43, B43, and C43, and they are designed in support of Annexes A, B, and C, respectively, in G.992.1 (full-rate ADSL) and G.992.2 (splitterless ADSL). Each carrier set contains three tones transmitting in the upstream direction and three tones transmitting in the downstream direction. The maximum transmit power for each upstream carrier set is “1.65 dBm and that for each downstream carrier set is “3.65 dBm.

5.1.2 The 4 kHz Signaling Family

Table 5.2 shows the 4 kHz signaling family carrier set. This set consists of one upstream tone at 12 kHz and one downstream tone at 20 kHz. The maximum transmit power for each tone is +5 dBm. G.991.2 (SHDSL) is the digital subscriber line technology that uses this carrier set during handshake preactivation.

Table 5.2. Carrier Set for the 4 kHz Signaling Family

Carrier Set

Upstream Carriers

 

Downstream Carriers

N u1

F u1 (kHz)

 

N d1

F d1 (kHz)

A4

3

12

 

5

20



   
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DSL Advances
DSL Advances
ISBN: 0130938106
EAN: 2147483647
Year: 2002
Pages: 154

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