25. A data signal coding method of coding data using a coder having a multilayered fixed codebook structure, comprising: extracting a linear prediction coding coefficient from an input data signal and generating a data signal corresponding to the input data signal by searching codebooks, in a first layer, wherein the generating the data signal corresponding to the restored input data signal using the codebook table of the first layer includes conducting a fixed codebook search and an adaptive codebook search; searching a codebook using parameters of the data signal and providing additional data to enhance a quality of restored data, in a second layer; obtaining bit streams of data by multiplexing data generated from both the first layer and the second layer; transmitting the bit stream of the second layer following the bit stream of the first layer, such that the respective bit streams are separated at a bit rate necessary for a decoding apparatus in a network according to network traffic conditions; searching respective codebooks of additional second layers using parameters of the data signal and providing additional data to enhance the quality of restored data, wherein when the additional second-layers to receive coding data from the first layer are used, quantization data regarding a difference between the fixed codebook gain values and fixed codebook indexes are multiplexed, the quantization data is output from each of the additional second-layers, wherein the parameters comprise: a first correlation d(n) between an impulse response h(i-n) and a target signal x′(n) obtained by the fixed codebook search of the base layer by the following equation, d ⁡ ( n ) = ∑ i = n 39 ⁢ ⁢ x ′ ⁡ ( n ) ⁢ h ⁡ ( i - n ) ⁢ ⁢ n = 0 , … ⁢ , 39 a second correlation C detected by using a sign s of each pulse and the first correlation d(n) by the following equation, C = ∑ i = 0 3 ⁢ ⁢ s 1 ⁢ d ⁡ ( m i ) =  d ⁡ ( m 0 )  +  d ⁡ ( m 1 )  +  d ⁡ ( m 2 )  +  d ⁡ ( m 3 )  , wherein m i represents an i th pulse position, and s i represents a sign of an i th pulse; and an amount of energy E of the impulse response h(n) detected by the following equation, E = ∑ i = 0 3 ⁢ ⁢ ϕ ⁡ ( m i , m j ) + 2 ⁢ ∑ i = 0 2 ⁢ ⁢ ∑ j = i + 1 3 ⁢ ⁢ s i ⁢ s j ⁢ ϕ ⁡ ( m i ⁢ m j ) , wherein Φ(m i , m j ) represents a correlation between the impulse response h(n) with respect to i th and j th pulse positions, s i is the sign of an i th pulse, and s j is a sign of a j th pulse.
25. A data signal coding method of coding data using a coder having a multilayered fixed codebook structure, comprising: extracting a linear prediction coding coefficient from an input data signal and generating a data signal corresponding to the input data signal by searching codebooks, in a first layer, wherein the generating the data signal corresponding to the restored input data signal using the codebook table of the first layer includes conducting a fixed codebook search and an adaptive codebook search; searching a codebook using parameters of the data signal and providing additional data to enhance a quality of restored data, in a second layer; obtaining bit streams of data by multiplexing data generated from both the first layer and the second layer; transmitting the bit stream of the second layer following the bit stream of the first layer, such that the respective bit streams are separated at a bit rate necessary for a decoding apparatus in a network according to network traffic conditions; searching respective codebooks of additional second layers using parameters of the data signal and providing additional data to enhance the quality of restored data, wherein when the additional second-layers to receive coding data from the first layer are used, quantization data regarding a difference between the fixed codebook gain values and fixed codebook indexes are multiplexed, the quantization data is output from each of the additional second-layers, wherein the parameters comprise: a first correlation d(n) between an impulse response h(i-n) and a target signal x′(n) obtained by the fixed codebook search of the base layer by the following equation, d ⁡ ( n ) = ∑ i = n 39 ⁢ ⁢ x ′ ⁡ ( n ) ⁢ h ⁡ ( i - n ) ⁢ ⁢ n = 0 , … ⁢ , 39 a second correlation C detected by using a sign s of each pulse and the first correlation d(n) by the following equation, C = ∑ i = 0 3 ⁢ ⁢ s 1 ⁢ d ⁡ ( m i ) =  d ⁡ ( m 0 )  +  d ⁡ ( m 1 )  +  d ⁡ ( m 2 )  +  d ⁡ ( m 3 )  , wherein m i represents an i th pulse position, and s i represents a sign of an i th pulse; and an amount of energy E of the impulse response h(n) detected by the following equation, E = ∑ i = 0 3 ⁢ ⁢ ϕ ⁡ ( m i , m j ) + 2 ⁢ ∑ i = 0 2 ⁢ ⁢ ∑ j = i + 1 3 ⁢ ⁢ s i ⁢ s j ⁢ ϕ ⁡ ( m i ⁢ m j ) , wherein Φ(m i , m j ) represents a correlation between the impulse response h(n) with respect to i th and j th pulse positions, s i is the sign of an i th pulse, and s j is a sign of a j th pulse. 26. The data signal coding method as claimed in claim 25 , wherein the second layer is a speech quality enhancement layer.