19. A data signal coding apparatus for coding data having a multilayered codebook structure, comprising: a first layer filtering an input data signal using linear prediction coding and generating a data signal corresponding to the restored input data signal using a codebook table; a second layer to receive the data signal from the first layer, search a codebook using parameters of the data signal, and provide additional data to the first layer to enhance a quality of restored data; and a multiplexer to multiplex data generated from both the first layer and the second layer to obtain bit streams, wherein the bit stream of the second layer is transmitted 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, 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, and the second layer further quantizes a difference between a first gain value obtained through the fixed codebook search performed by the first layer and a second gain value obtained by the fixed codebook search performed by the second layer, wherein the parameters include a first correlation d(n) between an impulse response h(i-n) and a target signal x′(n) obtained in the fixed codebook searching unit in 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.
19. A data signal coding apparatus for coding data having a multilayered codebook structure, comprising: a first layer filtering an input data signal using linear prediction coding and generating a data signal corresponding to the restored input data signal using a codebook table; a second layer to receive the data signal from the first layer, search a codebook using parameters of the data signal, and provide additional data to the first layer to enhance a quality of restored data; and a multiplexer to multiplex data generated from both the first layer and the second layer to obtain bit streams, wherein the bit stream of the second layer is transmitted 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, 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, and the second layer further quantizes a difference between a first gain value obtained through the fixed codebook search performed by the first layer and a second gain value obtained by the fixed codebook search performed by the second layer, wherein the parameters include a first correlation d(n) between an impulse response h(i-n) and a target signal x′(n) obtained in the fixed codebook searching unit in 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. 20. The data signal coding apparatus as claimed in claim 19 , wherein the data is speech.