var SRMP = new byte[4];

Array.Copy(Encoding.UTF8.GetBytes(1.ToString("x2")), SRMP, Encoding.UTF8.GetBytes(1.ToString("x2")).Length);

Array.Copy(Encoding.UTF8.GetBytes((j + 1).ToString("x2")), 0, SRMP, 1, Encoding.UTF8.GetBytes((j + 1).ToString()).Length);

 ushort leng = 1400;

byte[] a = new byte[2];

a[0] = (byte)(leng>>8);

a[1] = (byte)leng;

var ccc = BitConverter.ToString(a).Replace("-", "");

var dddd = Convert.ToInt32(ccc, 16);

Array.Copy(a, 0, SRMP, 2, a.Length);