How to send multipart long SMS using gsm modem

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C#

.NET

I am using AT command set.
I want to send long sms but i m able to send only 160 chars msg.
How can I send long msg having more than 160 chars from gsm modem ?

public bool sendSms(string cellNo, string sms)
       {
           string messages = null;
           messages = sms;

           if (this.serialPort.IsOpen == true)
           {
               try
               {
                   this.serialPort.WriteLine("AT" + (char)(13));
                   Thread.Sleep(4);
                   this.serialPort.WriteLine("AT+CMGF=1" + (char)(13));
                   Thread.Sleep(5);
                   this.serialPort.WriteLine("AT+CMGS=\"" + cellNo + "\"");
                   Thread.Sleep(10);
                   this.serialPort.WriteLine(messages + (char)(26));
               }
               catch (Exception ex)
               {
                   MessageBox.Show(ex.Source);
               }
               return true;
           }
           else
               return false;
       }