diff --git a/lib/biju/pdu.rb b/lib/biju/pdu.rb
index afbf491..e0c1faf 100644
--- a/lib/biju/pdu.rb
+++ b/lib/biju/pdu.rb
@@ -4,6 +4,7 @@ require 'biju/pdu/encoding/ucs2'
 require 'biju/pdu/user_data'
 require 'biju/pdu/data_coding_scheme'
 
+require 'biju/pdu/first_octet'
 require 'biju/pdu/timestamp'
 require 'biju/pdu/phone_number'
 require 'biju/pdu/type_of_address'
@@ -13,15 +14,21 @@ module Biju
     def self.encode(phone_number, message, type_of_address: :international)
       phone_number = PhoneNumber.encode(phone_number)
       user_data = UserData.encode(message)
+      first_octet = FirstOctet.new.message_type_indicator!(:sms_submit)
 
       [
+        # Length of SMSC information
+        # 0 means the SMSC stored in the phone should be used
+        '00',
+        # First octet
+        '%02x' % first_octet.binary,
+        # TP-Message-Reference
         '00',
-        '01',
-        '00', # TP-Message-Reference
         "%02x" % phone_number.length,
         "%02x" % phone_number.type_of_address.hex,
         phone_number.number,
-        '00', # TP-PID: Protocol identifier
+        # TP-PID: Protocol identifier
+        '00',
         "%02x" % user_data.encoding.hex,
         "%02x" % user_data.length,
         user_data.message
diff --git a/lib/biju/pdu/first_octet.rb b/lib/biju/pdu/first_octet.rb
new file mode 100644
index 0000000..3296afd
--- /dev/null
+++ b/lib/biju/pdu/first_octet.rb
@@ -0,0 +1,68 @@
+module Biju
+  module PDU
+    class FirstOctet
+      FIRST_OCTET = {
+        reply_path: 0b10000000,
+        user_data_header: 0b01000000,
+        status_report_request: 0b00100000,
+        validity_period_format: 0b00011000,
+        reject_duplicates: 0b00000100,
+        message_type_indicator: 0b00000011,
+      }
+
+      MESSAGE_TYPE_INDICATOR = {
+        sms_deliver: 0b00000000,
+        sms_submit: 0b00000001,
+        sms_status: 0b00000010,
+        reserved: 0b00000011,
+      }
+
+      VALIDITY_PERIOD_FORMAT = {
+        not_present: 0b00000000,
+        reserved: 0b00001000,
+        relative: 0b00010000,
+        absolute: 0b00011000,
+      }
+
+      attr_accessor :binary
+
+      def initialize(first_octet = 0)
+        self.binary = first_octet
+      end
+
+      def get(field)
+        binary & FIRST_OCTET[field]
+      end
+
+      [:reply_path, :user_data_header, :status_report_request,
+       :reject_duplicates].each do |sym|
+        define_method :"#{sym}?" do
+          get(sym) > 0
+        end
+
+        define_method :"#{sym}!" do |value = true|
+          if value
+            self.binary |= FIRST_OCTET[sym]
+          else
+            self.binary &= (FIRST_OCTET[sym] ^ 0b11111111)
+          end
+          self
+        end
+      end
+
+      [:message_type_indicator, :validity_period_format].each do |sym|
+        define_method sym do
+          self.class.const_get(sym.upcase).key(get(sym))
+        end
+
+        define_method :"#{sym}!" do |value|
+          hash = self.class.const_get(sym.upcase)
+
+          self.binary = ((binary & (FIRST_OCTET[sym] ^ 0b11111111)) |
+                         hash[value]) unless hash[value].nil?
+          self
+        end
+      end
+    end
+  end
+end
diff --git a/spec/biju/pdu/first_octet_spec.rb b/spec/biju/pdu/first_octet_spec.rb
new file mode 100644
index 0000000..c24ca52
--- /dev/null
+++ b/spec/biju/pdu/first_octet_spec.rb
@@ -0,0 +1,12 @@
+require 'spec_helper'
+require 'biju/pdu'
+
+describe Biju::PDU::FirstOctet do
+  its(:reply_path?) { should be_false }
+  its(:user_data_header?) { should be_false }
+  its(:status_report_request?) { should be_false }
+  its(:reject_duplicates?) { should be_false }
+
+  its(:message_type_indicator) { should eq(:sms_deliver) }
+  its(:validity_period_format) { should eq(:not_present) }
+end