I am using the following Apex Web Service class.

Global class SuryaDhuravSoftWebservice
{
 webservice static String saywelcome(String Name)
 {
 return 'Hello' +Name+' welcome to salesforce Web Services';
 }
 }

I generated the WSDL and created another apex class using that WSDL, code pasted below

<!-- 
 Web Services API : SuryaDhuravSoftWebservice
 -->
<definitions xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns="http://schemas.xmlsoap.org/wsdl/" xmlns:soap="http://schemas.xmlsoap.org/wsdl/soap/" xmlns:tns="http://soap.sforce.com/schemas/class/SuryaDhuravSoftWebservice" targetNamespace="http://soap.sforce.com/schemas/class/SuryaDhuravSoftWebservice">
<types>
<xsd:schema elementFormDefault="qualified" targetNamespace="http://soap.sforce.com/schemas/class/SuryaDhuravSoftWebservice">
<xsd:element name="DebuggingInfo">
<xsd:complexType>
<xsd:sequence>
<xsd:element name="debugLog" type="xsd:string"/>
</xsd:sequence>
</xsd:complexType>
</xsd:element>
<xsd:simpleType name="ID">
<xsd:restriction base="xsd:string">
<xsd:length value="18"/>
<xsd:pattern value="[a-zA-Z0-9]{18}"/>
</xsd:restriction>
</xsd:simpleType>
<xsd:simpleType name="LogCategory">
<xsd:restriction base="xsd:string">
<xsd:enumeration value="Db"/>
<xsd:enumeration value="Workflow"/>
<xsd:enumeration value="Validation"/>
<xsd:enumeration value="Callout"/>
<xsd:enumeration value="Apex_code"/>
<xsd:enumeration value="Apex_profiling"/>
<xsd:enumeration value="Visualforce"/>
<xsd:enumeration value="System"/>
<xsd:enumeration value="All"/>
</xsd:restriction>
</xsd:simpleType>
<xsd:simpleType name="LogCategoryLevel">
<xsd:restriction base="xsd:string">
<xsd:enumeration value="Internal"/>
<xsd:enumeration value="Finest"/>
<xsd:enumeration value="Finer"/>
<xsd:enumeration value="Fine"/>
<xsd:enumeration value="Debug"/>
<xsd:enumeration value="Info"/>
<xsd:enumeration value="Warn"/>
<xsd:enumeration value="Error"/>
</xsd:restriction>
</xsd:simpleType>
<xsd:complexType name="LogInfo">
<xsd:sequence>
<xsd:element name="category" type="tns:LogCategory"/>
<xsd:element name="level" type="tns:LogCategoryLevel"/>
</xsd:sequence>
</xsd:complexType>
<xsd:simpleType name="LogType">
<xsd:restriction base="xsd:string">
<xsd:enumeration value="None"/>
<xsd:enumeration value="Debugonly"/>
<xsd:enumeration value="Db"/>
<xsd:enumeration value="Profiling"/>
<xsd:enumeration value="Callout"/>
<xsd:enumeration value="Detail"/>
</xsd:restriction>
</xsd:simpleType>
<xsd:element name="DebuggingHeader">
<xsd:complexType>
<xsd:sequence>
<xsd:element name="categories" minOccurs="0" maxOccurs="unbounded" type="tns:LogInfo"/>
<xsd:element name="debugLevel" type="tns:LogType"/>
</xsd:sequence>
</xsd:complexType>
</xsd:element>
<xsd:element name="CallOptions">
<xsd:complexType>
<xsd:sequence>
<xsd:element name="client" type="xsd:string"/>
</xsd:sequence>
</xsd:complexType>
</xsd:element>
<xsd:element name="SessionHeader">
<xsd:complexType>
<xsd:sequence>
<xsd:element name="sessionId" type="xsd:string"/>
</xsd:sequence>
</xsd:complexType>
</xsd:element>
<xsd:element name="AllowFieldTruncationHeader">
<xsd:complexType>
<xsd:sequence>
<xsd:element name="allowFieldTruncation" type="xsd:boolean"/>
</xsd:sequence>
</xsd:complexType>
</xsd:element>
<xsd:element name="saywelcome">
<xsd:complexType>
<xsd:sequence>
<xsd:element name="Name" type="xsd:string" nillable="true"/>
</xsd:sequence>
</xsd:complexType>
</xsd:element>
<xsd:element name="saywelcomeResponse">
<xsd:complexType>
<xsd:sequence>
<xsd:element name="result" type="xsd:string" nillable="true"/>
</xsd:sequence>
</xsd:complexType>
</xsd:element>
</xsd:schema>
</types>
<!--  Message for the header parts  -->
<message name="Header">
<part name="AllowFieldTruncationHeader" element="tns:AllowFieldTruncationHeader"/>
<part name="CallOptions" element="tns:CallOptions"/>
<part name="DebuggingHeader" element="tns:DebuggingHeader"/>
<part name="DebuggingInfo" element="tns:DebuggingInfo"/>
<part name="SessionHeader" element="tns:SessionHeader"/>
</message>
<!--  Operation Messages  -->
<message name="saywelcomeRequest">
<part element="tns:saywelcome" name="parameters"/>
</message>
<message name="saywelcomeResponse">
<part element="tns:saywelcomeResponse" name="parameters"/>
</message>
<portType name="SuryaDhuravSoftWebservicePortType">
<operation name="saywelcome">
<input message="tns:saywelcomeRequest"/>
<output message="tns:saywelcomeResponse"/>
</operation>
</portType>
<binding name="SuryaDhuravSoftWebserviceBinding" type="tns:SuryaDhuravSoftWebservicePortType">
<soap:binding style="document" transport="http://schemas.xmlsoap.org/soap/http"/>
<operation name="saywelcome">
<soap:operation soapAction=""/>
<input>
<soap:header use="literal" part="SessionHeader" message="tns:Header"/>
<soap:header use="literal" part="CallOptions" message="tns:Header"/>
<soap:header use="literal" part="DebuggingHeader" message="tns:Header"/>
<soap:header use="literal" part="AllowFieldTruncationHeader" message="tns:Header"/>
<soap:body use="literal" parts="parameters"/>
</input>
<output>
<soap:header use="literal" part="DebuggingInfo" message="tns:Header"/>
<soap:body use="literal"/>
</output>
</operation>
</binding>
<service name="SuryaDhuravSoftWebserviceService">
<documentation/>
<port binding="tns:SuryaDhuravSoftWebserviceBinding" name="SuryaDhuravSoftWebservice">
<soap:address location="https://ap1.salesforce.com/services/Soap/class/SuryaDhuravSoftWebservice"/>
</port>
</service>
</definitions>

In Remote Site Setting As end point URL 


Now I am executing the following code in Developer Console.

soapSforceComSchemasClassSuryadhura.SuryaDhuravSoftWebservice  proxyClass = new soapSforceComSchemasClassSuryadhura.SuryaDhuravSoftWebservice();

String retVal = proxyClass.saywelcome('prayank');

System.debug('------retVal----'+retVal);

But It shows Error 

System.CalloutException: Web service callout failed: WebService returned a SOAP Fault: INVALID_SESSION_ID: Invalid Session ID found in SessionHeader: Illegal Session faultcode=sf:INVALID_SESSION_ID faultactor=

What i am doing wrong here ?