Internetwork Packet Exchange/Sequenced Packet Exchange (IPX/SPX) The Internetwork Packet Exchange/Sequenced Packet Exchange (IPX/SPX) protocol is from the Xerox Network System IDP and SPP protocols. IPX/SPX has been thought of as the protocol for Novell Networks. Of course, with the popularity of the Internet, the default protocol for most networks is TCP/IP. The IPX/SPX protocol is based on the Open Systems Interconnect (OSI) Model. IPX is the Network Layer (Layer 3 and 4) and SPX is the Transport Layer (Layer 4) of the OSI Model. FIGURE 1 NOTE: mapping the IPX/SPX protocol to the OSI Model has many various renditions. Figure 1 is only one to show how the protocol works. The mapping is more of a server side map. Let’s look at the protocol through its parts starting with SAP. The Service Advertising Protocol (SAP) is used by servers to let the client systems know what Network Services are available. One server may be advertising file sharing, while another may be file sharing and printer sharing. Any combination can exist and SAP is used to inform the network clients which servers have which Network Services. The Netware Core Protocol (NCP) has various uses. It allows clients to log into the servers, request files and printer resources and other requests to Network Services. NOTE: it is possible to use these protocols on a client which is sharing Network Resources. As you can see from Figure 1, SAP and NCP reside on Layers 5, 6 and 7. Requests made here then go to Layer 4 through IPX or SPX. Sequenced Packet Exchange (SPX) works on Layer 4 and can guarantee delivery of the packets with acknowledgments. SPX is also responsible for reassembling the data packets into a full file. SPX allows only for transmission between two systems since it cannot perform broadcasts. IPX exists on Layers 3 and 4. It is used for connectionless communications. That is, the connections are not guaranteed and used for broadcasts. In this manner, when someone is watching a video from a server, some sections of the video may be jumpy. The lack of smooth video flow is usually due to loss of some data not arriving at the client system, or arriving out of turn so the complete stream cannot be pieced back together properly. Layer 3 consists of part of IPX and mainly the Routing Information Protocol (RIP). IPX/SPX can be used in a WAN and needs to be able to determine systems on the Local Area Network (LAN) and those systems which are not local. Part of the system address is made up of the MAC address which is usually hard coded on the Network Adapter. NOTE: some network adapters allow you to use your own MAC address and override the built-in address. Network addresses consist of a Network Number and a Node Number. The Network Number is a 32-bit number that represents the network in which the system resides to a system which it is communicating. The value is set to zero if the other system is on a (LAN). The Node Number is a 48-bit number which signifies the physical system. If the hexadecimal value of the Node Number is all “F’s”, then the information is a broadcast to the whole network specified by the Network Number. Otherwise, the 48-bits are the MAC address taken from the Network Interface Card (NIC). Layer 1 and 2 are dependent on the physical network. If an Ethernet network is used (802.3), the scheme used to detect network traffic is Carrier Sense Multiple Access with Collision Detection (CSMA/CD). This basically means that each system attempts to transmit on the network. If two systems send data at once, this is a collision, and both systems wait a random time before trying again. These two Layers can be made up of many different network schemes as follows: 802.1 Internetworking 802.2 Logical Link Control (LLC) 802.3 CSMA/CD 802.4 Token Passing Bus 802.5 Token Ring network 802.6 Metropolitan Area Network (MAN) 802.7 Broadband 802.8 Fiber Optic 802.9 Integrated Voice and Data 802.10 Network Security 802.11 Wireless 802.12 100BaseVG-AnyLAN As you can see, the IPX/SPX protocol is a routable protocol which can be used on a WAN. A routable protocol is one which can be passed over a router. Not all protocols are routable. IPX/SPX can be used on numerous network mediums. It is a very robust protocol, but unfortunately not used on the Internet for its inability to work over WANs as well as TCP/IP.