Test Strategies for ISO/OSI Network Model Layers

The standard model for networking protocols and distributed applications is the International Standard Organization's Open System Interconnect (ISO/OSI) model. It defines seven network layers. At each of these layers difefrent test assets and strategies are relevant and test coverage should reflect all layers.

Layer	Description	Test Strategies
1:Physical	Defines the cable or physical medium itself, e.g., thinnet, thicknet, unshielded twisted pairs (UTP). All media are functionally equivalent. The main difference is in convenience and cost of installation and maintenance. Converters from one media to another operate at this level.	Monitoring of system up status for outages; review of power out recovery plans Mapping of physical media for hubs and nodes Tests of wireless connectivity VOIP & WAN interoperability testing (Such as users are under ADSL, DSL, modem, Cellular network(GSM/CDMA/3G)...) VOIP Network capacity calculation and evaluation
2:Data Link	Defines the format of data on the network. A network data frame, aka packet, includes checksum, source and destination address, and data. The largest packet that can be sent through a data link layer defines the Maximum Transmission Unit (MTU). The data link layer handles the physical and logical connections to the packet's destination, using a network interface. A host connected to an Ethernet would have an Ethernet interface to handle connections to the outside world, and a loopback interface to send packets to itself. Ethernet addresses a host using a unique, 48-bit address called its Ethernet address or Media Access Control (MAC) address. MAC addresses are usually represented as six colon-separated pairs of hex digits, e.g., 8:0:20:11:ac:85. This number is unique and is associated with a particular Ethernet device. Hosts with multiple network interfaces should use the same MAC address on each. The data link layer's protocol-specific header specifies the MAC address of the packet's source and destination. When a packet is sent to all hosts (broadcast), a special MAC address (ff:ff:ff:ff:ff:ff) is used. Puts data in frames and ensures error free transmission. Also controls the timing of the network transmission. Adds frame type, address, and error control information. IEEE divided this layer into the two following sublayers. Logical Link control (LLC) - Maintains the Link between two computers by establishing Service Access Points (SAPs) which are a series of interface points. Media Access Control (MAC) - Used to coordinate the sending of data between computers. The 802.3, 4, 5, and 12 standards apply to this layer.	View and recording of SIP and RTP messaging through tcpdump and Ethereal. Tests of provisioning system evaluate MAC address resolution. Verification of proper function of third party authorization functions
3:Network	NFS uses Internetwork Protocol (IP) as its network layer interface. IP is responsible for routing, directing datagrams from one network to another. The network layer may have to break large datagrams, larger than MTU, into smaller packets and host receiving the packet will have to reassemble the fragmented datagram. The Internetwork Protocol identifies each host with a 32-bit IP address. IP addresses are written as four dot-separated decimal numbers between 0 and 255, e.g., 129.79.16.40. The leading 1-3 bytes of the IP identify the network and the remaining bytes identifies the host on that network. The network portion of the IP is assigned by InterNIC Registration Services, under the contract to the National Science Foundation, and the host portion of the IP is assigned by the local network administrators. For large sites, usually subnetted, the first two bytes represents the network portion of the IP, and the third and fourth bytes identify the subnet and host respectively. Even though IP packets are addressed using IP addresses, hardware addresses must be used to actually transport data from one host to another. The Address Resolution Protocol (ARP) is used to map the IP address to it hardware address. Routes messages using the best path available.	Review and monitoring of gateway routing for call completion.
4:Transport	Subdivides user-buffer into network-buffer sized datagrams and enforces desired transmission control. Two transport protocols, Transmission Control Protocol (TCP) and User Datagram Protocol (UDP), sits at the transport layer. Reliability and speed are the primary difference between these two protocols. TCP establishes connections between two hosts on the network through 'sockets' which are determined by the IP address and port number. TCP keeps track of the packet delivery order and the packets that must be resent. Maintaining this information for each connection makes TCP a stateful protocol. UDP on the other hand provides a low overhead transmission service, but with less error checking. Ensures properly sequenced and error free transmission.	RTP is UDP: monitoring and evaluation of call quality implies evaluation of UDP transmission Evaluation of packet sequencing through jitter analysis TCP and SIP also operate at the Transport layer
5:Session	Defines the format of the data sent over the connections. For VOIP this is SIP and RTP. The user's interface to the network. Determines when the session is begun or opened, how long it is used, and when it is closed. Controls the transmission of data during the session. Supports security and name lookup enabling computers to locate each other.	Call completion verifies SIP protocal signalling Call stability verifies Session handling Verification of H.323 header information found in AAA and Auth packets in B2B Core RBR parameters testing(such as TTL, BCL) SIP RFC3261 conformance verification
6:Presentation	External Data Representation (XDR) sits at the presentation level. It converts local representation of data to its canonical form and vice versa. The canonical uses a standard byte ordering and structure packing convention, independent of the host. ASCII or EBCDEC data syntax. Makes the type of data transparent to the layers around it. Used to translate date to computer specific format such as byte ordering. It may include compression. It prepares the data, either for the network or the application depending on the direction it is going.	Testing and verification of codecs and codec negotiation. Codec performance testing - such as gateway capability, bandwidth and call quality
7:Application	Provides network services to the end-users. Mail, ftp, telnet, DNS, NIS, NFS are examples of network applications. Provides services software applications need. Provides the ability for user applications to interact with the network.	Applications such as Skype, Jah Jah, or Net Meeting operate over the web at the application layer level. Softphone testing Video VOIP testing Subscriber self care web interface(Esther) testing Admin management web interface(Salma and Hat) testing Lawful interception testing User call function testing (such as Voice to email, call transfer, call forwarding, conference call, call blocking.)