s need to be analyzed and routed to proper destinations by the network layer. Error messaging: Some protocols for communication error handling resides in the network layer. They generate error messages about connection, route traffic, and health of devices in the network. A popular error handling pro- tocol that is part of the IP suite that resides in the network layer is ICMP (Internet control message protocol). Fundamentals of EMS, NMS and OSS/BSS Packet Packet data header Network layer Frame Frame Frame data header footer Data link layer Figure 10.2 Packet in the network layer. Though the network layer protocol is usually connectionless, it can also be con- nection oriented. That is, the network layer protocol supports both connectionless and connection-oriented protocols. When connectionless protocol like IP is used in the network layer, data consistency and reliability are handled using connection- oriented protocols like TCP in the transport layer. IP (Internet protocol) is the most popular network layer protocol. There are sev- eral other supporting protocols of IP for security, traffic control, error handling, and SO on that reside in the network layer. These protocols include IPsec, IP NAT, and ICMP. Another network layer protocol outside the realm of the Internet protocol is IPX (inter-network packet exchange) and DDP (datagram delivery protocol). 10.5 Transport Layer This is the fourth lowest layer (L4) in the ISO/OSI model. While the physical layer is concerned with transmission of bits, the data link layer with transmission in local area, and the network layer with transition even with remote systems, the trans- port layer handles transmission of data between processes/applications. There will be multiple applications running on the same system and there would be scenarios when more than one application from a specific source machine wants to send data to multiple applications on a destination machine. Transport layer ensures that mul- tiple applications on the same system can communicate to another 