Transport and Session are layers 4 and 5 of the OSI model. The Transport Layer (Layer 4) establishes, maintains, and terminates communication between networked machines. The Session Layer (Layer 5) initiates contact between applications and governs the exchange of data between programs. The IP Suite compresses the upper three layers (Application, Presentation, and Session) into one layer called Application. The transport layer remains the same. The Network layer is called the Internet Layer (but performs the same functions), and the bottom two layers, Data Link and Physical, are grouped into one layer called the Network Interface Layer. In the IP suite transport layer there are two methods of delivery: TCP and UDP. TCP stands for Transmission Control Protocol. This is the most common method of delivery in data networks. Each packet sent is acknowledged by the receiving end and missing packets are re-transmitted. UDP, or User Datagram Protocol, is referred to as a connectionless protocol. It does not try to retransmit missing data; instead it assumes that all the data that was sent was received. While this may not sound like the best idea it is an optimal solution for VoIP and other time-sensitive applications, as it does not waste bandwidth with acknowledgements and retransmission of packets. Using this best-effort method, UDP is able to present our VoIP applications with a steady stream of voice packets with minimal delay.
The IP suite is a connectionless Layer 3 protocol with no continuing connection between endpoints. As such, there can be multiple Layer 2 protocols used, including Ethernet, ATM, PPP, Multi-Protocol Label Switching (MPLS), Frame Relay, and Wireless protocols. I would like to take some time now to discuss some of these protocols.
Frame Relay is a cost-effective alternative to a leased line. It can work at speeds from 56kbps to 45Mbps. It provides some QoS capabilities, including prioritization (802.11p) and DiffServ. It exists at Layer 2 so there is less processing to perform at each node. It is considered to be a best-effort protocol, as once a packet enters the network it is not guaranteed any priority over other packets. The delays are higher and more variable than the PSTN. This can lead to an increased chance of serialization delay. Frame relay does offer error correction. When purchasing a Frame Relay connection you purchase a Converted Information Rate (CIR). This is your guaranteed throughput on the relay. If the CIR is too low there is a chance the packets may be dropped, resulting in poor voice quality.
Asynchronous Transfer Mode, or ATM, is available from service providers in four distinct levels of service. The first, Constant Bit Rate (CBR) specifies a fixed bit rate so that data is sent at a constant rate, similar to a leased line. A Variable Bit Rate (VBR) provides a specified throughput capacity, but the data is not sent evenly. VBR is a popular choice for voice and video conferencing data. Unspecified Bit Rate (UBR) does not guarantee throughput rates. It is generally used for applications that can tolerate delays, such as file transfer and other data applications. Available Bit Rate provides a guaranteed minimum capacity and allows for the ability to burst data throughput rates when the network capacity is available. ATM speeds range from 25Mbps to 622Mbps. It has sophisticated QoS services, including DiffServ. Using CBR or VBR you can also receive a guarantee of prioritization. ATM is, unfortunately, not always available and is both difficult to set up and very costly. The equipment required to run ATM is very costly on its own. ATM does have a larger header than many alternatives, and is also subject to serialization delays. ATM will have a lower serialization delay than Frame Relay, however.
Point-to-Point Protocol, or PPP, is used to transmit datagrams between peer routers or other devices over dedicated switched lines. This also the method used to connect dial-up Internet connections. It uses a dedicated link, and therefore can provide the greatest QoS. It is full duplex and can be dedicated or circuit switched.
Wireless LAN is a relatively new technology. WLANs function on radio broadcast frequencies. Devices share the physical layer (the radio band). They must contend for use of the media. There is no collision detection with a WLAN, so collision avoidance methods are used – devices must wait for an inter-frame spacing interval before they transmit, and they must not transmit while other devices are transmitting.
Multi-Protocol Label Switching (MPLS) is a newer transport method. It provides a data transport mechanism for both circuit-based and packet-switched environments, and supports more than one transport method. It can use IP, ATM, SONET, and Ethernet.
Finally, there are common residential connections, namely Cable and DSL. These are best-effort methods, and do not provide and QoS. They are able to provide high-bandwidth Internet access, but minimum transmission speeds are not guaranteed.
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