WAN PROTOCOLS

Frame Relay
 

• Frame Relay is a high-performance WAN protocol that operates at the physical and data link layers of the OSI reference model. Frame Relay originally was designed for use across Integrated Services Digital Network (ISDN) interfaces. Today, it is used over a variety of other network interfaces as well. This chapter focuses on Frame Relay’s specifications and applications in the context of WAN services.Frame Relay is an example of a packet-switched technology. Packet-switched networks enable endstations to dynamically share the network medium and the available bandwidth. The following twotechniques are used in packet-switching technology:

• Variable-length packets

• Statistical multiplexing

• Variable-length packets are used for more efficient and flexible data transfers. These packets are switched between the various segments in the network until the destination is reached.Statistical multiplexing techniques control network access in a packet-switched network. The advantage of this technique is that it accommodates more flexibility and more efficient use of bandwidth. Most of today’s popular LANs, such as Ethernet and Token Ring, are packet-switched networks. Frame Relay often is described as a streamlined version of X.25, offering fewer of the robust capabilities, such as windowing and retransmission of last data that are offered in X.25. This is because Frame Relay typically operates over WAN facilities that offer more reliable connection services and a higher degree of reliability than the facilities available during the late 1970s and early 1980s that served as the common platforms for X.25 WANs. As mentioned earlier, Frame Relay is strictly a Layer 2 protocol suite, whereas X.25 provides services at Layer 3 (the network layer) as well. This enables Frame Relay to offer higher performance and greater transmission efficiency than X.25, and makes Frame Relay suitable for current WAN applications, such as LAN interconnection

  

What is CIDR?

CIDR stands for Classless Inter-Domain Routing. CIDR was developed in the 1990s as a standard scheme for routing IP addresses.

CIDR Notation

CIDR specifies an IP address range by the combination of an IP address and its associated network mask. CIDR notation uses the following format -

xxx.xxx.xxx.xxx/n

where n is the number of (leftmost) '1' bits in the mask. For example,

192.168.12.0/23

applies the network mask 255.255.254.0 to the 192.168 network, starting at 192.168.12.0. This notation represents the address range 192.168.12.0 - 192.168.13.255. Compared to traditional class-based networking, 192.168.12.0/23 represents an aggregation of the two Class C networks 192.168.12.0 and 192.168.13.0 each using the default network mask 255.255.255.0.

CIDR supports Internet address allocation and message routing independent of the traditional class of a given IP address range. For example,

10.4.12.0/22

represents the address range 10.4.12.0 - 10.4.15.255 by employing the network mask 255.255.252.0. This effectively represents an apportioning of four Class C networks within the much larger Class A space.

CIDR notation is sometimes adopted even on non-CIDR networks. In non-CIDR IP subnetting, however, the value of n is restricted to either 8 (Class A), 16 (Class B) or 24 (Class C) from the Internet address allocation and routing TCP SUMMARY

TCP provides a connection oriented, reliable, byte stream service. The term connection-oriented means the two applications using TCP must establish a TCP connection with each other before they can exchange data. It is a full duplex protocol, meaning that each TCP connection supports a pair of byte streams, one flowing in each direction. TCP includes a flow-control mechanism for each of these byte streams that allows the receiver to limit how much data the sender can transmit. TCP also implements a congestion-control mechanism. 

 

Two processes communicating via TCP sockets. Each side of a TCP connection has a socket which can be identified by the pair < IP_address, port_number >. Two processes communicating over TCP form a logical connection that is uniquely identifiable by the two sockets involved, that is by the combination < local_IP_address, local_port, remote_IP_address, remote_port>. 

TCP provides the following facilities to:

Stream Data Transfer

From the application's viewpoint, TCP transfers a contiguous stream of bytes. TCP does this by grouping the bytes in TCP segments, which are passed to IP for transmission to the destination. TCP itself decides how to segment the data and it may forward the data at its own convenience.

Reliability
TCP assigns a sequence number to each byte transmitted, and expects a positive acknowledgment (ACK) from the receiving TCP. If the ACK is not received within a timeout interval, the data is retransmitted. The receiving TCP uses the sequence numbers to rearrange the segments when they arrive out of order, and to eliminate duplicate segments.

Flow Control
The receiving TCP, when sending an ACK back to the sender, also indicates to the sender the number of bytes it can receive beyond the last received TCP segment, without causing overrun and overflow in its internal buffers. This is sent in the ACK in the form of the highest sequence number it can receive without problems.

Multiplexing
To allow for many processes within a single host to use TCP communication facilities simultaneously, the TCP provides a set of addresses or ports within each host. Concatenated with the network and host addresses from the internet communication layer, this forms a socket. A pair of sockets uniquely identifies each connection.

Logical Connections

The reliability and flow control mechanisms described above require that TCP initializes and maintains certain status information for each data stream. The combination of this status, including sockets, sequence numbers and window sizes, is called a logical connection. Each connection is uniquely identified by the pair of sockets used by the sending and receiving processes.

Full Duplex
TCP provides for concurrent data streams in both directions.