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Data
Data link frame
Frame header
IP header
Data
Frame trailer
Packet terminology. (Courtesy of Feit, 1997.)
referred to as data. The packet comprising the TCP header, and the data are a TCP segment. The packet comprising the UDP header and the data is a UDP message. The packet comprising the IP header, the TCP or UDP header, and the data is an IP datagram. Finally, the packet comprising the data-link frame header, the frame trailer (used for error control), and the IP datagram is a frame. It should be noted here that the preceding definitions are those used in version 4 of the IPv4. IPv6 is a more recent version being brought on-stream in which the IP datagram is in fact called an IP packet. Some of the units used in data transmission are:
Byte. Common usage has established the byte (symbol B) as a unit of 8 bits, and this practice will be followed here. It should be noted, however, that in computer terminology, a byte can mean a unit other than 8 bits, and the 8-bit unit may be called an octet. Kilobyte. The kilobyte (symbol kB) is 1024 bytes. Transmission rates may be stated in kilobytes per second or kB/s. Megabyte. The megabyte (symbol MB) is 1024 kilobytes. Transmission rates may be stated in megabytes per second or MB/s.
The TCP/IP suite is shown in Fig. 15.12, and an excellent detailed description of these protocols will be found in Feit (1997). The present text will be concerned more with the special enhancements needed on TCP/IP for successful satellite transmission.
Fifteen
NEWS
SMTP
Telnet
SNMP
TCP End-to-end connections IP Route datagrams Underlying communications Ethernet
UDP Single messages
Token ring
FDDI
Frame relay
Point-topoint
The TCP/IP suite. (Courtesy of Feit, 1997.)
15.8 The TCP Link A virtual communications link exists between corresponding layers in a network. The header in the TCP segment (see Fig. 15.11) carries instructions that enable communication between the send and receive TCP layers. Of course, the communication has to pass through the other layers and along the physical link, but only the TCP layers act on the TCPs contained in the segment header. There is no direct physical link between the TCP layers, and for this reason, it is called a virtual link. The send and receive TCP layers have buffer memories (usually just called buffers). The receive buffer holds incoming data while they are being processed. The send buffer holds data until they are ready for transmission. It also holds copies of data already sent until it receives an acknowledgment that the original has been received correctly. The receive window is the amount of receive buffer space available at any given time. This changes as the received data are processed and removed from the buffer. The receive TCP layer sends an acknowledgment (ACK) signal to the send TCP layer when it has cleared data from its buffer, and the ACK signal also provides an update on the current size of the receive window. The send TCP layer keeps track of the amount of data in transit and, therefore, unacknowledged. It can calculate the amount of receive buffer space remaining, allowing for the data in transit. This remaining buffer space represents the amount of data that can still be sent and is termed the send window. The send TCP layer also sets a timeout period, and failure to receive an ACK signal within this period results in a duplicate packet being sent. On terrestrial networks, the probability of bit error (see Chap. 10) is extremely low, and congestion is the most likely
Satellites in Networks
reason for loss of ACK signals. Because a network carries traffic from many sources, traffic congestion can occur. The IP layer of the TCP/IP discards packets when congestion occurs, and hence the corresponding ACK signals from the TCP layer do not get sent. Rather than continually resending packets, the send station reduces its rate of transmission, this being known as congestion control. A congestion window is applied, which starts at a size of one segment for a new connection. The window is doubled in size for each ACK received until it reaches a maximum value determined by the number of failed ACKs experienced. For normal operation, the congestion window grows in size to equal the receive window. The congestion window increases slowly at first, but as each doubling takes effect, the size increases exponentially. This controlling mechanism is known as slow start. If congestion sets in, this will be evidenced by an increase in the failure to receive ACKs, and the send TCP will revert to the slow start. 15.9 Satellite Links and TCP Although satellite links have formed part of the Internet from its beginning, the rapid expansion of the Internet and the need to introduce congestion control have highlighted certain performance limitations imposed by the satellite links. Before discussing these, it should be pointed out that the increasing demand for Internet services may well be met best with satellite direct-to-home links, and many companies are actively engaged in setting up just such systems. In the ideal case, the virtual link between TCP layers should not be affected by the physical link, and certainly the TCP is so well established that it would be undesirable (some would say unacceptable) to modify it to accommodate peculiarities of the physical link. The factors that can adversely affect TCP performance over satellite links are as follows: Bit error rate (BER). Satellite links have a higher bit error rate (BER; see Chap. 10) than the terrestrial links forming the Internet. Typically, the satellite link BER without error-control coding is around 10 6, whereas a level of 10 8 or lower is needed for successful TCP transfer (Chotikapong and Sun, 2000). The comparatively low BER on terrestrial links means that most packet losses are the result of congestion, and the TCP send layer is programmed to act on this assumption. When packets are lost as a result of high BER, therefore, as they might on satellite links, the TCP layer assumes that congestion is at fault and automatically invokes the congestion control measures. This slows the throughput. Round-trip time (RTT). The round-trip time (RTT) of interest here is the time interval that elapses between sending a TCP segment and
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