Data Networks (Dimitri.P) - 阅读笔记

阅读与摘抄chap1, 3, 5.

Chapter1. Introduction and Layered Network Architecture

1.2 Messages and Switching

The purpose of data compression is to reduce the length of the bit string representing
the message.

From the standpoint of information theory, a message is regarded as one
of a collection of possible messages, with a probability distribution on the likelihood of
different messages.

It will also be shown that transmitting very long messages as units in a subnet is harmful in several ways, including delay, buffer management, and congestion control. Thus, messages represented by long strings of bits are usually broken into shorter bit strings called packets. These packets can then be transmitted through the subnet as individual entities and reassembled into messages at the destination.

The purpose of a subnet, then, is to receive packets at the nodes from sites outside
the subnet, then transmit these packets over some path of communication links and
other nodes, and finally deliver them to the destination sites.
子网的目的是：在节点接收来自子网外网站的数据包，然后经过一些通信链路和其他节点传输这些数据包，最后分发到正确的目的地。

The characteristics of sessions

From the standpoint of network users, the messages within a session are typically
triggered by particular events. From the standpoint of the subnet, however, these message
initiation times are somewhat arbitrary and unpredictable. — to model the sequence of times at which messages or packets arrive for a given session as a random process.
从子网的角度，消息到达的时间是随机、相互独立的。

common characteristics:

• Message arrival rate and variability of arrivals. Typical arrival rates for sessions
  vary from zero to more than enough to saturate the network. Simple models for the variability of arrivals include Poisson arrivals , deterministic arrivals (i.e., a fixed time interval from each message to the next message), and uniformly distributed arrivals (i.e., the time interval between successive messages has a uniform probability density between some minimum and maximum interval).

• Session holding time.

• Expected message length and length distribution. Simple models for length distribution include an exponentially decaying probability density , a uniform probability density between some minimum and maximum, and fixed length.

• Allowable Delay.

• Reliability.

• Message and packet ordering.

four types of applications

• Interactive terminal to computer sessions :
  messages are short, the message rate is low, the delay requirement is moderately stringent, and the need for reliability is high.

• file transfer sessions
  messages are very long, the message arrival rate is typically low, the delay requirement is very
  relaxed, and the need for reliability is very high.

• high-resolution graphics
  messages are again long, sometimes up to 109 bits, the delay requirement is stringent, and the arrival rate is low

• packetized voice

Circuit Switching and Store-and-Forward Switching

For the circuit switching approach, when a session s is initiated, it is allocated a given transmission rate r_s in bits per second (this could be different in the two directions of a two-way session, but we focus on a one-way session here). A path is then created from the transmitting site through the subnet and to the destination site. Each communication link on this path then allocates a portion T_s of its total transmission capacity in the given direction for that session. This allocation of transmission rates to different sessions on a communication link is usually done by time-division multiplexing (TDM) or frequency-division multiplexing (FDM) (通过时分复用或频分复用分配不同会话在通信链路上的传输速率)

the sum of the rates for all the sessions using a link cannot exceed the total capacity of the link.
所有会话的速率总和不可超过链路容量。

Circuit switching is rarely used for data networks. 电路交换很少用于数据网络。
Why:
For many of the interactive terminal sessions carried by data networks, AT is on the order of 0.01 or less. Thus, with circuit switching, that fraction of a link allocated to such sessions is utilized at most 1% of the time. If link costs are a dominant part of the cost of a network and if bursty sessions require a dominant fraction of link capacity using circuit switching, then circuit switching is an unattractive choice for data networks.

In the future, it appears that link costs will become less important in the overall cost of a network. Also, with optical fiber, the marginal cost of link capacity is quite small, so that the wasted capacity of circuit switching will become less important.

若数据网承载的终端交互会话使用在电路交换传输，则分配给这些会话的链路占了很小的比例。并且处理突发式会话会占据链路开销绝大部分。这与未来的趋势是不符的: 链路开销并不占据网络开销重要的部分。

Modification：Store-and-Forward Switching
each session is initiated without necessarily making any reserved allocation of transmission rate for the session. Similarly, there is no conventional multiplexing of the communication links. Rather, one packet or message at a time is transmitted on a communication link, using the full transmission rate of the link. The link is shared between the different sessions using that link, but the sharing is done on an as needed basis (i.e., demand basis) rather than a fixed allocation
basis. Thus, when a packet or message arrives at a switching node on its path to the
destination site, it waits in a queue for its tum to be transmitted on the next link in its
path.
每个会话不需要分配与预留传输速率，不需要复用。数据包在传输时使用全链路速率。链路被所有的会话分享，但是每个回话需要的时间基于需求来分享，而不是电路交换那样固定分配。数据包需排队等待。

Store-and-forward switching has the advantage over circuit switching that each
communication link is fully utilized whenever it has any traffic to send. However, has the disadvantage that the queueing delays in the nodes are hard to control.
存储转发方式的缺点在于其排队延迟难以控制，尤其是在排队延迟过长，而缓冲容量(buffering capacity) 即将到达极限时。

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1.3 Layering

1.3.1 The Physical Layer

The function of the physical layer is to provide a virtual link for transmitting a sequence
of bits between any pair of nodes (or any node and external site) joined by a physical
communication channel. Such a virtual link is called a virtual\quad bit \quad pipe.
three common situations for timing:

• synchronous : The higher-layer DLC module must supply bits at this synchronous rate
  whether or not it has any real data to send.

• intermittent synchronous : the DLC module supplies bits at a synchronous rate when it
  has data to send and stops sending bits when there are no data to send.

• asynchronous characters

The physical interface module that performs these mapping functions is often called a modem (digital data modulator and demodulator).

Known standards : RS-232-C & X.21

1.3.2 The Data Link Control Layer

In essence, the sending DLC module places some overhead control bits called a header at the beginning of each packet and some more overhead bits called a trailer at the end of each packet, resulting in a longer string of bits called a frame. Some of these overhead bits determine if errors have occurred in the transmitted frames, some request retransmissions when errors occur, and some delineate the beginning and ending of frames.

The MAC sublayer
need:
a DLC layer to provide a virtual error-free packet link to higher layers
a physical layer to provide a bit pipe
an intermediate layer to manage the multiaccess link

1.3.3 The Network Layer

There is one network layer process associated with each node and with each external site of the network. All these processes are peer processes and all work together in implementing routing and flow control for the network. When a frame enters a node or site from a communication link, the bits in that frame pass through the physical layer to the DLC layer.

The network layer module uses the packet header of an incoming packet, along with stored information at the module, to accomplish its routing and flow control functions.

For networks using virtual circuit routing, the routing function at the network layer module consists of two parts. The first is to select a route when the virtual circuit is being initiated, and the second is to ensure that each packet of the session follows the assigned route.