随着数字通信技术和计算机技术的快速发展以及通信网与计算机网络的相 互融合,信息科学技术已成为21世纪国际社会和世界经济发展的新的强大推动 力。信息作为一种资源,只有通过广泛地传播与交流,才能产生利用价值、促进 社会成员之间的合作、推动社会生产力的发展、创造出巨大的经济效益。 本章将简要介绍通信系统的基本概念、组成和分类、信息的度量以及评价通 信系统性能的指标

目前，数字技术已渗透到科研、生产和人们日常生活的各个领域。从计算机到家用电器，从手机到数字电话，以及绝大部分新研制的医用设备、军用设备等，无不尽可能地采用了数字技术。数字系统是对数字信息进行存储、传输、处理的电子系统。通常把门电路、触发器等称为逻辑器件，将由逻辑器件构成，能执行某单一功能的电路，如计数器、译码器、加法器等，称为逻辑功能部件，把由逻辑功能部件组成的能实现复杂功能的数字电路称数字系统。复杂的数字系统可以分割成若干个子系统，例如计算机就是一个内部结构相当复杂的数字系统

Outline Network Layer and Internetworking The TCP/IP protocol suit ATM

Must choose routes for various origin destination pairs(O/D pairs) or for various sessions - Datagram routing: route chosen on packet by packet basis Using datagram routing is an easy way to split paths Virtual circuit routing: route chosen a session by session basis Static routing: route chosen in a prearranged way based on O/D pairs Eytan Modiano

Flow control: end-to-end mechanism for regulating traffic between source and destination Congestion control: Mechanism used by the network to limit congestion The two are not really separable, and I will refer to both as flow control In either case, both amount to mechanisms for limiting the amount of traffic entering the network Sometimes the load is more than the network can handle Eytan Modiano

Optimal Routing View routing as\global\ optimization problem Assumptions: The cost of using a link is a function of the flow on that link The total network cost is the sum of the link costs The required traffic rate between each source-destination pair is known in advance Traffic between source-destination pair can be split along multiple paths with infinite precision

A packet switch consists of a routing engine(table look-up), a switch scheduler, and a switch fabric. The routing engine looks-up the packet address in a routing table and determines which output port to send the packet. Packet is tagged with port number The switch uses the tag to send the packet to the proper output port Eytan Modiano

Carrier Sense Multiple Access(CSMA) In certain situations nodes can hear each other by listening to the channel “Carrier Sensing CSMA: Polite version of Aloha Nodes listen to the channel before they start transmission Channel idle=> Transmit Channel busy = Wait (join backlog) When do backlogged nodes transmit? When channel becomes idle backlogged nodes attempt transmission with probability q=1 Persistent protocol, q=1 Non-persistent protocol,< 1

Broadcast Routing Route a packet from a source to all nodes in the network Possible solutions: Flooding: Each node sends packet on all outgoing links Discard packets received a second time Spanning Tree Routing: Send packet along a tree that includes all of the nodes in the network

Multiple Access shared transmission medium a receiver can hear multiple transmitters a transmitter can be heard by multiple receivers the major problem with multi-access is allocating the channel between the users the nodes do not know when the other nodes have data to send Need to coordinate transmissions