一、SIM卡应用工具 二、无线应用协议

一、主要承载技术GPRS 二、短消息业务

MIT OCW DATA COMMUNICATION CONTENTS Lectures 2 The Data Link Layer: Framing and Error Detection Lectures 3 4 The Data link layer: arQ Protocols Lectures 5 6 Introduction to Queueing Theory Lectures 8&9 M/G/I Queues

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

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

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

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

An interesting property of an M/M/ 1 queue, which greatly simplifies combining these queues into a network, is the surprising fact that the output of an M/M/ queue with arrival rate is a Poisson process of rate This is part of Burke's theorem, which follows from reversibility A Markov chain has the property that P[future present, past] P[future present] Conditional on the present state, future states and past states are independent

Service times Poisson arrivals at rate n Service time has arbitrary distribution with given E[X] and E[X2I Service times are independent and identically distributed (ID) Independent of arrival times E[service time]=1/u Single Server queue

Data Link Layer (DLC) Responsible for reliable transmission of packets over a link Framing: Determine the start and end of packets(sec 2.5) Error Detection: Determine when a packet contains errors ( sec 2.3) Error recovery: Retransmission of packets containing errors(sec 2. 4) DLC layer recovery May be done at higher layer