专业术语-GPRS_电脑电信
general packet radio service (gprs) is a mobile data service available to users of gsm mobile phones. it is often described as "2.5g", that is, a technology between the second (2g) and third (3g) generations of mobile telephony. it provides moderate speed data transfer, by using unused tdma channels in the gsm network. originally there was some thought to extend gprs to cover other standards, but instead those networks are being converted to use the gsm standard, so that is the only kind of network where gprs is in use. gprs is integrated into gsm standards releases starting with release 97 and onwards. first it was standardised by etsi but now that effort has been handed onto the 3gpp.
gprs basics
gprs is different from the older circuit switched data (or csd) connection included in gsm standards releases before release 97 (from 1997, the year the standard was feature frozen). in csd, a data connection establishes a circuit, and reserves the full bandwidth of that circuit during the lifetime of the connection. gprs is packet-switched which means that multiple users share the same transmission channel, only transmitting when they have data to send. this means that the total available bandwidth can be immediately dedicated to those users who are actually sending at any given moment, providing higher utilisation where users only send or receive data intermittently. web browsing, receiving e-mails as they arrive and instant messaging are examples of uses that require intermittent data transfers, which benefit from sharing the available bandwidth.
usually, gprs data are billed per kilobytes of information transceived while circuit-switched data connections are billed per second. the latter is to reflect the fact that even during times when no data are being transferred, the bandwidth is unavailable to other potential users.
gprs originally supported (in theory) ip, ppp and x.25 connections. the latter has been typically used for applications like wireless payment terminals although it has been removed as a requirement from the standard. x.25 can still be supported over ppp, or even over ip, but doing this requires either a router to do encapsulation or intelligence built into the end terminal.
gprs speeds and profile
packet-switched data under gprs is achieved by allocating unused cell bandwidth to transmit data. as dedicated voice (or data) channels are setup by phones, the bandwidth available for packet switched data shrinks. a consequence of this is that packet switched data has a poor bit rate in busy cells. the theoretical limit for packet switched data is approx. 160.0 kbit/s (using 8 time slots and cs-4). a realistic bit rate is 30–80 kbit/s, because it is possible to use max 4 time slots for downlink. a change to the radio part of gprs called edge allows higher bit rates of between 160 and 236.8 kbit/s. the maximum data rates are achieved only by allocation of more than one time slot in the tdma frame. also, the higher the data rate, the lower the error correction capability. generally, the connection speed drops logarithmically with distance from the base station. this is not an issue in heavily populated areas with high cell density, but may become an issue in sparsely populated/rural areas.
the gprs capability classes
class a
can be connected to gprs service and gsm service (voice, sms), using both at the same time. no such devices are known to be available today.
class b
can be connected to gprs service and gsm service (voice, sms), but using only one or the other at a given time. during gsm service (voice call or sms), gprs service is suspended, and then resumed automatically after the gsm service (voice call or sms) has concluded. most gprs mobile devices are class b.
class c
are connected to either gprs service or gsm service (voice, sms). must be switched manually between one or the other service.
gprs multislot classes
gprs speed is a direct function of the number of tdma time slots assigned, which is the lesser of (a) what the particular cell supports and (b) the maximum capability of the mobile device expressed as a gprs multislot class.
the most common gprs multislot classes are:
class 2
minimal gprs implementation
class 4
modest gprs implementation, 50% faster download than class 2
class 6
modest implementation, but with better uploading than class 4
class 8
better implementation, 33% faster download than classes 4 & 6
class 10
better implementation, and with better uploading than class 8, seen in better cell phones and pc cards
class 12
best implementation, with maximum upload performance, typically seen only in high-end pc cards
gprs coding
transfer speed depends also on the channel encoding used. the least robust (but fastest) encoding scheme (cs-4) is available near the base transceiver station (bts) while the most robust encoding scheme (cs-1) is used when the mobile station (ms) is further away from the bts.
using the cs-4 it is possible to achieve a user speed of 20.0 kbit/s per time slot. however, using this scheme the cell coverage is 25% of normal. cs-1 can achieve a user speed of only 8.0 kbit/s per time slot, but has 98% of normal coverage. newer network equipment can adapt the transfer speed automatically depending on the mobile location.
speed (kbit/s) cs-1 8.0 cs-2 12.0 cs-3 14.4cs-4 20.0
download (kbit/s) upload (kbit/s) csd9.69.6hscsd28.8 14.4 (2+1) hscsd43.2 14.4 (3+1) gprs 4+180.020.0 (class 8 & 10 and cs-4) gprs 3+2 60.040.0 (class 10 and cs-4)
note: like csd, hscsd establishes a circuit and is usually billed per minute. for an application such as downloading, hscsd may be preferred, since circuit-switched data are usually given priority over packet-switched data on a mobile network, and there are relatively few seconds when no data are being transferred.
gprs is packet based. when tcp/ip is used, each phone can have one or more ip addresses allocated. gprs will store and forward the ip packets to the phone during cell handover (when you move from one cell to another). a radio noise induced pause can be interpreted by tcp as packet loss, and cause a temporary throttling in transmission speed.
gprs services
gprs upgrades gsm data services providing:
point-to-point (ptp) service: internetworking with the internet (ip protocols) and x.25 networks.
point-to-multipoint (pt2mp) service: point-to-multipoint multicast and point-to-multipoint group calls.
short message service (sms): bearer for sms.
anonymous service: anonymous access to predefined services.
future enhancements: flexible to add new functions, such as more capacity, more users, new accesses, new protocols, new radio networks.
gprs in practice
telephone operators have priced gprs relatively cheaply (compared to older gsm data transfer, csd and hscsd) in many areas, such as finland. most mobile phone operators don't offer flat rate access to the internet (with the notable exceptions of t-mobile and cingular in the united states), instead basing their tariffs on data transferred, usually rounded off per 100 kilobyte.
typical rates vary wildly, ranging from eur 1 per megabyte to over 20 per megabyte. in the u.s., t-mobile offers us$30 per month unlimited gprs. in india, bpl mobile (bombay) offers unlimited gprs for rs.500 (usd 11) per month. airtel offers nation-wide unlimited gprs and edge for rs. 600 (usd 13.5). orange (uk) offers a 1 gigabyte package for 128 a month, and a £1 per day unlimited use package for pre-paid users. in poland, era gsm offers a 2 gigabyte package of gprs and umts transmission under blueconnect brand for 30 a month and plus gsm offerts a 1 gigabyte package for 15 a month.
gprs data on pre-paid packages is usually expensive, and limited to wap and mms. full internet access, allowing web browsing, access to pop/imap mail, ftp and other mainstream web applications is usually rest
ricted to contract packages, and are made available at lower cost.
the maximum speed of a gprs connection (as offered in 2003) is the same as modem connection in an analog wire telephone network, about 4–5 kb/s (depending on the phone used). latency is very high; a round-trip ping being typically about 600–700 ms and often reaching one second round trip time. gprs is typically prioritised lower than speech, and thus the quality of connection varies greatly.
in order to set up a gprs connection, a user needs to specify access point name (apn), user name and password, and very often an ip address, all provided by the network operator.
general packet radio service (gprs)通用分组无线服务 是gsm移动电话用户可用的一种移动数据业务。 它经常被描述成 "2.5g",也就是说这项技术位于第二代 (2g)和第三代(3g)移动通讯技术之间。它通过利用gsm网络中未使用的tdma信道，提供中速的数据传递。最初有人想通过扩展gprs来覆盖其他标准，只是这些网络都正在转而使用gsm标准，这样gsm就成了gprs唯一能够使用的网络。gprs在release 97之后被集成进gsm标准,起先它是由etsi标准化的，但是当前已经移交3gpp负责。
gprs原理
gprs 区别于旧的 电路交换 (or csd) 连接，连接在release 97之前（gsm电话功能还没怎么开发）就已经包含进gsm标准中。在旧有系统中一个数据连接要创建并保持一个电路连接，在整个连接过程中这条电路被独占直到连接被拆除。 gprs 基于报文交换，也就是说多个用户可以共享一个相同的传输通道，每个用户只有在传输数据的时候才会占用信道。这就意味着所有的可用带宽可以立即分配给当前发送数据的用户，这样更多的间隙发送或者接受数据的用户可以共享带宽。 web浏览，收发电子邮件和即时消息都是共享带宽的间歇传输数据的服务。
通常gprs数据的计费是不是按照电路交换方式的秒，而是千字节kb。电路交换方式下，即使网络上没有数据传输，其他用户也不能使用空闲的信道。
gprs最初支持(理论上)互联网协议ip,点到点协议ppp和x.25连接。后者典型的应用是无线付费终端，尽管它已经作为标准需求被去除。x.25依然可以通过ppp甚至是ip得到支持, 但是这样做既不需要重新封装也不用集成什么到终端里。
gprs速度和属性
基于gprs的报文数据交换使用未使用的蜂窝网络带宽传输数据。 而作为专门为电话系统设计的语音信道（或者数据信道）一旦被报文数据交换使用，将降低可用带宽，其结果是如果在一个忙碌的电话域内，报文传输速度极慢。理论上报文数据交换速度是大约170千比特/秒，而实际速度是30-70千比特/秒。在gprs的射频部分的改进，取名为edge技术，将支持从20至200千比特/秒的更高速度传输。最大数据速率取决于同时分配到的tdma帧的时隙。因此，数据速率越高，纠错可靠性就越低。一般来说，连接速度随着与距离的增加迅速下降。在人口密集的高网络密度城区这倒不是什么大问题，但是在人口比较少的郊区这就真是问题了。
gprs class 8 也就是平常所说的4+1。这表示4个时隙用于下行流量，1个时隙用于上行流量。 this profile is appropriate for 这样做是为了优化象web浏览器这样的大部分是下载流量的应用。如果用户阅读邮件量大于他发送的量，这个也适用。一般来说gprs手机默认使用 class 8 来传输。
gprs class 10也就是4+2。4个时隙下行，2个时隙上行。不过同时使用的时隙不能超过5个。 这个方案适用于双向数据差不多相等的情况下，例如即时消息.
其他存在的级别，包括gprs class 6 (3+2) 和gprs class 4 (3+1)，只有老设备才使用。有些个别设备能够做到 4+4 (四个时隙用于上行和下行，最多5个同时工作).这只是工业应用，超过2个上行时隙电磁辐射就会对人体产生一定的影响。.
传输速率还依赖于信道编码。最佳编码方案（cd-4）适用于在基站附近的时候，最差编码方案（cd-1）用于离基站比较远的地方。.
使用cs-4 有可能达到21,4 kbps每时隙的速度。但是如果使用这个方案先有网络只能覆盖一般情况的25%的区域。. cs-1能达到9.05 kbit/s 每时隙的速率而且可以覆盖98%的正常区域。
每一个时间片可以达到最高14.4kbps的速率。
下载 上传gprs 4+1 57.6 kbps14.4kbps(class 8 & 10)gprs 3+243.2 kbps 28.8kbps(class 10)csd9.6 kbps9.6kbpshscsd28.8 kbps14.4kbps (2+1)hscsd43.2 kbps14.4kbps (3+1)
注意！csd及hscsd这类服务通常都按使用时间来计费,就像通话时间般。假如要长时进行下载档案的动作,则会比gprs优胜，因为在移动电话网络中csd]及hscsd的优先次序都会比gprs为高,较少有数据传输中断的情况出现。
gprs报文数据交换基于数据包。 当使用tcp/ip协议时，每个电话分配到有一个或多个ip地址。 当电话切换扇区或者基站时，gprs要暂时存储转发数据包到电话里。当因为无线电噪声干扰导致传输暂停和丢包可以由tcp来处理，这将导致临时的传输速率调整。
gprs服务
- gprs提升gsm的数据服务性能:
- 点到点 (p2p) 服务: 连接 (ip protocols)ip网络 and x.25网络。
- 多播 (p2mp)服务 : 一点到多点的组播和多方通话。
- 短信服务 (sms): 发送sms。
- 彩信服务mms: 发送携带语音和图像的短消息。
- 因特网服务提供商服务: 提供互联网内容服务。
- 邮件服务通过pop3或者imap协议检查阅读发送电子邮件
- 匿名服务: 匿名访问预定服务
- 未来功能: 灵活加入新的功能，例如更大容量，更多用户，新的资源和无线网络。
现实中的gprs
运营商已经给gprs制定了相对便宜的价格(相对于旧的gsm数据通讯、蜂窝数据交换、高速电路交换数据）在很多地方例如芬兰，多数运营商不支持包价收费访问因特网 (但是美国的t-mobile是一个著名的例外), 取而代之的是按照数据流量，通常以100kb或者1kb（中国移动通信）作为计费单位。
典型的费率比较高昂，在美国t-mobile提供每月20美元无限量gprs使用的服务。 其他运营商象at&t wireless也提供包月。orange (英国)提供88英镑每月1gb流量套餐。中国运营商中国移动通信的费率提供多种选择方式，比如神州行用户可以选择15元人民币包月gprs上网（这相当便宜）而动感地带用户可以选择包月限定流量，或者不缴纳月租费而以每kb0.03元人民币计费。
gprs预付费数据服务一般比较贵, 局限在wapmms。
gprs最大连接速度跟模拟电话网络上的调制解调器一样大约4-5 kb/s (依赖于电话)。 延迟比较高，在回路测试ping中，典型的要大约600-700毫秒，往返时间经常达到 1 秒 。 gprs的实际性能十分普遍的低于理论值，而且连接质量很不稳定，很易受到影响产生大幅波动。
gprs的性能
gprs分为三级性能， 第一个类别是a级性能，指可以同时连接使用gprs和gsm的服务传送语音和数据，移动电台需具备双工以支援上述功能。
第二个类别是b级实施，即同时支持gprs附件和gsm附件，但是不同时支持以上两种服务。移动电台可以自动转换使用gprs服务或是gsm服务，但不能同时传送voice与传送data。所以当移动电台在传送资料收到来电话的呼叫时，会暂停传送资料，接听电话后在继续传送火数据资料。
第三个类别是c级实施，功能是只能使用gprs或是gsm的服务（语音、短信）。必须靠手动在两种服务中进行切换。