对NSMutableURLRequest的大响应适用于Wifi，而不适用于3G或4G

Question

这可能与iOS JSON Error: NSDebugDescription=Garbage at end重复，但没有得到答案。

这是我的代码：

    NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:MY_URL]];

    [request:@"POST"];
    [request setAllowsCellularAccess:YES]; // not really needed
    [request setValue:@"application/x-www-form-urlencoded" forHTTPHeaderField:@"Content-Type"];
    [request setHTTPBody:[postData dataUsingEncoding:NSASCIIStringEncoding allowLossyConversion:NO]];

    // Create url connection and fire request
    NSURLConnection *conn = [[NSURLConnection alloc] initWithRequest:request delegate:self];

然后：

- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response {
    _jsonData = [[NSMutableData alloc] init];
}

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
    [_jsonData appendData:data];
}


- (NSCachedURLResponse *)connection:(NSURLConnection *)connection
              willCacheResponse:(NSCachedURLResponse*)cachedResponse {
    return nil;
}

当JSON响应很大（大约110k左右）时，可以在有wifi连接的情况下完美解析，但在运营商网络上出错：iOS JSON错误：NSDebugDescription =垃圾邮件结束时。在这两种情况下，都会调用didReceiveData两次。

有什么想法吗？谢谢！