diff --git a/src/main.rs b/src/main.rs
index 0b2c8e2..bcb3da9 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,10 +1,35 @@
 mod modem;
 
+use std;
+use std::io::{BufReader, BufWriter, Cursor};
+use std::io::{Read, Write};
 use esp_idf_sys as _; // If using the `binstart` feature of `esp-idf-sys`, always keep this module imported
 
+fn start_loop<R: Read, W: Write>(_m: &mut modem::Modem<R, W>) -> () {
+    println!("Starting rx/tx loop (not implemented yet) ...");
+}
+
+struct Ap<'a> {
+    apn: &'a str,
+    username: &'a str,
+    password: &'a str,
+}
+
+const A1: Ap = Ap {
+    apn: "internet",
+    username: "internet",
+    password: "internet",
+};
+
 fn main() {
     println!("Starting GPRS ...");
-    let sim800l = modem::Modem::new();
-    sim800l.connect_to_gprs_ap("internet", "internet", "internet");
-    println!("GPRS started ...");
+
+    let rx = Cursor::new(Vec::new());
+    let tx = Cursor::new(Vec::new());
+    let mut sim800l = modem::Modem::new(BufReader::new(rx), BufWriter::new(tx));
+
+    match sim800l.connect_to_gprs_ap(A1.apn, A1.username, A1.password) {
+        Ok(_) => start_loop(&mut sim800l),
+        Err(e) => println!("{}", e),
+    }
 }
diff --git a/src/modem.rs b/src/modem.rs
index 4e98b58..e2664b8 100644
--- a/src/modem.rs
+++ b/src/modem.rs
@@ -1,29 +1,64 @@
 use std;
+use std::io::{Read, Write};
 
-pub struct Modem {
+pub struct Modem<R: Read, W: Write> {
     is_connected: bool,
+    rx: R,
+    tx: W,
 }
 
-#[derive(Debug, Clone)]
-pub struct ModemConnectionError;
+pub enum ModemError {
+    GprsAPConnectionError(String),
+    ATCommandError(String),
+    CommandError(String),
+}
 
-impl std::fmt::Display for ModemConnectionError {
+impl std::fmt::Display for ModemError {
     fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
-        write!(f, "gprs connection error")
-    }
-}
-
-type ModemConnection = Result<Modem, ModemConnectionError>;
-
-impl Modem {
-    pub fn new() -> Self {
-        Self {
-            is_connected: false,
+        match self {
+            Self::GprsAPConnectionError(msg) => write!(f, "gprs connection error {}", msg),
+            Self::ATCommandError(msg) => write!(f, "AT command error {}", msg),
+            Self::CommandError(msg) => write!(f, "Modem command error {}", msg),
         }
     }
-    pub fn connect_to_gprs_ap(mut self, apn: &str, username: &str, password: &str)-> ModemConnection {
-        // TODO: set AT command for connecting to gprs
-        self.is_connected = true;
-        Ok(self)
+}
+
+pub type Result<T> = std::result::Result<T, ModemError>;
+
+impl<R: Read, W: Write> Modem<R, W> {
+    pub fn new(rx: R, tx: W) -> Self {
+        Self {
+            is_connected: false,
+            rx,
+            tx,
+        }
+    }
+
+    fn at_command(&mut self, cmd: &str) -> Result<String> {
+        let mut msg = "AT+".to_owned();
+        msg.push_str(cmd);
+        self.send_command(msg.to_owned())
+            .map_err(|err| ModemError::ATCommandError(format!("{}", err)))
+    }
+
+    fn send_command(&mut self, cmd: String) -> Result<String> {
+        self.tx.write(cmd.as_bytes())
+            .map_err(|err| ModemError::CommandError(format!("{}", err)))?;
+        let mut response = String::new();
+        self.rx.read_to_string(&mut response)
+            .map_err(|err| ModemError::CommandError(format!("{}", err)))?;
+        if cfg!(debug_assertions) {
+            println!("{} = {}", cmd, response);
+        }
+        Ok(response)
+    }
+
+    pub fn connect_to_gprs_ap(&mut self, apn: &str, username: &str, password: &str)-> Result<()> {
+        self.at_command("CGATTCGATT=1")
+            .map(|_| {
+                println!("connecting to {} with {}:{}", apn, username, password);
+                self.is_connected = true;
+            })
+            .map_err(|err| ModemError::GprsAPConnectionError(format!("{}", err)))
     }
 }